Diversity in plant functions

Class 11 Biology, unit 9, book pages 234–270

Practise 72 MCQs Take the unit quiz Review 61 cards

This unit explains how a plant lives: how it takes in mineral nutrients and, in a few cases, insects; how it exchanges gases through stomata; how water, minerals and sugars are moved around the body by xylem and phloem; and how it keeps its water and temperature balance. It then shows how plants are supported and how they grow, and how hormones, light, gravity, touch, day length and cold control growth, movement and flowering. Many FBISE questions come from the transport mechanisms (water potential, TACT, stomata, pressure flow) and from growth and photoperiodism.

Learning outcomes (24)

List the macro and micronutrients of plants highlighting the role of each nutrient.
State examples of carnivorous plants.
Explain the role of stomata and palisade tissues in exchange of gases in plants.
Relate transpiration with gas exchange in plants.
Assess the structure of xylem vessel elements, sieve tube elements, companion cells, tracheids and related their structures with functions.
Describe the movement of water between plant cells and between the cells and their environment in terms of water potential.
Describe the movement of water through the roots in terms of symplast, apoplast and vacuolar pathways.
Explain the movement of water in xylem through TACT mechanism.
Describe the mechanism involved in opening and closing of stomata.
Explain the movement of sugar within the plants.
Define osmotic adjustment.
State movement of water into or out of the cell in isotonic, hypotonic and hypertonic conditions.
Explain the osmotic adjustment in hydrophytic (marine and freshwater), xerophytic and mesophytic plants and plants in the saline soil.
List adaptations in plants to cope with low and high temperatures.
Explain the turgor pressure and its significance in providing support to herbaceous plants.
Describe the structure of supporting tissues in plants.
Define growth and explain primary and secondary growth in plants.
Justify the formation of annual rings.
Explain influence of apical meristem on the growth of lateral shoots.
Outline the role of important plant growth regulators.
Explain the types of movement in plants in response to light, force of gravity, touch and chemicals.
Define photoperiodism.
Classify with examples plants on the basis of photoperiodism and describe the mechanism of photoperiodism with reference to the mode of action of phytochromes.
Explain the role of low temperature treatment on flower production, especially to biennial and perennial plants.

9.1Nutrition in plants

Diversity in plant functions covers how a plant grows from a seed to the whole body, how it absorbs water and minerals and carries them to the leaves, how it makes organic solutes and distributes them, how it exchanges carbon dioxide and oxygen with its surroundings, how it adapts to changing conditions and how it produces young ones to continue the species. Some of these functions were met in Grade 9 (Plant Physiology).

Nutrient: Any substance that supplies the essential elements an organism needs for its growth and metabolic functions. Plant nutrients are mainly inorganic compounds.
Nutrition: All the processes involved in the utilisation of nutrients to support growth and the various metabolic activities within an organism's body.

Plants are autotrophs: they make their own organic compounds. To do so they take inorganic nutrients (water, carbon dioxide and specific minerals) from their surroundings, and then convert them into essential organic compounds through biochemical processes inside the plant.

From inorganic nutrients to organic compounds

The plant absorbs inorganic nutrients: water and minerals through the roots, carbon dioxide through the leaves.
Biochemical processes (photosynthesis and the reactions that follow it) change these inorganic nutrients into organic compounds.
The organic compounds are used for growth and metabolism of the plant body.

9.1.1Types of plant nutrientsOften examined

Sixteen elements have been found essential for plant growth. Nine of them are needed in fairly large quantities (more than $0.05%$ of the dry weight) and are called macronutrients. The remaining seven are needed only in traces or small amounts (less than $0.05%$ of the dry weight) for normal plant growth and development and are called micronutrients.

Macronutrients and micronutrients
Group	Amount needed	Elements named by the book
Macronutrients (9)	More than $0.05%$ of dry weight	Carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, potassium, calcium, magnesium
Micronutrients (7)	Less than $0.05%$ of dry weight	Iron, boron, manganese, copper, molybdenum, chlorine, zinc

Role of plant nutrients and effect of their deficiencies

Each mineral nutrient, whether macro or micro, is very important for normal plant growth. Even the deficiency of a micronutrient can cause complications in the plant body, generally called deficiency diseases. Table 9.1 gives the details.

Table 9.1 (a): Macronutrients in plants
Element (form primarily absorbed)	% mass in dry tissue	Major functions	Early visual symptoms of deficiency
Carbon ( $CO X_{2}$ )	$45%$	Major component of plant's organic compounds	Poor growth
Oxygen ( $O X_{2}$ )	$45%$	Major component of plant's organic compounds	Poor growth
Hydrogen ( $H X_{2} O$ )	$6%$	Major component of plant's organic compounds	Wilting, poor growth
Nitrogen ( $NO X_{3} X^{-}$ , $NH X_{4} X^{+}$ )	$1.5%$	Component of nucleic acids, proteins and chlorophyll	Chlorosis at tips of older leaves (common in heavily cultivated soils or soils low in organic material)
Potassium ( $K X^{+}$ )	$1.0%$	Cofactor of many enzymes; major solute functioning in water balance; operation of stomata	Mottling of older leaves, with drying of leaf edges; weak stems; roots poorly developed (common in acidic or sandy soils)
Calcium ( $Ca X^{2 +}$ )	$0.5%$	Important component of middle lamella and cell walls; maintains membrane function; signal transduction	Crinkling of young leaves; death of terminal buds (common in acidic or sandy soils)
Magnesium ( $Mg X^{2 +}$ )	$0.2%$	Component of chlorophyll; cofactor of many enzymes	Chlorosis between veins, found in older leaves (common in acidic or sandy soils)
Phosphorus ( $H X_{2} PO X_{4} X^{-}$ , $HPO X_{4} X^{2 -}$ )	$0.2%$	Component of nucleic acids, phospholipids, ATP	Healthy appearance but very slow development; thin stems; purpling of veins; poor flowering and fruiting (common in acidic, wet or cold soils)
Sulphur ( $SO X_{4} X^{2 -}$ )	$0.1%$	Component of proteins	General chlorosis in young leaves (common in sandy or very wet soils)

Table 9.1 (b): Micronutrients in plants
Element (form primarily absorbed)	% mass in dry tissue	Major functions	Early visual symptoms of deficiency
Chlorine ( $Cl X^{-}$ )	$0.01%$	Photosynthesis (water-splitting); functions in water balance	Wilting; stubby roots; leaf mottling (uncommon)
Iron ( $Fe X^{3 +}$ , $Fe X^{2 +}$ )	$0.01%$	Respiration; photosynthesis; chlorophyll synthesis; $N X_{2}$ fixation	Chlorosis between veins, found in young leaves (common in basic soils)
Manganese ( $Mn X^{2 +}$ )	$0.005%$	Active in formation of amino acids; activates some enzymes; required for the water-splitting step of photosynthesis	Chlorosis between veins, found in young leaves (common in basic soils rich in humus)
Boron ( $H X_{2} BO X_{3} X^{-}$ )	$0.002%$	Cofactor in chlorophyll synthesis; role in cell wall function; pollen tube growth	Death of meristems; thick, leathery and discoloured leaves (occurs in any soil; most common micronutrient deficiency)
Zinc ( $Zn X^{2 +}$ )	$0.002%$	Active in formation of chlorophyll; cofactor of some enzymes; needed for DNA transcription	Reduced internode length; crinkled leaves (common in some geographic regions)
Copper ( $Cu X^{+}$ , $Cu X^{2 +}$ )	$0.001%$	Component of many redox and lignin-biosynthetic enzymes	Light green colour throughout young leaves, with drying of leaf tips; roots stunted and excessively branched (common in some geographic regions)
Nickel ( $Ni X^{2 +}$ )	$0.001%$	Nitrogen metabolism	General chlorosis in all leaves; death of leaf tips (common in acidic or sandy soils)
Molybdenum ( $MoO X_{4} X^{2 -}$ )	$0.0001%$	Nitrogen metabolism	Death of root and shoot tips; chlorosis in older leaves (common in acidic soils in some geographic areas)

Science, Technology and Society connections: symptoms of mineral deficiency

Three major symptoms of mineral deficiency are named by the book:

Symptoms of mineral deficiency (as listed in the book)
Symptom	Meaning	Deficiency of
Necrosis	Death of a part of the plant body	Calcium, magnesium, copper and molybdenum
Chlorosis	Yellowing of the green parts of a plant, showing lack of chlorophyll	Nitrogen and magnesium
Stunted growth	Less than normal growth	Phosphorus, sulphur and potassium

Symptoms of mineral deficiency on a leaf. The symptoms are drawn in the accent colour.

Worked example: diagnose the deficiency

A farmer finds that the young leaves of his crop are yellow between the veins while the veins stay green. The soil is basic. A second field has older leaves yellow between the veins and the soil is sandy and acidic. Which nutrient is probably deficient in each field, and why?

Look at the age of the leaves first. Nutrients that cannot move easily show symptoms in young leaves, so the table gives young-leaf symptoms for iron and manganese and older-leaf symptoms for magnesium.
Field 1: chlorosis between veins, in young leaves, in a basic soil. Table 9.1 gives this for iron (and also for manganese in basic soils rich in humus). Iron is needed for chlorophyll synthesis, so chlorosis follows.
Field 2: chlorosis between veins, in older leaves, in an acidic or sandy soil. This is the symptom of magnesium deficiency. Magnesium is the central atom of chlorophyll, so without it the leaf turns yellow.
Remedy for both fields: add a fertilizer that contains the missing element, or use crop rotation so that the same crop is not grown again and again in the same soil.

Field 1: iron (or manganese); Field 2: magnesium

9.1.2Nutrition of carnivorous plants

Carnivorous (or insectivorous) plants are a unique category of photosynthetic autotrophs. In spite of being able to photosynthesise, they have evolved to include insects in their diet. This adaptation is useful in environments where nitrogenous salts are scarce, such as marshy areas. In such nitrogen-deficient habitats these plants have developed specialised mechanisms to capture insects and use them as a supplementary source of nitrogen.

Their modified leaves serve as traps to capture and digest insects.
The leaves have various mechanisms and structures for insect capture, digestion and nutrient absorption.
Some carnivorous plants form symbiotic relationships with specific bacteria. These bacteria help to break down insect proteins, so the plant extracts the essential nutrients more easily.

Figure 9.1: (a) Pitcher plant, (b) Venus flytrap, (c) Sundew (simplified drawings of the book's photographs).

Examples of carnivorous plants
Carnivorous plant	Scientific name (as in the book)
Pitcher plant	Nepenthes pupurea
Venus flytrap	Dionaea muscipula
Sundew	Drosera intermedia

9.2Gaseous exchange in plants

Gaseous exchange in plants is the movement of $CO X_{2}$ and $O X_{2}$ between the plant body and its environment. It occurs mainly by diffusion. Plants have no special organs for it, so every cell carries out the exchange of oxygen and carbon dioxide according to its own needs. The exchange mainly occurs through two openings: the stomata and the lenticels.

Gas exchange during the day and the night

Gas exchange at different times of the day
Time	What the plant is doing	Gas exchange with the surroundings
Daytime	Photosynthesis is active and faster than respiration	Takes in $CO X_{2}$ and releases $O X_{2}$
Night	Photosynthesis stops; respiration continues	Absorbs $O X_{2}$ and releases $CO X_{2}$
Dawn (sunrise) and dusk (sunset)	Rates of photosynthesis and respiration are balanced	No net exchange of gases (the compensation point of photosynthesis)

Gas exchange of a leaf during the day, at the compensation point and at night. Accent arrows show the net movement of gases.

Compensation point of photosynthesis: The time at dawn and dusk when the rates of photosynthesis and respiration are balanced, so there is no net exchange of gases between the plant and its surroundings.

Root epidermal cells also get oxygen from the soil air and release carbon dioxide to it, so that cellular respiration can occur in the roots.

9.2.1Role of palisade and spongy mesophyll in exchange of gasesOften examined

Mesophyll cells are thin-walled living cells found between the two epidermal layers within a leaf. They are adapted to carry out photosynthesis. Mesophyll cells are very busy with metabolic activities and play a vital role in gas exchange, which happens through the structures called stomata.

Types of mesophylls in dicot and monocot leaves

In dicot plants there are two mesophyll layers:

Palisade mesophyll: tightly packed, elongated cells with no spaces in between. It lies next to the upper epidermis.
Spongy mesophyll: loosely arranged cells, larger, with large spaces between them. It lies next to the lower epidermis.

A leaf with these two types of mesophyll and a distinguishable upper and lower epidermis is called a bifacial or dorsiventral leaf. In monocot plants the leaf is different: it is monofacial or isobilateral and has only spongy mesophyll cells between both epidermal layers.

Figure 9.2: Bifacial leaf (transverse section).

Palisade and spongy mesophyll compared
Feature	Palisade mesophyll	Spongy mesophyll
Position	Just below the upper epidermis	Just above the lower epidermis
Arrangement of cells	Tightly packed, elongated cells	Loosely arranged, larger cells
Spaces between cells	No spaces in between	Large spaces between the cells

Bifacial and monofacial leaves compared
Feature	Bifacial (dorsiventral) leaf	Monofacial (isobilateral) leaf
Occurs in	Dicot plants	Monocot plants
Mesophyll	Palisade and spongy layers	Only spongy mesophyll
Epidermal surfaces	Upper and lower sides can be told apart	Both sides alike

Worked example: identify the parts of a leaf section

The figure shows a transverse section of a dicot leaf with parts labelled by letters. Name each part and say which layer is next to the upper epidermis.

A transverse section of a dicot leaf (parts lettered).

Start from the top of the drawing. A is the thin layer on the outside: the cuticle. B is the single row of cells below it: the upper epidermis.
Below B is a layer of tightly packed elongated cells with no spaces: C, the palisade mesophyll. This is the layer next to the upper epidermis.
Below C are loosely arranged cells with large air spaces: F, the spongy mesophyll, and G, the intercellular space between its cells.
The cluster of thick-walled cells at the left is the vascular bundle: D is a xylem vessel and E is phloem.
At the bottom, H is the lower epidermis. The gap in it is K, the stoma, and the two bean-shaped cells on either side are the guard cells, J.

Letter	Part
A	Cuticle
B	Upper epidermis
C	Palisade mesophyll
D	Xylem vessel
E	Phloem
F	Spongy mesophyll
G	Intercellular space
H	Lower epidermis
J	Guard cell
K	Stoma

A cuticle, B upper epidermis, C palisade mesophyll, D xylem, E phloem, F spongy mesophyll, G intercellular space, H lower epidermis, J guard cell, K stoma

9.2.2Relationship of stomata and transpiration with exchange of gasesOften examined

Stomata are tiny, curved openings found on the leaf surface. A single opening is called a stoma. Each stoma is made by two specialised kidney-shaped (bean-shaped) guard cells of the leaf's epidermis.

Stomata in different leaves
Leaf type	Distribution of stomata
Bifacial leaf (distinguishable upper and lower sides)	More stomata on the lower epidermis, which has spaces between its cells
Isobilateral leaf (equal surfaces)	An even distribution of stomata on both epidermises

Transpiration: The process of water loss in the form of vapour from the above-ground parts of plants, primarily through stomata.

Transpiration does more than lose water. It helps the transport of water and minerals and it plays a crucial role in gas exchange. During daylight hours the stomata are wide open, which gives plenty of passage for gas exchange.

Putting the three together

Stomata, gas exchange and transpiration

By day the guard cells open the stomata, so air from outside is in contact with the moist air spaces of the leaf.
$CO X_{2}$ diffuses in through the open stomata for photosynthesis, and $O X_{2}$ diffuses out.
Through the same open pores, water vapour leaves the leaf. This loss of water vapour from the above-ground parts is transpiration.
The pull produced by this transpiration moves water and minerals up the plant (see the TACT mechanism).

One open stoma: carbon dioxide enters, while oxygen and water vapour leave through the same pore. The arrows show the net movement by day.

9.3Transport in plants

Plant roots have a dual role: they anchor the plant in the soil and they absorb essential water and minerals from it. The process begins when the root epidermal cells take up these nutrients. They are then transported through the xylem, a plant tissue, and finally reach the leaves.

In the leaves these nutrients are used for photosynthesis, where the plant makes its food. After photosynthesis the plant has organic solutes that must be distributed to the various parts of the plant. All these movements of material within the plant are together called transport in plants.

Transport in plants at a glance
What is moved	From	To	Through
Water and minerals	Root epidermal cells (from the soil)	Leaves	Xylem
Organic solutes (food made in photosynthesis)	Leaves and other food-making or storage organs	Other parts of the plant	Phloem

9.3.1Movement of water between plant cells and their environmentOften examined

The movement of water in and out of cells happens by osmosis. Osmosis is the movement of water from where it is in higher concentration to where it is in lower concentration through a semipermeable membrane. When water moves into a cell it is called endosmosis, and when it moves out of a cell it is called exosmosis. This movement of water involves three aspects: water potential, solute potential and pressure potential.

Water potential ( $Ψ_{w}$ )

Water potential is a measure of the potential energy in water that enables it to move from one place to another. Potential or pressure is denoted by the Greek letter $Ψ$ (psi) and is expressed in units of pressure (pressure is a form of energy) called megapascal (MPa). Water potential is therefore written $Ψ_{w}$ .

The water potential of a medium is directly proportional to the concentration of water in that medium, so pure water has the highest water potential.
The water potential of pure water is given the value zero (although pure water contains plenty of potential energy, that energy is ignored). It is the reference against which all other solutions are compared.
The water potential of all solutions or cells must therefore be less than zero, that is, in the negative range.
Water potential in plant solutions is mainly influenced by solute concentration and by pressure greater than atmospheric pressure.

Water potential can be broken into its two components by the equation:

Ψ_{w} = Ψ_{s} + Ψ_{p}

Water potential of a cell

where $Ψ_{s}$ is the solute potential and $Ψ_{p}$ is the pressure potential. As the components change they raise or lower the total water potential of a system. Water then moves to equilibrate: it moves from the system or compartment with the higher water potential to the one with the lower water potential.

The water potential gradient from the soil to the atmosphere. Water moves in the direction of falling water potential.

Figure 9.3: Changes in

Ψ_{w}

of two adjacent systems and their effects. A semipermeable membrane separates two aqueous systems in a U-shaped tube. Water moves from the side of higher water potential to the side of lower water potential until equilibrium is reached.

Solute potential ( $Ψ_{s}$ )

Solute potential ( $Ψ_{s}$ ): The decrease in water potential of a medium due to the addition of solutes. It is also called osmotic potential.

Solutes reduce water potential (make $Ψ_{w}$ more negative) by using up some of the potential energy available in the water.
Solute potential is negative in a plant cell and zero in distilled water. Typical values for cell cytoplasm are $- 0.5$ to $- 1.0 MPa$ .
Solute molecules dissolve in water because water molecules can bind to them by hydrogen bonds. A hydrophobic molecule such as oil cannot bind to water, so it cannot go into solution.
The energy in the hydrogen bonds between solute and water is tied up in the bond and is no longer available to do work in the system. So $Ψ_{s}$ decreases as solute concentration increases, and a decrease in $Ψ_{s}$ causes a decrease in total $Ψ_{w}$ .

The internal water potential of a plant cell is more negative than that of pure water because of the high solute content of the cytoplasm. Because of this difference in water potential, water moves from the soil into the plant's root cells by osmosis. This is why solute potential is sometimes called osmotic potential.

Pressure potential ( $Ψ_{p}$ )

Pressure potential ( $Ψ_{p}$ ): The increase in water potential of a medium due to the addition of water or due to pressure greater than atmospheric pressure.

If a pressure greater than atmospheric pressure is applied to pure water or to a solution, its water potential increases. This is like pumping water from one place to another, and such a situation may arise in living systems. When water enters plant cells by osmosis, pressure may build up inside the cell, making the cell turgid and increasing its water potential.

Book example: two adjacent vacuolated cells

The book gives two adjacent vacuolated cells with their values of $Ψ_{w}$ , $Ψ_{s}$ and $Ψ_{p}$ and asks four questions.

The book's two adjacent cells. Cell B has the higher (less negative) water potential, so water moves from B to A.

Solving the book's example

Check the values first. For A: $- 2000 + 600 = - 1400$ kPa. For B: $- 1400 + 800 = - 600$ kPa. Both agree with $Ψ_{w} = Ψ_{s} + Ψ_{p}$ .
(i) Higher water potential: $- 600$ is greater than $- 1400$ , so cell B has the higher water potential.
(ii) Direction of movement: water moves from the higher to the lower water potential, so from B to A by osmosis.
(iii) At equilibrium: water leaves B and enters A until both cells have the **same $Ψ_{w}$ **, a value between $- 1400$ and $- 600$ kPa. If the two cells are of equal size and change by equal amounts, this is the mean, $- 1000$ kPa.
(iv)(a) In pure water ( $Ψ_{w} = 0$ ): this is higher than the water potential of either cell, so water enters both cells. They become more turgid, because $Ψ_{p}$ rises.
**(iv)(b) In the sucrose solution of $- 3510$ kPa: this is lower than the water potential of either cell, so water leaves both** cells (exosmosis). They lose turgor and become flaccid.

Worked example: direction of water movement in root cells

A root cell near the soil has

Ψ_{s} = - 1000

kPa and

Ψ_{p} = 300

kPa. The neighbouring cell, nearer the xylem, has

Ψ_{s} = - 1200

kPa and

Ψ_{p} = 400

kPa. (a) Calculate the water potential of each cell. (b) In which direction does water move between them? (c) What happens to the water potential of the first cell if it loses water and its

Ψ_{p}

falls to zero?

Use $Ψ_{w} = Ψ_{s} + Ψ_{p}$ for each cell.
$Cell 1: Ψ_{w} = - 1000 + 300 = - 700 kPa$
Second cell.
$Cell 2: Ψ_{w} = - 1200 + 400 = - 800 kPa$
Water moves from the higher to the lower water potential. $- 700$ is higher than $- 800$ , so water moves from cell 1 to cell 2, towards the xylem. This is the same direction as the gradient soil > root > stem.
If $Ψ_{p} = 0$ the cell has no turgor and $Ψ_{w} = Ψ_{s}$ .
$Ψ_{w} = - 1000 + 0 = - 1000 kPa$
The water potential is now lower (more negative) than before, so the flaccid cell can take in water again from a neighbour or from the soil.

(a) - 700 kPa and - 800 kPa; (b) cell 1 to cell 2; (c) - 1000 kPa

Worked example: finding the water potential of potato tissue

Potato strips were placed in sucrose solutions of different concentrations and their percentage change in mass was measured after one hour (practice readings, made up for this example). The water potential of the solutions is

0

kPa at

0.0

, and falls by about

270

kPa for every

0.1 mol d m^{- 3}

increase in concentration. Use the graph to find the water potential of the potato tissue.

Sucrose (mol dm $^{- 3}$ )	0.0	0.1	0.2	0.3	0.4	0.5
Change in mass (%)	+16	+9.5	+3	-3.5	-10	-16.5

Sucrose (mol dm $^{- 3}$ )	Change in mass (%)
0.0	+16
0.1	+9.5
0.2	+3
0.3	-3.5
0.4	-10
0.5	-16.5

Plot change in mass (y-axis) against sucrose concentration (x-axis) and join the points with a straight line.
The strips gain mass in dilute solutions (water enters by osmosis because the solution has a higher water potential) and lose mass in concentrated solutions.
The line crosses zero change at about $0.25 mol d m^{- 3}$ . At this concentration there is no net movement of water, so the solution and the tissue have the same water potential.
Convert the concentration to water potential.
$0.25 mol dm^{- 3} \approx 2.5 \times (- 270) kPa \approx - 670 kPa$

Change in mass of potato strips

0.1

0.2

0.3

0.4

0.5

−20

−10

Sucrose concentration (mol dm^{- 3})

Change (%)

no change

readings

Ψ_{w} of the tissue \approx - 670 kPa

9.3.2Uptake of water by roots and pathwaysOften examined

The cell wall of the epidermal cells of roots is freely permeable to water and other minerals, while the cell membrane is differentially permeable. From the root hairs water enters the epidermal cells by osmosis, moving along the concentration gradient. It passes through the cortex, endodermis and pericycle and reaches the xylem vessels. Water takes three pathways to reach the xylem tissues: (a) the apoplast pathway, (b) the symplast pathway and (c) the vacuolar pathway.

Apoplast pathway

The apoplast is the system of adjacent cell walls, which is continuous throughout the plant. When water moving through the spaces in the cell walls reaches the endodermis, its progress is stopped by the Casparian strips, a band of suberin and lignin bordering four sides of the root endodermal cells. Water and solutes, particularly salts in the form of ions, must therefore pass through the cell surface and into the cytoplasm of the endodermal cells. In this way the endodermal cells can control and regulate the movement of solutes into the xylem.

Symplast pathway

The movement of cell sap that involves the cytoplasmic connection of adjacent cells is the symplast pathway. The symplast is the system of interconnected protoplasts in the plant. The cytoplasm of neighbouring protoplasts is linked by plasmodesmata. Once water and any solutes it contains are taken into the cytoplasm of one cell, they can move through the symplast without crossing further membranes. The movement may be aided by cytoplasmic streaming. The symplast is an important pathway of water movement.

Vacuolar pathway

In the vacuolar pathway water moves from vacuole to vacuole through neighbouring cells. In the process it crosses the symplast and the apoplast and moves through membranes and the tonoplast by osmosis. It moves down a water potential gradient.

Figure 9.4: Pathways of water in the root. The three pathways are in the accent colour, each with its own line style.

The three pathways compared
Feature	Apoplast pathway	Symplast pathway	Vacuolar pathway
Route	Along the cell walls (and the spaces in them)	Through the cytoplasm of cells joined by plasmodesmata	From vacuole to vacuole through neighbouring cells
Membranes crossed	None until the endodermis	None after the first cell	Plasma membrane and tonoplast of each cell
Special structure	Stopped by the Casparian strips of the endodermis	Plasmodesmata (and cytoplasmic streaming)	Tonoplast

Differentiate between apoplast and symplast
Point	Apoplast	Symplast
Made of	The system of adjacent cell walls, continuous throughout the plant	The system of interconnected protoplasts of the plant
Linked by	Spaces in the cell walls	Plasmodesmata between the cytoplasm of neighbouring cells
At the endodermis	Water is stopped by the Casparian strips, so it must enter the cytoplasm	Water passes on through the cytoplasm

9.3.3Structure and function of xylem and phloemOften examined

There are two types of conducting tissues in plants, xylem and phloem. Together they form the vascular tissues. Xylem conducts mainly water and minerals from the roots up to the other parts of the plant. Phloem conducts organic food from the leaves both up and down the plant.

Components of xylem and their functions

Xylem is a complex tissue (it is made up of different kinds of cells) and a permanent tissue (it lacks the power of cell division). It consists of four cell types: tracheids, vessel elements, xylem fibres and xylem parenchyma.

Components of xylem
Cell type	Living or dead	Structure	Function
Tracheids	Dead	Elongated cells with tapering ends and lignified walls	Mechanical strength and support; conduction of water and minerals. Present in the xylem of all vascular plants, which are therefore called tracheophytes
Vessel elements (forming xylem vessels)	Dead	Vessel elements are shorter than tracheids. Several are fused end to end in a row, one above the other, to form a very long, highly thick-walled tubular xylem vessel	Form a pipeline in the plant body; conduction of water and minerals; mechanical support. Found only in angiosperms
Xylem fibres	Dead	Narrow, highly elongated, highly thick-walled cells with tapering ends	Mainly mechanical support for the plant body
Xylem parenchyma	Living (the only living cells of xylem)	Thin-walled, broad cells	Temporary storage of water and minerals during conduction

Figure 9.5: Vessels and tracheids. A xylem fibre is added for comparison.

Components of phloem and their functions

Phloem is also a complex permanent tissue. It is made of three kinds of living cells (sieve tube elements, companion cells and phloem parenchyma) and one dead cell (phloem fibre).

Sieve tubes are long, tube-like structures that translocate solutions of organic solutes (sucrose) throughout the plant body. They are formed by the end-to-end fusion of cells called sieve tube elements.
Sieve tube elements are thin-walled living cells but they have no nuclei; the nuclei are lost as the cells mature. The cells are narrow from the centre and wide at the ends.
A transverse or oblique partition bearing pores, the sieve plate, separates two successive sieve tube elements.
Because the sieve tube elements have no nuclei, their functions are controlled by the adjacent companion cells. A sieve element and its companion cell together form a functional unit that acts as a conducting channel in the transport of organic solutes. Companion cells provide energy to the sieve tube elements for their activities.
Like xylem, phloem also has parenchyma and fibre cells. Phloem parenchyma are storage cells and phloem fibres are very thick-walled supporting cells.

Figure 9.6: Phloem. Two sieve tube elements are separated by a sieve plate; each has a companion cell beside it.

Components of phloem
Cell	Living or dead	Function
Sieve tube element	Living, no nucleus	Translocation of sucrose and other organic solutes
Companion cell	Living	Controls the sieve tube element and provides it with energy
Phloem parenchyma	Living	Storage
Phloem fibre	Dead	Support (very thick walls)

Differentiate between xylem and phloem
Point	Xylem	Phloem
Material conducted	Water and minerals	Organic food, mainly sucrose
Direction	From the roots up to the other parts	From the leaves both up and down
Living cells	Only xylem parenchyma	Sieve tube elements, companion cells and phloem parenchyma
Dead cells	Tracheids, vessel elements and fibres	Phloem fibres
Conducting cells	Tracheids and vessels (vessels only in angiosperms)	Sieve tubes with sieve plates

9.3.4Ascent of sapOften examined

Water and minerals are absorbed by the root epidermal cells of the soil. From there they are first moved to the root xylem cells and then to the leaves. This upward movement of water and dissolved minerals from the root to the leaves through the xylem tissue is called ascent of sap. It occurs against gravity, so a considerable force is required to conduct water and minerals in tall plants. The most acceptable theory that explains this movement is the TACT theory.

TACT theory

According to this theory four factors, Transpiration pull, Adhesion, Cohesion and Tension, combine to form a collective force that is mainly responsible for the ascent of sap.

Components of TACT force
Component	What it is	What it does for the ascent of sap
Transpiration pull	The pulling force produced by evaporation of water from the leaf (transpiration)	Compels water and minerals to move upwards through the xylem
Adhesion	Attractive force between water molecules and other substances (here the polar cellulose of the xylem walls)	Water sticks to the walls of the xylem cells; helps to move sap upwards against gravity and to hold water in the xylem when transpiration is not occurring
Cohesion	Attraction among water molecules, by hydrogen bonds	Holds the molecules together as a continuous column (a string) of water
Tension	The hydrogen bonds formed between water molecules (book's definition); the string of water is kept tense	Keeps the column tight, so it is pulled up as one unit

1. Transpiration pull

How the pull is produced

When the stomata are open, water molecules move from a region of high water potential (the intercellular spaces of the leaf) to a region of low water potential (the air).
This evaporation of water is transpiration.
The dry intercellular spaces pull water from the surrounding cells, and these cells in turn pull water from the xylem tissue.
So transpiration develops a pulling force, the transpiration pull, that makes the water and minerals move upward through the xylem.
Water and dissolved minerals in the xylem form a continuous column, which is maintained by adhesion, cohesion and tension.

2. Adhesion

Adhesion is the attractive force between water molecules and other substances. Water and cellulose are both polar molecules, so there is a strong attraction for water within the hollow capillaries of the xylem. The adhesion of the string of water molecules to the wall of the xylem cells helps the upward movement of xylem sap against the downward pull of gravity. Adhesion also helps to hold water in the xylem when transpiration is not occurring.

3. Cohesion and tension

Recall from Chapter 2 that water is a polar molecule: oxygen carries a slight negative charge and hydrogen a slight positive charge. Nearby water molecules therefore attract one another and form weak hydrogen bonds. This attraction among water molecules is called cohesion, and the hydrogen bonds formed between them are termed tension.

Mechanism of TACT force

The column of water molecules in the xylem is at least as strong and as unbreakable as a steel wire of the same diameter. The hydrogen bonds among the water molecules provide the cohesion that holds the 'string' of water together over the entire height of the plant within the xylem. As long as transpiration continues, the string is kept tense and is pulled upward as one molecule. The bulk flow of water to the top of the plant is driven by solar energy, since evaporation from the leaves is responsible for the transpiration pull.

Figure 9.7: Movement of water in the xylem through the TACT mechanism. Leaf: transpiration pull. Stem: cohesion between water molecules and adhesion to the cell wall (enlarged circle). Root: water uptake from the soil.

The ascent of sap in order, from root to leaf

Root: the root hair absorbs water from the soil by osmosis (water potential of soil higher than that of the root).
Stem: water moves up the xylem as a continuous column. Adhesion to the walls and cohesion between the molecules stop the column from breaking or falling back.
Leaf: water evaporates from the mesophyll into the air spaces and leaves through the open stomata. This transpiration pulls on the column and the whole column moves up.

Worked example: what happens if transpiration is stopped?

Two leafy shoots of the same plant are cut and stood in beakers of coloured water. Shoot A is left in the open air. Shoot B has a transparent polythene bag tied over its leaves (the air inside the bag soon becomes full of water vapour). After two hours the dye has climbed much higher in A than in B. Explain this with the TACT mechanism.

Dye rising in the xylem shows how far the sap has been pulled up. The driving force for the ascent of sap is the transpiration pull.
Shoot A: the air outside is dry, so it has a low water potential. Water vapour leaves the leaves through the stomata, the leaf cells pull water from the xylem, and the column is pulled up. Cohesion and adhesion keep the column unbroken.
Shoot B: the air inside the bag is full of water vapour, so its water potential is almost as high as that of the leaf spaces. There is little difference in water potential, so little water evaporates, the transpiration pull is weak and little dye is drawn up.
So A shows a larger rise of dye because it has a stronger transpiration pull.

\text{A: strong transpiration pull; B: weak pull because the humid air has a high water potential}

9.3.5Opening and closing of stomataOften examined

Stomata are generally open in the day and close (incompletely) at night. Opening and closing depend on changes in the turgidity of the guard cells: when the guard cells are turgid the stoma is fully open, and when they become flaccid the stoma is closed. Two theories explain how this happens: (a) the starch sugar interconversion theory and (b) the **\ce{K+} ion influx/efflux theory**.

Guard cells and the stoma
Guard cells	State of the stoma
Turgid	Open
Flaccid	Closed

Starch sugar interconversion theory

According to this theory the guard cells are the only photosynthesising cells of the leaf epidermis, because they have a high chlorophyll content compared with the surrounding epidermal cells.

Opening in the morning

Morning: the plant is exposed to light and photosynthesis starts in the guard cells.
The concentration of glucose (the product of photosynthesis) increases, so the solute potential and the water potential of the guard cells become low (more negative).
The surrounding epidermal cells have high water content, so water moves from the epidermal cells into the guard cells.
The guard cells become turgid and the stomata open.

Closing in the evening

Evening: photosynthesis stops and the concentration of glucose falls. Some glucose is used in respiration of the guard cells, some is changed into sucrose so that it can be transported to other parts, and the rest is converted into insoluble starch and stored for later use.
The fall in glucose concentration raises the solute potential and therefore the water potential of the guard cells.
Water moves from the guard cells to the surrounding epidermal cells.
The guard cells lose water, become flaccid, and the stomata close.

\ce{K+} ions influx/efflux theory

Opening in the morning

Morning: photosynthesis begins in the light and the level of \ce{CO2} in the guard cells decreases.
Low \ce{CO2} stimulates the influx (inward movement) of \ce{K+} ions into the guard cells from the surrounding epidermal cells by active transport.
At the same time, malic acid is ionised into malate ions and \ce{H+} ions, because of the blue light (a part of visible light). The accumulating \ce{H+} ions lower the pH of the guard cells, so they are pumped out to the surrounding epidermal cells to maintain the pH.
The incoming \ce{K+} ions combine with malate ions to form potassium malate, which is highly soluble in water. This decreases the solute potential and the water potential of the guard cells.
Water moves in from the surrounding epidermal cells, the guard cells become turgid and the stomata open.

Closing in the evening

Evening: photosynthesis stops and the level of \ce{CO2} in the guard cells rises.
High \ce{CO2} stimulates the efflux (outward movement) of \ce{K+} ions from the guard cells into the surrounding epidermal cells by active transport.
Without blue light, malic acid is reformed by the combination of malate and \ce{H+} ions.
With no potassium malate, the solute potential and water potential of the guard cells increase.
Water moves out to the surrounding epidermal cells, the guard cells become flaccid and the stomata close.

Figure 9.8: (i) Stoma opening, (ii) stoma closing. The \ce{K+} ions are shown as accent dots. When \ce{K+} ions are inside the guard cells, water enters and the stoma opens; when they leave, water leaves and the stoma closes.

The two theories compared
Point	Starch sugar interconversion	\ce{K+} influx/efflux
What changes the solute potential	Glucose made in photosynthesis (morning) or removed as sucrose and starch (evening)	\ce{K+} ions entering (with malate, as potassium malate) or leaving the guard cells
Trigger	Light and photosynthesis	Level of \ce{CO2} and blue light
How the movement of ions or sugar happens	Photosynthesis, respiration and conversion of glucose	Active transport of \ce{K+}
Result in the morning	Solute and water potential fall, water enters, guard cells turgid, stoma open	Solute and water potential fall, water enters, guard cells turgid, stoma open
Result in the evening	Solute and water potential rise, water leaves, guard cells flaccid, stoma closed	Solute and water potential rise, water leaves, guard cells flaccid, stoma closed

9.3.6Translocation of organic solutesOften examined

Translocation of organic solutes: The movement of prepared food (organic solutes) to the different parts of the plant body through the phloem tissue.

Pattern or direction of translocation

Like the ascent of sap, this movement cannot be described as only upward or only downward, because the prepared food has to move in different directions. The direction is therefore defined by saying that translocation always occurs from a source towards a sink.

Source: The area of supply of food: a food-manufacturing organ, or a storage organ when it supplies the food.
Sink: The area of utilisation of food: a metabolising organ, or a storage organ when it stores the food.

Examples of source and sink (as in the book)
Plant part	Source or sink
Leaf	Purely a source
Fruit	Particularly a sink
Root and stem	Act as both source and sink

Composition of translocating fluid (phloem sap)

Studies have revealed that phloem sap consists of 10 to 25% dry matter. About 90% of this dry matter is sucrose (cane sugar) and the rest is other organic compounds.

Mechanism of translocation: pressure flow (mass flow) theory

The most acceptable theory that explains the translocation of organic solutes is the pressure flow or mass flow theory.

Pressure flow mechanism

At the source regions, such as photosynthesising leaves or storage places, sugars are loaded into the sieve tube elements of the phloem by the companion cells. This loading is by active transport and needs ATP.
The sugar concentration in the phloem increases, so the solute potential and water potential decrease.
Water moves into the phloem by osmosis from the nearby xylem cells. This increases the hydrostatic pressure (pressure due to the presence of water) in the phloem cells, which pushes the sugary solution away from the source.
The pressure gradient from source to sink causes translocation from the area of higher hydrostatic pressure (the source) to the area of lower hydrostatic pressure (the sink).
At the sink, such as the roots, the root cells actively absorb (unload) the organic solutes from the solution. The loss of solutes raises the solute and water potential of the phloem sap.
The water from the phloem then flows back to the xylem.

Figure 9.9: Pressure flow mechanism for phloem transport. Sugar is actively loaded into the sieve tube at the source, so water moves into the sieve tube by osmosis. At the sink the sugar is actively unloaded and water leaves the sieve tube by osmosis. Sucrose is shown as accent dots.

Source and sink compared
Point	Source	Sink
Meaning	Area of supply of food	Area of utilisation or storage of food
Sucrose	Actively loaded into the sieve tube	Actively unloaded from the sieve tube
Water	Moves into the phloem by osmosis (from xylem)	Moves out of the phloem by osmosis (back to xylem)
Hydrostatic pressure in phloem	High	Low
Examples	Leaf; storage organ that supplies food	Fruit; root; storage organ that stores food

9.3.6STEAM activity: investigating plant vascular systems

The book's STEAM activity is titled Investigating plant vascular systems: xylem and phloem, the plant's arteries and veins.

Objective: To explore and understand the function of the plant vascular systems (xylem and phloem) in transporting water, nutrients and sugars within the plant, and to show how important these systems are for the life processes of the plant. The activity encourages hands-on learning and group discussion.

Materials required

Fresh celery or other leafy stems with intact veins
Red and blue food colouring
Clear containers or jars
Scalpel or razor blade
Microscope (optional)
Water
Absorbent paper or towels
Markers and paper for labelling and recording observations
Plastic gloves (optional)

Procedure

Steps of the activity

Introduction. Discuss the importance of the vascular systems (xylem and phloem) in transporting water, nutrients and sugars. Introduce the concept of capillary action and how it helps the movement of water in plants.
Preparation of materials. Cut the bottom of the celery stems to expose the xylem vessels (Figure 9.10 in the book). Fill clear containers with water. Add red food colouring to one container and blue to another.
Dye uptake demonstration. Place the celery stems in the coloured water containers. Observe and discuss how the coloured water moves through the stem. Record the observations on the uptake of the different coloured dyes by the celery (Figure 9.11 in the book).
Microscopic observation. If available, use a microscope to examine cross sections of the celery stems or to look at the bottom end of the stems. Identify and discuss the xylem and phloem tissues (Figure 9.12 in the book).
Discussion and analysis. Discuss the results and observations. Relate the experiment to real plant systems and their roles. Explore how environmental factors might affect the efficiency of water transport.

Conclusion and extension

Conclusion: summarise the key findings and stress the importance of xylem and phloem for plant survival.
Extension (optional): compare the findings with other plant materials, such as flowers or leaves, and explore how temperature, light or other external factors influence water transport.

The set-up of the STEAM activity (a simple drawing of the book's Figures 9.10 to 9.12): a celery stem stands in coloured water and the dye rises in the xylem, which shows up as dots in a cross section.

9.4Homeostasis in plants

Homeostasis: The ability to maintain a steady state within a constantly changing environment, which contributes to the success of living systems.

Outer environmental conditions such as water availability, nutrients and temperature often show fluctuations that also affect these components inside the plant body. Plants have several adaptations to cope with these challenges. The book teaches two examples:

Homeostasis in plants
Type of homeostasis	What is kept steady	Section
Osmotic adjustment (osmoregulation)	Water and solute level	9.4.1 and 9.4.2
Thermoregulation	Temperature suitable for enzymes	9.4.3

9.4.1Osmotic adjustment (osmoregulation)Often examined

Osmotic adjustment (osmoregulation): The maintenance of water and solute level in the body of the plant.

Water and solute relations create three kinds of osmotic situation: hypotonic, hypertonic and isotonic.

Table 9.2: Movement of water in and out of cells in different osmotic conditions
Medium	Water potential compared with cell sap	Solute concentration compared with cell sap	Movement of water	Effect on the plant cell
Hypotonic	Higher	Lower	Endosmosis: water moves into the cell	Cell becomes turgid
Hypertonic	Lower	Higher	Exosmosis: water moves out of the cell	Cell becomes flaccid
Isotonic	Same	Same	No net movement of water	No net change

Table 9.2 (drawn as a figure): movement of water in and out of a plant cell in a hypotonic, a hypertonic and an isotonic medium. The accent arrows show the water movement. Each cell has a cell wall (outer outline), a cell membrane around the cytoplasm and a large vacuole.

Movement of water in different osmotic conditions

Different osmotic conditions have different effects on the net movement of water in and out of cells.

A cell kept in a medium with higher water potential and lower solute concentration than its cell sap takes in water (endosmosis) and becomes turgid. Such a medium is called a hypotonic medium.
A cell kept in a medium with lower water potential and higher solute concentration than its cell sap loses water (exosmosis) and becomes flaccid. Such a medium is called a hypertonic medium.
Hypotonic and hypertonic conditions make osmotic adjustment necessary.
The ideal situation for a cell is isotonic, in which the water potential of the cell and of the outer environment are equal, so there is no net movement of water. This situation is rarely available to plants in the natural environment.

Osmotic adjustments in plants of different environments

On the basis of the availability of water, plants are classified into hydrophytes, mesophytes and xerophytes. Each group is exposed to different osmotic conditions and therefore shows different osmotic adjustments.

Hydrophytes

Hydrophytes are plants found in areas where water is abundant. Aquatic plants of freshwater habitats are examples. They are exposed to hypotonic conditions, so they face the problem of flooding (excess of water). They show the highest rate of transpiration, because of broad leaves with a large surface area and stomata on the upper epidermis, which are kept open day and night. They do not store water and have a very thin cuticle or almost none.

Mesophytes

Mesophytes are plants found where a moderate supply of water is available. Common crop plants such as wheat, rice and corn are examples. They are exposed to nearly isotonic conditions, so they face no drastic problem of flooding or dehydration. They show a moderate rate of transpiration, because of medium-sized leaves with stomata on the lower epidermis, which are generally open in the day and close at night, although they can also close their stomata in the day. They store a very small amount of water.

Xerophytes

Xerophytes are plants found where very little water is available. Desert plants such as cactus and Opuntia are examples. They are exposed to severely hypertonic conditions and face extreme dehydration. They show a very reduced rate of transpiration because of narrow needle-like leaves or the complete modification of leaves into spines. Their stomata lie in depressions (sunken stomata) and are generally closed in the day and open at night. They store a very large amount of water, so they are also called succulent plants. They have a very thick cuticle.

Table 9.3: Comparison of osmotic adjustment in different groups of plants
Feature	Hydrophytes	Mesophytes	Xerophytes
Habitat	Aquatic	Terrestrial (moderate)	Terrestrial (severely dry)
Osmotic conditions of medium	Hypotonic	Nearly isotonic	Strongly hypertonic
Problems faced by plant	Flooding	No drastic problem	Severe dehydration
Rate of transpiration	Very high	Medium	Very low
Stomatal distribution	On upper epidermis	On lower epidermis	Sunken stomata
Stomatal opening	Remain open all the time	Open in day and close at night	Close in day and open at night
Thickness of cuticle	Very thin or none	Less thick	Very thick
Storage of water	None	Little storage	Succulent

Skills box of the book: the structure and position of stomata in a hydrophyte, a mesophyte and a xerophyte (diagrams of a leaf in section). The stomata are drawn in the accent colour.

9.4.2Osmotic adjustments of plants (halophytes) in saline soils

A saline soil is a soil with an excessive salt concentration. Highly saline soils are characteristic of salt marshes. In saline soils the solute potential and water potential are very low, which creates a physiological drought: the plants are unable to absorb water even though water is available in the soil. Plants living in saline environments, the halophytes, have adapted mechanisms to deal with this problem.

Osmotic adjustment in halophytes
Mechanism	What the halophyte does
Taking up salt ions	Takes up sodium and chloride at high rates and accumulates these ions in the vacuoles of the leaf cells. This keeps the salt concentration in the cytoplasm and organelles low, so that it does not interfere with the functions of the enzymes and the metabolic machinery of the leaves. The plants use the accumulated salt for osmotic adjustment to the low water potential in the soil.
Making compatible solutes	Some halophytes do not take up salts from the soil. They make osmotic adjustment by accumulating their own dissolved substances that are compatible with enzymes and metabolism. These compatible solutes are mostly organic compounds such as the nitrogenous compound glycine and, in some plants, sugar alcohols such as sorbitol.

9.4.3Thermoregulation in plantsOften examined

All organisms need a suitable temperature for the optimum activity of their enzymes. Thermoregulation is a type of homeostasis by which an organism makes the necessary adjustment in response to low and high temperature stress. Plants also have adaptations that let them tolerate low and high temperature changes.

Adaptations in plants to cope with low temperature stress

Low temperature affects the fluidity of the cell membrane because the lipids of the membranes become locked into crystalline structures. The membranes become porous, which affects the transport of solutes. The structure of membrane proteins is also affected. In addition, freezing temperature causes ice crystals to form within the cytoplasm, which interferes with the metabolic activities of the cell.

Adaptations to low temperature
Problem at low temperature	Adaptation
Crystallisation of membrane lipids (membranes become porous)	The plant increases the proportion of unsaturated fatty acids, which have a low freezing point. This helps the membrane to keep its structure at low temperature and prevents crystal formation
Ice crystal formation within the cytoplasm	Plants native to cold regions, such as oaks, maples and roses, change the solute composition of their cells so that the cytosol supercools without ice formation (ice crystals may still form in the cell wall)

Adaptations in plants to cope with high temperature stress

High temperature stress causes excessive evaporative loss of water by transpiration, which can lead to dehydration and finally wilting (loss of turgor). It also causes denaturation of enzymes and damages metabolism, so it harms or kills the plant.

Adaptations to high temperature
Adaptation	How it helps
Shiny cuticle (in hot areas)	Reflects much of the incident light, so it prevents heat absorption and overheating of the plant
Numerous stomatal openings in the leaves	Allow the loss of water (transpiration), which removes heat from the plant
Closing the stomata above 45^\circ\mathrm{C}	At very high temperature the plants close their stomata to prevent water loss
Heat shock proteins	Most plants survive very high temperature by making large quantities of these special proteins. They are heat tolerant and help to protect (the book says 'embrace') enzymes and other proteins, so that denaturation is prevented

Low and high temperature stress compared
Point	Low temperature stress	High temperature stress
Harm done	Membrane lipids crystallise, membrane proteins are affected, ice crystals form in cytoplasm	Loss of water (dehydration, wilting), denaturation of enzymes, damage to metabolism
Main adaptations	More unsaturated fatty acids; change in solute composition (supercooling)	Shiny cuticle; many stomata (cooling by transpiration); closing stomata above 45^\circ\mathrm{C}; heat shock proteins

9.5Support in plants

When life started on land from water, one of the very important needs of organisms was to gain some support and strength to keep their bodies in shape. Plants have a variety of mechanisms for support.

Mechanisms of support in plants
Mechanism of support	Where it works	Section
Turgor mechanism	Parenchyma cells (large water-filled vacuoles)	9.5.1
Mechanical tissues (thick-walled cells)	Collenchyma and sclerenchyma	9.5.2
Arrangement of vascular bundles	Ring of bundles in dicot stems	9.5.3
Secondary growth	Wood formed by the vascular cambium	9.6.3

9.5.1Role of turgor mechanism in supportOften examined

In plants the parenchyma cells have large central vacuoles filled with water. When the cells are turgid, the water presses on the surrounding walls. This pressure on the walls keeps the cells stiff and hard and is called turgor pressure.

Turgor pressure: The pressure of the water in the vacuole on the cell wall of a turgid cell, which keeps the cell stiff and hard.

In herbaceous plants, where specialised supporting tissues are not common, the turgidity of the cells provides support and strength, and the plant grows upright. If turgidity decreases (a fall in the internal hydrostatic pressure), the plant may wilt or collapse. Besides the hydrostatic pressure, plants (especially shrubs and trees) have supporting tissues such as collenchyma, sclerenchyma and conducting tissues.

Turgor and support in a herbaceous plant. Turgid cells are stiff because of turgor pressure (accent arrows), so the plant stands upright; when the cells lose water they become flaccid and the plant wilts.

How turgor gives support

Water enters the parenchyma cells by osmosis, so the large vacuole fills with water.
The vacuole presses on the cell wall: this is the turgor pressure (pressure potential).
The cell wall resists, so the cell becomes stiff and hard. Many stiff cells together hold up the soft stem and leaves.
When the plant loses more water than it takes in, the turgor pressure falls, the cells go flaccid and the plant wilts.

9.5.2Role of mechanical tissues in supportOften examined

The term mechanical tissues is used for the tissues made of thick-walled cells, that is, collenchyma and sclerenchyma.

Collenchyma

Collenchyma is characterised by extra cellulose deposition at the corners of its cells. It is a mechanical tissue that gives support especially in young plants, herbs and leaves (where secondary growth does not occur). It is living, so it can grow and stretch freely. In stems and petioles it plays a more important role in support because of its peripheral location near the epidermis.

Figure 9.10: Collenchyma (transverse section). The extra cellulose is deposited at the corners of the cells.

Sclerenchyma

Sclerenchyma tissue is the sole means of giving support and mechanical strength to the plant. The mature cells are dead, because a very thick secondary lignified wall is formed (lignin is a macromolecule that provides rigidity to the wall). Sclerenchyma is of two types:

Types of sclerenchyma
Type	Shape	Where found
Fibres	Elongated cells	In the pericycle of stems, forming a solid rod of tissue; also in xylem and phloem tissues
Sclereids	Roughly spherical or variously shaped cells	Common in fruit walls and seed coats

Both fibres and sclereids have heavily thickened walls with lignin and great tensile strength.

Figure 9.11: (a) T.S. and (b) L.S. of sclerenchyma cells. The walls are very thick and the lumen of a mature fibre is narrow and empty.

Differentiate between collenchyma and sclerenchyma
Point	Collenchyma	Sclerenchyma
Wall thickening	Extra cellulose at the corners	Very thick secondary lignified wall all round
Living or dead	Living	Dead when mature
Where found	Young plants, herbs, leaves, peripheral part of stems and petioles	Pericycle, xylem, phloem (fibres); fruit wall and seed coat (sclereids)
Function	Support in young parts that can still grow and stretch	The sole means of mechanical strength; great tensile strength

9.5.3Arrangement of vascular bundles

In dicot plants the vascular bundles are arranged in the form of a ring in the stem. In monocot plants the vascular bundles are scattered in the stem. The ring arrangement of the vascular bundles in dicot plants also provides support to the plant body, as compared with the scattered bundles of monocots.

Vascular bundles in dicot and monocot stems
Plants	Arrangement of vascular bundles in the stem
Dicot	In the form of a ring
Monocot	Scattered

Arrangement of vascular bundles (diagrammatic transverse sections of stems). The bundles are shown in the accent colour.

9.6Growth and development in plants

Development: A programmed series of changes by which an organism is converted from a simpler form to a more complex one.
Growth: An irreversible increase in the size and mass of an organism. It is a part of the development process.

In plants, growth and development involve four phases:

Phases of growth and development in plants
Phase	What happens
1. Cell division	Production of new cells
2. Cell elongation	Enlargement of cell size
3. Cell maturation	The cells mature
4. Differentiation	Cells become structurally and functionally specialised to perform particular functions

Plants add new organs, such as branches, leaves and roots, throughout life. This unique pattern is called open growth.

9.6.1Meristematic tissues (meristems)Often examined

In lower plants the entire plant body is capable of growing. In higher plants growth is limited to certain regions called growing points, which have specialised tissues for growth: the meristematic tissues or meristems. Their cells remain forever young and divide actively throughout the life of the plant.

Meristematic tissues (meristems): Tissues of actively dividing cells at the growing points of higher plants.
Initial: When a meristematic cell divides in two, the new cell that remains in the meristem.
Derivative: The other new cell, which is pushed away from the zone of active division and differentiates into other types of tissue.

Initials and derivatives

A meristematic cell divides by mitosis into two cells.
One cell remains in the meristem: this is the initial.
The other cell is the derivative. With repeated divisions of the initials the derivatives are pushed farther from the zone of active division.
The derivatives stretch, enlarge and differentiate into other types of tissues as they mature.

Meristematic cells are generally small and cuboidal, with large nuclei, small vacuoles and thin walls. A plant has two major kinds of meristems: the apical meristem and the lateral meristem.

Apical meristems

Apical meristems are located at the opposite ends of the plant axis, in the tips of the roots and shoots. Cell division and the following cell enlargement in these areas lengthen the above-ground and below-ground parts of the plant. These meristems are present in plants right from embryonic life, so they are also known as primary meristems.

Lateral meristems

Lateral meristems are cylinders of dividing cells on the lateral sides in the stems and roots of dicots and gymnosperms. They increase the thickness and diameter of the plant. They are derived from the apical meristem after embryonic life, so they are also called secondary meristems. The book names three types of lateral meristem:

Types of lateral meristem
Lateral meristem	Where it is found	What it does
Vascular cambium	A cylinder of tissue extending throughout the length of the plant, from the tips of the shoots to the tips of the roots	Some plants grow in diameter by producing new tissues laterally from this cylinder
Cork cambium (also called phellogen)	In the bark of roots and stems of woody plants	Contributes to the width of the plant body
Intercalary meristem	Originates from apical meristem but is separated from it; located along the stems near the nodes	Growth here gives rise to side branches, leaves and lateral buds, which grow up to a certain size (determinate growth)

Tissues produced by cell division of any lateral meristem are called secondary tissues.

Apical and lateral meristems compared
Point	Apical meristem	Lateral meristem
Position	Tips of roots and shoots	Cylinders on the lateral sides of stems and roots
Effect	Increases length	Increases thickness and diameter
Also called	Primary meristem	Secondary meristem
Present from	Embryonic life	Derived from apical meristem after embryonic life
Examples	Shoot tip and root tip meristems	Vascular cambium, cork cambium (and intercalary meristem)

9.6.2Types of plant growthOften examined

On the basis of origin there are two types of plant growth: primary growth and secondary growth.

Primary growth

When a plant starts its life after the germination of the seed, it begins to increase its length first. This increase in the length of the root and the shoot is called primary growth. It continues throughout the life of the plant (indeterminate growth), although the rate of growth may vary in different periods of life. Primary growth is carried out by cell division in the apical meristem.

Secondary growth

The increase in thickness and diameter of a plant is called secondary growth. It is characteristic of the stems and roots of dicots and gymnosperms, where it has an important role in the support of the plant body.

Secondary tissues: All the tissues produced during secondary growth.
Primary tissues: The tissues that are already present in the plant before the onset of secondary growth.

Primary and secondary growth compared
Point	Primary growth	Secondary growth
What increases	Length of root and shoot	Thickness and diameter of stem and root
Meristem responsible	Apical meristem	Lateral meristems (vascular cambium and cork cambium)
Where it occurs	All plants, from germination	Stems and roots of dicots and gymnosperms
Tissues formed	Primary tissues	Secondary tissues
Duration	Continues throughout life (indeterminate growth)	Adds to the girth each year

Figure 9.13: Shoot apex in longitudinal section (a drawing of the book's photomicrograph): the apical meristem lies between the leaf primordia.

Figure 9.14: Root apex in longitudinal section (a drawing of the book's photomicrograph): the root cap covers the apical meristem; behind it lie the cortex and the region where vascular tissue will form.

9.6.3Role of lateral meristems in secondary growthOften examined

Two lateral meristems, the cork cambium and the vascular cambium, mainly take part in secondary growth.

Role of vascular cambium

As the plant grows lengthwise by the activity of the apical meristem, the vascular cambium is also produced. It is present in the vascular bundle between the primary xylem and the primary phloem. Its rate of cell division is faster than that of the cork cambium, so it contributes more tissues to secondary growth. It adds two new tissues to the plant body:

Tissues added by the vascular cambium
New tissue	Where it is added
Secondary xylem	Towards the inner side of the vascular cambium
Secondary phloem	Towards the outer side of the vascular cambium

Formation of annual (growth) rings

Secondary xylem is produced faster than secondary phloem and it is produced in two different ways during the whole year of growth:

Seasonal growth of secondary xylem
Season	Cells produced	Probable reason
Autumn and winter	Comparatively smaller cells (late wood)	Deficiency of water
Spring and summer	Larger cells (early wood)	Availability of water

How annual rings form

In spring and summer the vascular cambium produces large cells of secondary xylem.
In autumn and winter it produces small cells.
Because of this difference in growth, a distinct ring of smaller cells and larger cells is formed. It represents the growth of one whole year.
These rings are called growth rings or annual rings. By counting the number of rings, the age of the plant can be found, as each ring is formed in one year.

Figure 9.15: Secondary growth: a radial strip through the stem of a woody plant (diagram based on the book's section of a three-year-old Tilia (linden) stem). The two cambia are drawn in the accent colour; the vascular ray lines run across the wood and phloem.

Role of cork cambium (phellogen)

The cork cambium or phellogen generally originates from the outermost layer of the primary cortex. It adds two new tissues during secondary growth:

Tissues added by the cork cambium
New tissue	Where it is added
Secondary cortex	Towards the inner side of the cork cambium
Cork	Towards the outer side of the cork cambium, beneath the epidermis

The cork cells become dead, and sometimes the epidermis also dies. Therefore the outer surface of mature trees looks rough.

Wood and bark

Wood: The bulk of the tissue added laterally (inner to the vascular cambium) during secondary growth, mainly secondary xylem.
Heartwood: The inner region of the wood (some secondary xylem, primary xylem and the central pith). It is blocked by the deposition of wastes in later life and therefore has a dark appearance.
Sapwood: The outer region of the wood, consisting of only secondary xylem. It remains functional and therefore has a light appearance.
Bark: The outer region of the stem from the vascular cambium to the epidermis.
Inner bark: The portion of bark inner to the cork cambium: secondary phloem, primary phloem, pericycle, endodermis, primary cortex and secondary cortex.
Outer bark: The portion of bark outer to the cork cambium: cork and epidermis.

Vascular cambium and cork cambium compared
Point	Vascular cambium	Cork cambium
Position	Between the primary xylem and primary phloem	Originates from the outermost layer of the primary cortex
Tissue added inside	Secondary xylem	Secondary cortex
Tissue added outside	Secondary phloem	Cork
Rate of cell division	Faster, contributes more tissue	Slower
Result	Wood and annual rings	Cork of the outer bark (rough surface)

Heartwood and sapwood compared
Point	Heartwood	Sapwood
Position	Inner region of the wood	Outer region of the wood
Made of	Some secondary xylem, primary xylem and pith	Only secondary xylem
Appearance	Dark (wastes are deposited and block it)	Light
Function	Not functional	Functional (remains functional)

9.6.4Growth correlationsOften examined

During the development of a plant the rate of growth at different growing points is not constant. For example, the growth of different meristems sometimes influences each other. Such interactions among different growing points are called growth correlations.

Types of growth correlation
Growth correlation	Meaning
Positive	The growth of one part promotes the growth of another part
Negative	The growth of one part inhibits the growth of another part

Apical dominance

Apical dominance: A kind of negative growth correlation in which the presence of a growing apical bud inhibits the growth of the lateral buds. It is also found in roots, where lateral root growth is inhibited by the growth of the main root.

Strong and weak apical dominance
Plant	Apical dominance	Appearance
Tall with short side branches	Strong	Apical bud strongly inhibits the lateral buds
Short with very dense growth of lateral branches	Weak	Very little apical dominance

Figure 9.16: (a) A plant showing strong apical dominance, (b) a plant showing weak apical dominance (drawn as diagrams of the book's photographs).

Botanists have performed several experiments to find the cause of apical dominance. They concluded that:

It is the auxin (a plant growth regulator) released from the apical bud that inhibits the growth of the lateral buds.
Cytokinin produced by the lateral buds (another growth regulator) works antagonistically to the auxin of the apical bud, so it neutralises the influence of the apical bud on the lateral buds.

Hormones and the strength of apical dominance
Dominance	Auxin from apical bud	Cytokinin from lateral buds
Strong apical dominance	More	Less
Weak apical dominance	Less	More

Apical dominance: auxin from the apical bud (accent line, ending in a bar) inhibits the lateral buds, and cytokinin from the lateral buds works against it.

9.7.1Plant growth regulatorsOften examined

Plants generally adjust themselves to a changing environment by growth. The changes in plant shape or function are often regulated by plant hormones (growth substances) produced in response to environmental factors. Plant hormones act at the level of the cells to induce cell division, enlargement or cell maturation.

Plants are co-ordinated by chemicals commonly known as plant hormones, which must move from their sites of synthesis to their sites of action. Because their effects are usually on some aspect of growth, they are also called growth regulators. Five major types are recognised: (a) auxins, (b) gibberellins, (c) cytokinins, (d) abscisic acid and (e) ethene.

Growth promoters and growth inhibitors
Group	Growth regulators	Role
Growth promoters	Auxins, gibberellins, cytokinins	General role in promotion of growth
Growth inhibitors	Abscisic acid, ethene	General role in inhibition of growth

Auxins

Auxin, also known as indole-3-acetic acid (IAA), is the first plant hormone ever discovered. It plays a pivotal role in the growth and development of plants. It is mainly made in the shoot tips, leaf primordia (the early stages of leaf development), young leaves, embryos and various parts of developing flowers and seeds. One of its most significant functions is promoting cell elongation: auxins are primarily responsible for increasing the length of plant cells, which is crucial for stem elongation, leaf expansion and overall plant height.

Gibberellins

Gibberellins are widespread throughout the plant kingdom, and more than 75 have been isolated to date. Instead of specific names, they are numbered (GA1, GA2 and so on). Gibberellic acid 3 (GA3) is the most widespread and most thoroughly studied. They are especially abundant in seeds and young shoots, where they control stem elongation by stimulating both cell division and elongation.

Cytokinins

Cytokinins are named because of their role in cell division (cytokinesis). They have a molecular structure similar to adenine. The naturally occurring zeatin, first isolated from corn (Zea mays), is the most active of the cytokinins. Cytokinins are found at sites of active cell division, for example in root tips, seeds, fruits and leaves.

Abscisic acid

Abscisic acid (ABA) is synthesised in the plastids from carotenoids during stress situations and diffuses in all directions through vascular tissues and parenchyma. Its principal effect is the inhibition of cell growth. ABA increases in developing seeds and promotes dormancy. If leaves experience water stress, ABA increases immediately and causes the stomata to close.

Ethylene or ethene

Ethylene (ethene) is a simple gaseous hydrocarbon produced from an amino acid and found in most plant tissues in large amounts when they are stressed. It diffuses from its site of origin into the air and affects the surrounding plants as well. Ethylene stimulates the ripening of fruit and initiates abscission of fruits and leaves.

The five growth regulators
Hormone	Main sites or source (as in the book)	Main effects (as in the book)
Auxin (IAA)	Shoot tips, leaf primordia, young leaves, embryos, developing flowers and seeds	Promotes cell elongation: stem elongation, leaf expansion, plant height; apical dominance
Gibberellins (GA3 most studied)	Seeds and young shoots (especially abundant)	Stem elongation by stimulating cell division and elongation
Cytokinins (zeatin)	Root tips, seeds, fruits, leaves (sites of active cell division)	Cell division; promote lateral bud growth
Abscisic acid	Plastids, from carotenoids, in stress	Inhibits growth, promotes seed dormancy, closes stomata in water stress
Ethene	Most plant tissues in large amounts when stressed	Stimulates fruit ripening; initiates abscission of fruits and leaves

Table 9.4: A comparison of effects of plant growth regulators on different plant parts (as printed in the book)
Part affected	Auxins	Gibberellins	Cytokinins	Abscisic acid	Ethene
Stem	Promote cell enlargement behind apex; promote cell division in cambium	Promote cell enlargement in the presence of auxin; promote leaf growth	Promote cell division in apical meristem and cambium; promote leaf growth	Inhibit growth during stress, e.g., drought, salinity and water logging	Inhibit growth during stress, e.g., drought, salinity and water logging
Root	In low concentration promote growth; in high concentration inhibit growth; promote growth from cuttings and calluses	Nil	Inhibit primary root growth; promote lateral root growth	Nil	Inhibit root growth
Floral buds	Promote bud initiation	Promote bud initiation	Promote bud initiation; promote lateral bud growth; break bud dormancy	Promote bud initiation	Promote bud initiation
Flowering	Nil	Promote in long-day plants; inhibit in short-day plants; act as substitute for red light; antagonistic to abscisic acid	Nil	Promote in short-day plants; inhibit in long-day plants; act as substitute for far-red light; antagonistic to gibberellin	Promote in pineapple
Apical dominance	Promote	Enhance the action of auxin	Inhibit	Nil	Nil
Fruit growth	Promote ripening	Promote ripening	Promote ripening	Nil	Promote ripening
Parthenocarpy	Rarely promote	Promote	Rarely promote	Nil	Nil
Leaf senescence	Delay	Delay	Delay	Promote	Nil
Seed dormancy	Nil	Break	Break	Promote	Nil
Abscission	Inhibit	Inhibit	Inhibit	Promote	Nil
Stomatal opening	Nil	Nil	Promote	Inhibit	Nil

9.7.2Growth movements in plants (tropic movements)Often examined

Plants do not show locomotion, but individual plant organs may show movements by differential growth in response to various stimuli (internal or external). Tropic movements or tropisms are movements towards or away from an external stimulus such as light, gravity, touch or a chemical.

Tropism (tropic movement): A growth movement of a plant part, towards or away from an external stimulus, caused by differential growth.

Phototropism

Phototropism is the movement of plant parts towards (positive) or away (negative) from a light source. It is found in shoots and roots. Shoots show positive phototropism (they grow upward above the ground) and roots show negative phototropism (they grow downward under the ground).

Geotropism (gravitropism)

Geotropism (gravitropism) is the movement of plant parts towards (positive) or away (negative) from the force of gravity. It is also found in shoots and roots. Shoots show negative geotropism (they grow upward) and roots show positive geotropism (they grow downward).

Thigmotropism

Thigmotropism is the movement due to the touch stimulus. It is shown by climbing plants that need a supportive structure such as a wall, a wooden stick or rod, or even a rope to climb over.

Chemotropism

In chemotropism the stimulus is a chemical (nutrients). It is shown by the fungal mycelium, in which the hyphae show more growth towards the nutrients.

Types of tropism
Tropism	Stimulus	Plant part and response
Phototropism	Light	Shoots: positive (towards light). Roots: negative (away from light)
Geotropism	Gravity	Shoots: negative (away from gravity, upward). Roots: positive (towards gravity, downward)
Thigmotropism	Touch	Climbing plants grow over a supportive structure
Chemotropism	Chemical (nutrients)	Fungal hyphae grow more towards the nutrients

Types of tropic movement. The accent colour marks the stimulus: light, gravity, the touched support and the nutrient.

9.7.3PhotoperiodismOften examined

Light influences living organisms through variation in intensity, quality and day length. The total duration of light in the whole day is called the photoperiod, and its effect on the development of flowers in some plants is called photoperiodism.

Photoperiod: The total duration of light in the whole day.
Photoperiodism: The effect of the photoperiod on the development of flowers in some plants. With the exposure to the appropriate photoperiod, their vegetative buds are transformed into floral buds and flowers begin to be produced. If such plants do not get the required photoperiod, their vegetative growth continues but flowers are not produced.

Classification of plants based upon photoperiodism

Plants are classified into three main groups on the basis of how photoperiodism affects their flowering.

Photoperiodic classification of plants
Group	They flower when	Examples (as in the book)
Short-day plants	The photoperiod is less than a certain critical length	Maryland mammoth, cocklebur, chrysanthemum, tobacco
Long-day plants	The photoperiod exceeds the critical length	Spinach, sugar beet, henbane, snapdragon, cabbage, spring wheat
Day-neutral plants	In any photoperiod, after a considerable vegetative growth. Flowering is not affected by variation in day length or darkness	Tomato, pansy, bean, sweet pea, rose

Concept of critical photoperiod

The critical length of the photoperiod is different and variable for every plant, even for plants of the same group. For example, for tobacco a photoperiod of 14 hours is critical and for cocklebur 15.5 hours is critical, yet both are short-day plants. The critical photoperiod depends on the level and relative proportion of the two forms of phytochromes (see below).

Critical photoperiod of a short-day plant: The maximum length of day which is required to stimulate or induce flowering in a short-day plant.
Critical photoperiod of a long-day plant: The minimum length of day which is required to stimulate or induce flowering in a long-day plant. For example, for henbane an 11-hour photoperiod is critical.

Figure 9.17: Concept of critical photoperiod. A short-day plant flowers when the day is shorter than the critical photoperiod; a long-day plant flowers when the day is longer. The accent words mark where flowers form. Each bar is split into its light and dark parts.

Significance of dark periods over photoperiods

It has now been discovered that the actual stimulus for flowering is the uninterrupted dark period, not the light period. It is really the length of the dark period that is critical.

If a short-day plant (cocklebur) is grown in appropriate short days (less than 15.5 hours), but its long night (more than 8.5 hours) is interrupted by a short light period, flowering is prevented.
Similarly, a long-day plant (henbane) can flower in inappropriate short days (less than 11 hours) if its long nights (more than 13 hours) are interrupted.
So short-day plants are actually long-night plants, and long-day plants are short-night plants.

Effect of dark period (as explained in the book)
Group	Photoperiod (days)	Night	Flowers?
Short-day plant	Short	Long, uninterrupted	Yes
Short-day plant	Short	Long, but interrupted by a short light period	No
Long-day plant	Short	Long, but interrupted by a short light period	Yes
Long-day plant	Short	Long, uninterrupted	No

Effect of quality of light and discovery of phytochromes

Further night-interruption experiments on short-day and long-day plants showed that besides the length of the dark period, the quality (wavelength) of light is also important in the promotion or inhibition of flowering.

Cocklebur (a short-day plant) will not flower if its long nights are interrupted, but red light (660 nm) was the effective light in preventing flowering.
Far-red light (730 nm) reversed the effect of red light.
If more than one type of light exposure is given to interrupt the night, the last light treatment always determines the response.
Long-day plants behave in the opposite way in these experiments.

From these observations the researchers concluded that plants must have some kind of photoreceptors so that they respond differently to the different lights. This led to the discovery of the photoreceptor pigments called phytochromes.

Figure 9.18: Night-interruption experiments on a short-day plant. Left: light and dark periods. Right: red (R) and far-red (FR) flashes in the dark period. Each bar is a 24-hour day with its light and dark parts labelled; a small open gap in the night is a flash of light.

Worked example: predicting flowering from light flashes

A short-day plant and a long-day plant are each grown with long nights. The night is interrupted by brief flashes of light, in the orders below. R is red light and FR is far-red light. Say whether each plant flowers.

Recall the rule: the last light flash decides the response. Red light (R) converts Pr to Pfr; far-red light (FR) converts Pfr back to Pr.
A short-day plant needs a low Pfr to Pr ratio at the end of the night, so it flowers only if the last flash was FR (or no flash) and fails to flower if the last flash was R.
A long-day plant needs a high Pfr to Pr ratio, so it flowers if the last flash was R and fails to flower if the last flash was FR.
Apply the rule to each sequence and fill in the table.

Flash sequence in the night	Last flash	Short-day plant	Long-day plant
None (long, uninterrupted night)	None	Flowers	Does not flower
R	R	Does not flower	Flowers
R, FR	FR	Flowers	Does not flower
R, FR, R	R	Does not flower	Flowers
R, FR, R, FR	FR	Flowers	Does not flower

\text{The last flash decides: R gives a short-day plant no flowers; FR gives it flowers; the long-day plant is the opposite}

Mechanism of photoperiodism

The mechanism of photoperiodism depends on the rate of interconversion and the accumulation of either form of phytochrome. Phytochromes are blue-green pigments that act as photoreceptors in photoperiodism. They are protein in nature but also contain an iron-containing non-protein part.

Interconversion of phytochromes

Phytochromes occur in two interconvertible forms:

The two forms of phytochrome
Form	Absorbs	Change
Pr or \mathrm{P_{660}}	Red light (660 nm), strongly	Is converted into the second form, Pfr
Pfr or \mathrm{P_{730}}	Far-red light (730 nm)	Reverts to the original form, Pr

Figure 9.19: Interconversion of the two forms of phytochrome. Pfr is the active form (accent outline); it leads to a physiological response such as seed germination, stomatal opening or flowering.

Daytime: red light is available, so the conversion of Pr to Pfr occurs in the day.
Far-red light (invisible heat radiation) is present in both day and night, but the conversion of Pfr to Pr occurs prominently at night, because Pr cannot stay in the daytime in the presence of red light.
In this way the accumulation of either form of phytochrome in the day and the night lets the plant find out whether it is day or night.
The rate of conversion of Pfr to Pr at night also gives the plant a biological clock to measure the length of the night.

Stimuli required for induction of flowering

Ratio of Pfr to Pr and flowering
Group	Needs at the end of the night	Example and dark length	Why
Short-day plant	A low ratio of Pfr (\mathrm{P_{730}}) to Pr (\mathrm{P_{660}})	Cocklebur flowers if given an appropriate dark length (8.5 hours or longer)	This dark length can convert all the accumulated Pfr to Pr, so the Pfr to Pr ratio becomes low
Long-day plant	A high ratio of Pfr to Pr	Henbane flowers if given an appropriate dark length (13 hours or less)	This dark length cannot convert all the accumulated Pfr to Pr, so at the end of the night the Pfr to Pr ratio is high

This is why red and far-red lights affect short-day and long-day plants differently.

Role of florigen hormone

The specific ratios of Pfr (\mathrm{P_{730}}) to Pr (\mathrm{P_{660}}) at the end of the nights trigger the production of a particular hormone known as florigen, which transforms the vegetative buds into floral buds, so flowers are produced.

From night length to flowers

The dark period (night) length is measured by the conversion of Pfr to Pr.
A particular ratio of Pfr to Pr at the end of the night is reached.
This ratio triggers the production of florigen.
Florigen changes vegetative buds into floral buds, so the plant flowers.

9.7.4VernalizationOften examined

Vernalization: The promotion of flowering by exposure to low temperature.

The low temperature stimulus is received by the shoot apex of a mature plant or by the embryo (not by the leaves, as in photoperiodism).
The exact temperature and amount of time vary among species, but most vernalization temperatures lie between 0^\circ\mathrm{C} and 10^\circ\mathrm{C}. A temperature around 4^\circ\mathrm{C} is found to be the most effective.
Vernalization is the requirement of biennial plants, the plants that complete their life cycle during any months of two different calendar years.

Winter wheat and spring wheat (as in the book)
Wheat	Type	Sown	Result
Winter wheat	Biennial	October or November; harvested in June or July of the next year	Flowers, because the winter gives the low temperature stimulus
Spring wheat	Annual	February or March	Shows vegetative growth but does not flower; however, it can flower even if sown in February or March if its seeds are exposed to several weeks of low temperature (4^\circ\mathrm{C}) first

Vernalization of wheat (a diagram built from the sowing and harvesting times given in the book).

In some plants the requirement of a low temperature period is absolute: they will not flower without vernalization. The low temperature stimulates the production of the hormone vernalin, which transforms the vegetative buds into floral buds, so flowers are produced. It has now been discovered that vernalin is actually gibberellin.

How low temperature leads to flowers

The shoot apex (or the embryo of the seed) receives a low temperature stimulus, most effectively about 4^\circ\mathrm{C}, for several weeks.
The low temperature stimulates the production of the hormone vernalin (now known to be gibberellin).
Vernalin transforms the vegetative buds into floral buds.
Flowers are produced.

Photoperiodism and vernalization compared
Point	Photoperiodism	Vernalization
Stimulus	Day length (the length of the dark period)	Low temperature
Where the stimulus is received	In the leaves (by phytochromes)	In the shoot apex of a mature plant or in the embryo, not in the leaves
Hormone triggered	Florigen	Vernalin (gibberellin)
Result	Vegetative buds become floral buds	Vegetative buds become floral buds

Formula sheet

Water potential of a plant cell

\Psi_w=\Psi_s+\Psi_p

\Psi_w is the water potential of the cell, \Psi_s the solute (osmotic) potential and \Psi_p the pressure potential. All are measured in units of pressure (kPa or MPa); pure water at atmospheric pressure has \Psi_w=0.

Units: kPa or MPa

Key terms

Nutrient: Any substance that supplies the essential elements an organism requires for its growth and metabolic functions.
Macronutrients: The nine essential elements needed in fairly large quantities (more than 0.05\% of the dry weight): carbon, hydrogen, oxygen, nitrogen, phosphorus, sulphur, potassium, calcium and magnesium.
Micronutrients: The essential elements needed only in traces or small amounts (less than 0.05\% of the dry weight), such as iron, boron, manganese, copper, molybdenum, chlorine and zinc.
Chlorosis: The yellowing of the green parts of a plant, showing a lack of chlorophyll, caused by mineral deficiency of nitrogen and magnesium.
Necrosis: The death of a part of the plant body, caused by deficiency of calcium, magnesium, copper and molybdenum.
Carnivorous plants: Photosynthetic autotrophs with modified leaves that trap and digest insects as a supplementary source of nitrogen, for example the pitcher plant, Venus flytrap and sundew.
Mesophyll: Thin-walled living cells between the two epidermal layers of a leaf, adapted for photosynthesis; in dicots palisade mesophyll and spongy mesophyll.
Bifacial (dorsiventral) leaf: A leaf with palisade and spongy mesophyll and distinguishable upper and lower epidermis, found in dicots.
Stoma: A tiny curved opening in the leaf surface, formed by two kidney-shaped guard cells of the epidermis (plural stomata).
Compensation point of photosynthesis: The time at dawn and dusk when the rates of photosynthesis and respiration are balanced, so there is no net exchange of gases.
Transpiration: The loss of water in the form of vapour from the above-ground parts of plants, primarily through stomata.
Water potential: A measure of the potential energy in water that enables it to move from one place to another, expressed in pressure units (\Psi_w); pure water is zero and all solutions are negative.
Solute potential: The decrease in water potential of a medium due to the addition of solutes (\Psi_s, also called osmotic potential).
Pressure potential: The increase in water potential of a medium due to the addition of water or to pressure greater than atmospheric pressure (\Psi_p).
Apoplast: The system of adjacent cell walls, continuous throughout the plant; water moving in it is stopped by the Casparian strips of the endodermis.
Symplast: The system of interconnected protoplasts of the plant, linked by plasmodesmata; water and solutes move through it without crossing further membranes.
Casparian strip: A band of suberin and lignin bordering four sides of the root endodermal cells, which stops the apoplast pathway.
Xylem: A complex permanent tissue of tracheids, vessel elements, xylem fibres and xylem parenchyma that conducts water and minerals from the roots upward.
Tracheids: Dead, elongated cells with tapering ends, mechanical strength and conduction of water and minerals, present in all vascular plants (tracheophytes).
Xylem vessels: Dead, very long, thick-walled tubes formed by the end-to-end fusion of vessel elements, found only in angiosperms.
Phloem: A complex permanent tissue of sieve tube elements, companion cells, phloem parenchyma and phloem fibres that conducts organic food.
Sieve tube: A long tube formed by sieve tube elements joined end to end, separated by sieve plates, which translocates sucrose; the elements lose their nuclei when mature.
Companion cells: Cells beside sieve tube elements that control their functions and provide them with energy.
Ascent of sap: The upward movement of water and dissolved minerals from the root to the leaves through the xylem.
TACT theory: The theory that transpiration pull, adhesion, cohesion and tension together form the force responsible for the ascent of sap.
Transpiration pull: The pulling force produced by evaporation of water from the leaves that makes water and minerals move upward through the xylem.
Adhesion: The attractive force between water molecules and other substances, such as the cellulose of the xylem walls.
Cohesion: The attraction among water molecules, by hydrogen bonds, that holds the column of water together.
Translocation of organic solutes: The movement of prepared food (organic solutes) to different parts of the plant through the phloem, always from a source towards a sink.
Source: The area of supply of food, such as a food-manufacturing organ or a storage organ when it supplies the food.
Sink: The area of utilisation of food, such as a metabolising organ or a storage organ when it stores the food.
Pressure flow theory: The theory that sugar loaded into the phloem at a source raises the hydrostatic pressure, which pushes the solution to a sink where the sugar is unloaded (also called mass flow).
Homeostasis: The ability to maintain a steady state within a constantly changing environment.
Osmotic adjustment: The maintenance of water and solute level in the body (osmoregulation).
Hypotonic medium: A medium with higher water potential and lower solute concentration than the cell sap, so water enters the cell (endosmosis) and it becomes turgid.
Hypertonic medium: A medium with lower water potential and higher solute concentration than the cell sap, so water leaves the cell (exosmosis) and it becomes flaccid.
Hydrophytes: Plants of areas with abundant water, with a very high rate of transpiration, stomata on the upper epidermis, no water storage and a very thin cuticle or none.
Xerophytes: Plants of very dry areas, with sunken stomata, a very thick cuticle and reduced leaves, which store much water and are called succulent plants.
Halophytes: Plants living in saline soils, which take up sodium and chloride into the vacuoles or accumulate compatible solutes.
Turgor pressure: The pressure of the water in the vacuole on the cell wall of a turgid cell, which keeps the cell stiff and hard.
Collenchyma: A living mechanical tissue with extra cellulose at the corners of its cells, supporting young plants, herbs and leaves.
Sclerenchyma: A mechanical tissue of dead cells with very thick lignified walls (fibres and sclereids), the sole means of mechanical strength.
Meristems: Tissues at the growing points of higher plants whose cells remain young and divide actively throughout the life of the plant.
Apical meristems: Meristems at the tips of roots and shoots that lengthen the plant; also called primary meristems.
Lateral meristems: Cylinders of dividing cells on the sides of stems and roots of dicots and gymnosperms that increase thickness; also called secondary meristems.
Vascular cambium: The lateral meristem between primary xylem and primary phloem that adds secondary xylem inside and secondary phloem outside.
Cork cambium (phellogen): The lateral meristem arising from the outermost layer of the primary cortex that adds secondary cortex inside and cork outside.
Primary growth: The increase in length of the root and shoot, carried out by the apical meristem.
Secondary growth: The increase in thickness and diameter of a plant, carried out by the lateral meristems in dicots and gymnosperms.
Growth rings (annual rings): The rings of smaller and larger xylem cells that represent the growth of one whole year.
Heartwood: The dark inner region of the wood (some secondary xylem, primary xylem and pith), blocked by deposits of waste.
Apical dominance: A negative growth correlation in which a growing apical bud inhibits the growth of lateral buds.
Plant growth regulators: Plant hormones whose effects are usually on some aspect of growth: auxins, gibberellins, cytokinins, abscisic acid and ethene.
Auxin: Indole-3-acetic acid (IAA), the first plant hormone discovered, which promotes cell elongation.
Tropism (tropic movement): A growth movement of a plant part towards or away from an external stimulus such as light, gravity, touch or a chemical.
Photoperiodism: The effect of the total duration of light in the day (photoperiod) on the development of flowers in some plants.
Short-day, long-day and day-neutral plants: Short-day plants flower when the photoperiod is less than a critical length, long-day plants when it exceeds the critical length, and day-neutral plants flower in any photoperiod.
Phytochromes: Blue-green photoreceptor pigments with two interconvertible forms, Pr (\mathrm{P_{660}}) and Pfr (\mathrm{P_{730}}).
Florigen: The hormone produced when specific ratios of Pfr to Pr are reached, which transforms vegetative buds into floral buds.
Vernalization: The promotion of flowering by exposure to low temperature; vernalin, the hormone involved, is gibberellin.

Multiple choice questionsp. 266–268

Select the correct answer.

It is found essentially in organic compounds:

Acalcium
Bnitrogen
CcarbonAnswer
Dphosphorus

Why

Carbon is the major component of all organic compounds, which the plant builds from it (Table 9.1 gives carbon as 45% of dry mass, the major component of the plant's organic compounds).

Nitrogen (B) and phosphorus (D) are found in some organic molecules (proteins, nucleic acids, ATP) but not in organic compounds in general; calcium (A) is an inorganic mineral nutrient.

Chlorosis does not occur due to the deficiency of:

Asulphur
Bmagnesium
Cphosphorus
DcalciumAnswer

Why

Chlorosis is yellowing of green parts from lack of chlorophyll. Calcium deficiency causes crinkling of young leaves and death of terminal buds, not yellowing, so D is the answer.

Magnesium (B) causes chlorosis between the veins of older leaves, and sulphur (A) causes general chlorosis of young leaves, so both are wrong. Phosphorus (C) also does not cause chlorosis (it causes purpling and slow growth), so it is a second option that fits this description.

Note. Table 9.1 lists two options that do not cause chlorosis: calcium (crinkled young leaves, dead terminal buds) and phosphorus (purpling of veins, slow growth). The printed question has one key, so calcium is taken as the intended answer; the item is ambiguous.

Carnivorous plants use insects as a source of:

Awater
Bglucose
Coxygen
DnitrogenAnswer

Why

Carnivorous plants live in nitrogen-poor habitats, so the insects they trap give a supplementary source of nitrogen (book, section 9.1.2).

Glucose (B) is made by the plant itself in photosynthesis, and water (A) and oxygen (C) are not the nutrient the insects supply.

Most of the uptake of water and minerals from soil takes place through:

Aepidermal cells
Broot cap
Croot
Droot hairAnswer

Why

Water enters the root through root hairs, which are outgrowths of the epidermal cells that greatly increase the surface area in contact with the soil (section 10.3.2, 'From root hairs water enters the epidermal cells').

The root (C) is the whole organ and the root cap (B) only protects the tip; epidermal cells (A) are the cells that bear root hairs, so root hair is the more exact answer.

Which of the following is closest to the centre of a woody stem?

Avascular cambium
Byoung xylem
Cold phloem
Dold xylemAnswer

Why

In a woody stem, new secondary xylem is added on the inner side of the vascular cambium, so the oldest xylem lies nearest the centre. This old, dead, lignified region is the heartwood (section 9.6.3).

The vascular cambium (A), young xylem (B) and old phloem (C) all lie outside the centre: the cambium is the growing layer between xylem and phloem, and phloem is the outer part of the stem.

The diagram shows part of a section through a leaf. Which arrow represents the diffusion of oxygen during photosynthesis?

The leaf section of question 6, with arrows A to D.

AA
BB
CCAnswer
DD

Why

Oxygen is made in the photosynthesising mesophyll cells and diffuses out of the leaf through the stoma into the air. Arrow C points out of the stoma to the outside air, so it shows oxygen leaving the leaf.

Arrow D is carbon dioxide entering through the stoma, B is carbon dioxide entering the palisade cell from the air spaces, and A is water moving from the xylem into the leaf cell.

Symplast is the movement of water through:

Avacuoles
Bcell walls
Ccytoplasm of cellsAnswer
Dinterspaces

Why

The symplast is the system of interconnected protoplasts (cytoplasm) linked by plasmodesmata, so water moves from cytoplasm to cytoplasm without crossing further membranes (section 10.3.2).

Cell walls (B) and interspaces (D) form the apoplast, and vacuoles (A) belong to the separate vacuolar pathway.

Guard cells are the only cells of epidermis which have:

Avacuole
BchloroplastsAnswer
Ccytoplasm
Dleucoplasts

Why

Guard cells are the only epidermal cells that contain chloroplasts, because they photosynthesise and have high chlorophyll content (section 9.3.5, starch sugar interconversion theory).

Other epidermal cells also have a vacuole (A), cytoplasm (C) and leucoplasts (D, colourless plastids), so these features are not unique to guard cells.

The sugar moves through phloem is mostly in the form of:

Aglucose
BsucroseAnswer
Cmaltose
Dlactose

Why

Phloem sap is about 90% sucrose (cane sugar) of its dry matter, so sucrose is the sugar that is translocated (section 9.3.6).

Glucose (A) is the first product of photosynthesis but is converted to sucrose for transport; maltose (C) and lactose (D) are not the transport sugars of plants.

Succulent tissues are formed in:

Ahydrophytes
Bthallophytes
Cmesophyll
DxerophytesAnswer

Why

Xerophytes live in very dry places and store a very high amount of water in their tissues, so they are called succulent plants (section 9.4.1 and table 10.3).

Hydrophytes (A) store no water, and thallophytes (B) are a plant group, not a habitat type; mesophyll (C) is leaf tissue and is not described as succulent.

Structures X and Y are both part of the same:

The leaf section of question 11, with X and Y.

Acell
BorganAnswer
Ctissue
Dvessel

Why

X is a palisade mesophyll cell and Y is a cell of the lower epidermis, a different tissue from the palisade. Both belong to the leaf, which is an organ made of several tissues, so the answer is B (organ).

Option A is wrong because X and Y are two different cells; option C is wrong because the palisade and epidermal cells are different tissues; option D is wrong because neither structure is a vessel.

Note. The printed question 11 has the same stem as question 14 (the phloem capillary tube) and then the diagram question. The stem is printed only once in the exercise: it belongs to question 14. Question 11 is answered from the diagram alone.

Collenchyma is a supporting tissue in:

Aseeds
Bseedlings
CherbsAnswer
Dtrees

Why

Collenchyma gives support particularly in young plants, herbs and leaves where secondary growth does not occur (section 9.5.2). The book names herbs in this statement, so C is the answer.

Seeds (A) have no collenchyma support role, and trees rely on secondary growth and sclerenchyma (D). Seedlings (B) are young plants, so they are a close second, but the book's own wording points to herbs.

Note. Seedlings (B) are young plants and would also fit the book's wording 'young plants, herbs and leaves'. The book's key is not printed; C is taken as the intended answer.

The phenomenon of growth includes:

Acell differentiation
Bcell elongationAnswer
Ccell maturation
Dcell decomposition

Why

Growth is an irreversible increase in size and mass. Cell elongation (enlargement of the cell) is the phase that increases the size of the plant body, so B is the answer.

Cell decomposition (D) is the breakdown of cells, which is not growth. Differentiation (A) and maturation (C) are later phases of growth and development that the book lists alongside elongation; they do not by themselves increase size, so the item has more than one defensible option.

Note. The book lists four phases of growth and development (division, elongation, maturation, differentiation). A and C are also phases in the book's list, so this item is ambiguous; B is chosen as the option that defines an increase in size.

A researcher, who wants to study the composition of a plant's sap, inserts a capillary tube into the phloem. What causes the sap to flow out of the tube?

Acapillarity
Bhydrostatic pressureAnswer
Croot pressure
Dtranspiration stream

Why

Sugars loaded into the sieve tubes at the source lower their water potential, so water enters by osmosis and raises the hydrostatic pressure inside the phloem. This pressure pushes the sugary sap out when the tube is opened (section 9.3.6, pressure flow theory).

Capillarity (A), root pressure (C) and the transpiration stream (D) describe movement of water in the xylem, not the flow of sap in the phloem.

Two sealed test-tubes containing aquatic plants and hydrogencarbonate indicator were set up. One of the sealed test-tubes was kept in the light for 24 hours and one was kept in the dark for 24 hours. What is the correct explanation of what has taken place?

APhotosynthesis and respiration both occur in the light, but the rate of photosynthesis is higher.Answer
BPhotosynthesis occurs in the light, but respiration does not.
CRespiration can only occur when photosynthesis is not taking place.
DThe amount of carbon dioxide used and produced in the light is equal.

Why

In the light the indicator changes from orange (medium) to red (low), so carbon dioxide was removed faster than it was added: photosynthesis used more CO2 than respiration made. Respiration still goes on, because in the dark the indicator changes from orange to yellow (high CO2) as the plant respires.

B is wrong because respiration happens in the light too, and C is wrong because respiration is not stopped by photosynthesis. D is wrong because the colour changed from orange to red, so the CO2 used was not equal to the CO2 produced.

Short answer questionsp. 269

Answer the following short answer questions.

13 marks

What are carnivorous plants? Give examples.

Solution

Carnivorous (insectivorous) plants are photosynthetic autotrophs that have evolved to capture and digest insects. They live in nitrogen-poor habitats such as marshy areas, and the insects give them a supplementary source of nitrogen.

Their modified leaves serve as intricate traps, fitted with mechanisms for capturing, digesting and absorbing insects.
Some carnivorous plants live in symbiosis with specific bacteria that help break down insect proteins.
Examples: pitcher plant (Nepenthes pupurea), Venus flytrap (Dionaea muscipula) and sundew (Drosera intermedia).

23 marks

Define osmosis and diffusion.

Solution

Osmosis is the movement of water from a region of higher water concentration (lower solute concentration, higher water potential) to a region of lower water concentration (higher solute concentration, lower water potential) through a semipermeable membrane.

Diffusion is the net movement of molecules (gases, solutes or water) from a region of higher concentration to a region of lower concentration, down their concentration gradient, until they are evenly spread.

Osmosis is therefore a special case of diffusion, applied to water crossing a selectively permeable membrane. Diffusion does not need a membrane and needs no energy from the cell.

33 marks

What is water potential?

Solution

Water potential (\psi_w) is a measure of the potential energy of water, expressed in pressure units (MPa), that determines whether water moves from one place to another. Pure water at atmospheric pressure has a water potential of zero, which is the reference point.

Adding solutes lowers water potential (solute potential, \psi_s, is negative), and pressure greater than atmospheric pressure raises it (pressure potential, \psi_p). Thus \psi_w=\psi_s+\psi_p.

Water always moves from a region of higher water potential to a region of lower water potential, so the water potential of soil, root, stem, leaf and air decreases in that order.

43 marks

Why exchange of gases occurs more efficiently in air than water?

Solution

Air contains a much higher concentration of oxygen and carbon dioxide than water, where these gases are only slightly soluble. The concentration gradient for diffusion is therefore steeper in air.
Gases diffuse much faster through air, in which molecules move freely, than through water, which slows their movement.
Plants on land have aerial surfaces with stomata and lenticels that open to the air, so gases reach the cells over short distances and the exchange is quicker.

Name the hormones involved in each of the following physiological processes:

Solution

(a) Germination of seeds

Gibberellins (GA) break seed dormancy and start germination. Cytokinins also break dormancy (table 10.4).

(b) Stem elongation

Gibberellins promote stem elongation by stimulating both cell division and cell elongation, and auxin (IAA) promotes cell enlargement behind the apex. The two act together.

Ethene (ethylene) is the hormone that stimulates ripening. Auxins and gibberellins also promote ripening of fruit (table 10.4).

(d) Abscission of leaves

Abscisic acid promotes abscission of leaves (table 10.4). Ethene is also said in the text to initiate abscission of fruits and leaves, while the table gives it no effect on abscission.

(e) Dormancy of seeds

Abscisic acid (ABA) promotes and maintains seed dormancy. Gibberellins and cytokinins break dormancy.

Note. The book prints '(c) dormancy of seeds' for the last item. It is treated here as (e). The text on ethene and abscission (ethene initiates abscission) disagrees with table 10.4, which lists abscisic acid as the promoter and ethene as nil; both are given in the answer.

Differentiate between: collenchyma and sclerenchyma, photoperiodism and phototropism, transpiration and evaporation.

Solution

(a) Collenchyma and sclerenchyma

Feature	Collenchyma	Sclerenchyma
Cell wall	Thickened at the corners by extra cellulose; living cells	Thick secondary wall impregnated with lignin; dead cells at maturity
Position and function	Peripheral parts of young stems, petioles and leaves; flexible support while growing	Fibres in pericycle, xylem and phloem; sclereids in fruit walls; rigid strength
Growth	Can grow and stretch freely	Cannot stretch; mature cells are fixed in shape

(b) Photoperiodism and phototropism

Feature	Photoperiodism	Phototropism
What it is	Response of flowering to the length of the day (photoperiod) or night	Growth movement of a plant organ towards or away from light
Stimulus	Length of the uninterrupted dark period, read through phytochromes	Direction of light, caused by unequal growth on the two sides
Result	Vegetative buds change into floral buds and flowers form	Shoots bend towards light and roots grow away from it

Feature	Transpiration	Evaporation
What it is	Loss of water vapour from the above-ground parts of a plant	Change of liquid water to vapour from any moist surface
Where it occurs	Mainly through stomata, and some through the cuticle	From soil, water surfaces and wet cell surfaces
Control	Controlled by the plant, which opens and closes its stomata	Not controlled by the plant; depends on environment alone

Define: nutrition, nutrients, osmotic adjustment, primary growth, secondary growth, homeostasis, cohesion and adhesion.

Solution

(i) Nutrition

Nutrition is the set of processes by which an organism obtains nutrients and uses them to support its growth and metabolic activities.

(ii) Nutrients

A nutrient is any substance that supplies essential elements required by an organism for its growth and metabolic functions. Plant nutrients are mostly inorganic compounds.

(iii) Osmotic adjustment

Osmotic adjustment (osmoregulation) is the maintenance of the water and solute level in the body of an organism, so that water is kept in balance in changing osmotic conditions.

(iv) Primary growth

Primary growth is the increase in length of roots and shoots, carried out by cell division in the apical meristem, that starts after germination.

(v) Secondary growth

Secondary growth is the increase in thickness and diameter of stems and roots of dicots and gymnosperms, carried out by the lateral meristems (vascular and cork cambium).

(vi) Homeostasis

Homeostasis is the ability of an organism to maintain a steady internal state within a constantly changing external environment.

(vii) Cohesion

Cohesion is the attractive force between water molecules, due to the hydrogen bonds between them. It holds the water column in xylem together.

(viii) Adhesion

Adhesion is the attractive force between water molecules and other substances, such as the cellulose walls of xylem cells, which helps water rise in the xylem.

83 marks

Why support is needed in terrestrial life?

Solution

When life moved from water to land, organisms lost the buoyancy of water and had to keep their bodies in shape against gravity. Support is therefore needed for three reasons:

To keep the body upright and in shape, so that leaves and stems can hold their form in air.
To gain strength to withstand gravity and wind, and to hold the organs of the plant in place.
To hold leaves in a position that exposes them to light and air, which is needed for photosynthesis and gas exchange.

93 marks

What is the path of salts and water in vascular plants?

Solution

Salts and water travel from the soil to the air through the following path, each step driven by a fall in water potential (\psi_w of soil > root > stem > leaf > atmosphere):

Soil water and dissolved minerals enter the root hairs and epidermal cells of the root by osmosis and active uptake.
They cross the cortex by the apoplast, symplast and vacuolar pathways and are regulated at the endodermis by the Casparian strip.
They enter the xylem vessels and tracheids of the root and are carried upward in the xylem by the TACT mechanism to the stem and leaves.
In the leaf, water moves into the mesophyll cells and is lost as vapour through the stomata (transpiration).

102025 Annual I, Set B, Q.2 (vii)3 marks

How does symplast differ from apoplast?

Solution

Feature	Symplast	Apoplast
Route	Cytoplasm of interconnected cells through plasmodesmata (intracellular route)	Cell walls and intercellular spaces (extracellular route)
Membranes	Water does not cross further membranes once inside the cytoplasm	Water does not enter the cytoplasm; it moves through the walls
Control at endodermis	Passes through the endodermal cells, so the endodermis can regulate it	Blocked by the Casparian strip, so salts must enter the cytoplasm

113 marks

Enlist the names of supporting tissues in plants.

Solution

Turgid parenchyma cells, which give support by turgor pressure.
Collenchyma, with extra cellulose at the corners of its living cells.
Sclerenchyma: fibres and sclereids with thick, lignified walls.
Xylem fibres and phloem fibres, which are part of the vascular tissues.
The ring arrangement of vascular bundles in dicot stems, and secondary growth.

123 marks

What are annual rings? Define primary and secondary growth in plants.

Solution

Annual rings (growth rings) are the concentric rings of secondary xylem seen in a cross-section of a dicot or gymnosperm stem. Each ring is one year of growth: wide, light early wood is formed in spring and summer, and narrow, dark late wood in autumn and winter.

Primary growth is the increase in length of roots and shoots, produced by cell division in the apical meristems at the tips after germination.

Secondary growth is the increase in thickness and diameter of stems and roots of dicots and gymnosperms, produced by the lateral meristems (vascular and cork cambium).

133 marks

What happens to the primary tissue of a stem when secondary growth occurs?

Solution

Primary tissues are not destroyed. As the secondary tissues increase in thickness, the primary tissues are stretched and pushed outwards.

The epidermis and the outer primary cortex are cut off by the cork cambium, which forms cork outwards; they die and become part of the outer bark.
The primary phloem, pericycle, endodermis and inner primary cortex stay in the inner bark, next to the new secondary phloem.
The primary xylem stays near the pith, inside the secondary xylem (wood).

143 marks

Why does the wood of many tropical trees lack annual rings?

Solution

Annual rings form because the cells of the vascular cambium change in size with the seasons. In cold or dry seasons, small cells are made, and in wet seasons large cells are made, so each year produces a visible boundary between dark late wood and light early wood.

Many tropical trees grow where water is available all year round and there are no sharply different seasons. Their cambium therefore produces cells of similar size throughout the year, the boundaries between years do not form, and no distinct rings appear.

153 marks

Why is hardwood more desirable than softwood for making furniture?

Solution

The book does not compare hardwood and softwood directly. The answer below uses the book's account of xylem tissue.

Hardwood comes from dicots (angiosperms), whose xylem contains xylem vessels and many xylem fibres. The fibres are narrow, highly thick-walled and lignified, so the wood is dense and strong.
Softwood comes from gymnosperms, whose xylem has mainly tracheids and no xylem vessels, so it is less dense and weaker.
Because of its fibres and lignified walls, hardwood holds its shape, resists bending and takes a polished finish, which makes it more desirable for furniture.

Note. The book does not discuss hardwood and softwood. This answer is built from the book's description of xylem vessels, tracheids and xylem fibres (section 9.3.3) and the strength of sclerenchyma (section 9.5.2).

162026 Annual I, Set B, Q.2 (v)2026 Annual I, Set D, Q.2 (v)3 marks

What are the types of movement in plants in response to stimuli?

Solution

Plants respond to stimuli by growth movements called tropisms, which bend an organ towards (positive) or away from (negative) the stimulus. The types are:

Phototropism: response to light. Shoots bend towards light (positive) and roots grow away from it (negative).
Geotropism (gravitropism): response to gravity. Shoots grow upward (negative) and roots grow downward (positive).
Thigmotropism: response to touch, shown by climbing plants that coil around a wall, stick or rope.
Chemotropism: response to chemicals, shown by fungal hyphae that grow towards nutrients.

Extensive answer questionsp. 269–270

List the macro and micronutrients in plants highlighting role of each.

Solution

Macronutrients

Nine elements are needed in amounts greater than 0.05% of dry weight. Table 9.1 gives the mass share, the form in which the plant absorbs each element, and its main role.

Macronutrient	Form absorbed	Role in the plant
Carbon (45%)	CO₂	Major component of the plant's organic compounds
Oxygen (45%)	CO₂ and water	Major component of the plant's organic compounds
Hydrogen (6%)	H₂O	Major component of the plant's organic compounds
Nitrogen (1.5%)	NO₃⁻, NH₄⁺	Component of nucleic acids, proteins and chlorophyll
Potassium (1.0%)	K⁺	Cofactor of many enzymes; main solute in water balance; opening of stomata
Calcium (0.5%)	Ca²⁺	Component of middle lamella and cell walls; maintains membrane function; signal transduction
Magnesium (0.2%)	Mg²⁺	Component of chlorophyll; cofactor of many enzymes
Phosphorus (0.2%)	H₂PO₄⁻, HPO₄²⁻	Component of nucleic acids, phospholipids and ATP
Sulphur (0.1%)	SO₄²⁻	Component of proteins

Micronutrients

Micronutrients are needed in traces, less than 0.05% of dry weight, but a deficiency still disturbs normal growth.

Micronutrients listed in table 9.1
Micronutrient	Form absorbed	Role in the plant
Chlorine (0.01%)	Cl⁻	Water splitting in photosynthesis; water balance
Iron (0.01%)	Fe³⁺, Fe²⁺	Respiration; chlorophyll synthesis; N₂ fixation
Manganese (0.005%)	Mn²⁺	Formation of amino acids; activates some enzymes; water-splitting step of photosynthesis
Boron (0.002%)	H₂BO₃⁻	Cofactor in chlorophyll synthesis; cell wall function; pollen tube growth
Zinc (0.002%)	Zn²⁺	Formation of chlorophyll; cofactor of enzymes; needed for DNA transcription
Copper (0.001%)	Cu⁺, Cu²⁺	Component of many redox enzymes and lignin-biosynthetic enzymes
Nickel (0.001%)	Ni²⁺	Nitrogen metabolism
Molybdenum (0.0001%)	MoO₄²⁻	Nitrogen metabolism

Note. Table 9.1 lists eight micronutrients; the text lists seven. Nickel is taken from the table.

Explain the role of stomata and palisade tissue in the exchange of gases in plants.

Solution

Definitions

Stomata are tiny pores in the epidermis of leaves, each bounded by two kidney-shaped guard cells. Palisade tissue is the upper layer of mesophyll made of tightly packed, elongated cells with many chloroplasts, lying just under the upper epidermis.

Role of stomata

They are the main openings through which carbon dioxide enters the leaf and oxygen leaves it, by diffusion.
They open in the day, when photosynthesis is faster than respiration, so CO₂ is taken in and O₂ is released.
They close at night, when photosynthesis stops, and the gases needed for respiration then move in and out.

Role of palisade tissue

Palisade cells lie close to the upper epidermis, so light reaches them with little loss, and they contain many chloroplasts.
They are the main site of photosynthesis: they take in CO₂ from the air spaces and release O₂ into them.
Their tight packing has no air spaces between them, so the gases must diffuse through the spongy mesophyll and the air spaces below.

How the two work together

Gas exchange in a leaf

Air enters the stoma by diffusion, down the concentration gradient of CO₂.
CO₂ moves through the air spaces of the spongy mesophyll to the palisade cells.
In the palisade cells the CO₂ is fixed and used for photosynthesis, which releases O₂.
\ce{6CO2 + 6H2O ->[light] C6H12O6 + 6O2}
O₂ diffuses out of the palisade and spongy cells into the air spaces and leaves through the stoma.

Gas exchange through a stoma: CO₂ enters, O₂ leaves; the palisade cells are the site of photosynthesis.

Relate transpiration with gas exchange in plants.

Solution

Definition

Transpiration is the loss of water vapour from the above-ground parts of a plant, mainly through the stomata. The same stomata are used for gas exchange, so the two processes are linked.

The link

A stoma that is open for carbon dioxide to enter also lets water vapour escape. The leaf air is moister than the outside air, so water diffuses out whenever the stomata are open. Water loss is therefore the price the plant pays for taking in CO₂.

Time	Stomata	Gas exchange	Transpiration
Day (light)	Open	CO₂ in, O₂ out (photosynthesis faster than respiration)	High: water vapour leaves through the open stomata
Night (dark)	Closed or partly open	O₂ in, CO₂ out (respiration only)	Low: little water is lost
Hot day	Closed (above about 45 °C)	Reduced CO₂ uptake	Reduced to prevent dehydration

Steps linking the two

Light activates photosynthesis, which lowers the CO₂ inside the leaf.
CO₂ diffuses in through the open stomata.
Water in the spongy mesophyll evaporates into the air spaces and diffuses out through the same stomata (transpiration).
The loss of water creates the transpiration pull that draws water up the xylem, which keeps the cells supplied with water for photosynthesis.

Transpiration and gas exchange share the stomata: water vapour and O₂ leave, CO₂ enters, and water comes up the xylem.

Describe the structure of xylem vessel elements, sieve tube elements, companion cells, tracheids and relate their structures with functions.

Solution

Xylem tissue

Xylem conducts water and minerals upward from the roots. It is made of four cell types: tracheids, vessel elements, xylem fibres and xylem parenchyma.

Tracheids are dead, elongated cells with tapering ends and lignified walls with pits. They give mechanical strength and conduct water; they occur in all vascular plants.
Vessel elements are dead, short, wide, thick-walled tubes with open ends (perforations). They are placed end to end to form long vessels, which give a wide, unobstructed pathway for water. Vessels are found only in angiosperms.

Phloem tissue

Phloem conducts sugars (sucrose) and other organic solutes from the leaves to the rest of the plant, both upward and downward.

Sieve tube elements are living, thin-walled cells joined end to end. Their nuclei are lost when they mature, and each pair is separated by a sieve plate with pores, which lets sap flow from cell to cell.
Companion cells lie beside each sieve tube element. They have a nucleus and many mitochondria and supply the sieve tube elements with energy and control their activities, because the sieve tube elements have no nucleus.

Structure and function compared

Cell	Living or dead	Key structure	Function
Tracheid	Dead	Tapering, lignified walls with pits	Conduction of water; support
Vessel element	Dead	Wide tube with perforated end walls	Fast conduction of water and minerals; support
Sieve tube element	Living (no nucleus)	Sieve plates between cells	Transport of sucrose and other solutes
Companion cell	Living (with nucleus)	Dense cytoplasm, many mitochondria	Provides energy and controls the sieve tube element

Conducting cells: tracheid, a stack of vessel elements, and a sieve tube element with its companion cell.

Explain movement of water between plant cells, and between the cells and their environment in terms of water potential.

Solution

Water potential

Water moves from a region of higher water potential to a region of lower water potential. Water potential is written \psi_w, and pure water at atmospheric pressure has \psi_w=0. The total is made of two components:

\psi_w=\psi_s+\psi_p

Solute potential (\psi_s) is always negative; it falls as solute concentration rises.
Pressure potential (\psi_p) is positive when the cell is turgid and pressed against its wall, and it raises \psi_w.

Movement between two cells

Worked case from the book (cells A and B)

Compare the total water potential of the two adjacent cells.
\psi_{w,A}=-1400\ \mathrm{kPa},\qquad \psi_{w,B}=-600\ \mathrm{kPa}
Water moves from the cell with the higher \psi_w to the one with the lower value. Cell B has the higher value.
Water therefore moves by osmosis from cell B to cell A, until the two water potentials are equal.
At equilibrium the cells have the same \psi_w. If their volumes were equal, the value would be the mean of the two.
\psi_w=\frac{-1400-600}{2}=-1000\ \mathrm{kPa}

Movement between a cell and its environment

In pure water (\psi_w=0), both cells have a lower \psi_w, so water enters both by osmosis. Their pressure potentials rise as they swell, until the water potential of the cells approaches zero.
In a sucrose solution of \psi_w=-3510 kPa, the solution has a lower water potential than both cells, so water leaves both by exosmosis. The cells lose turgor and become flaccid.

Two adjacent cells: water moves from cell B (higher water potential) to cell A (lower water potential).

Explain the movement of water in xylem through roots in terms of symplast, apoplast and vacuolar pathways.

Solution

The three pathways

Water enters the root epidermis by osmosis from the soil through the root hairs. It then crosses the cortex to the endodermis and the stele, and reaches the xylem by one of three pathways.

Pathway	Route	What stops or controls it
Apoplast	Through the continuous cell walls and intercellular spaces	Stopped by the Casparian strip in the endodermis, so water and ions must enter the cytoplasm
Symplast	Through the cytoplasm of cells, linked by plasmodesmata	Passes through the endodermal cells, which regulate solutes
Vacuolar	From vacuole to vacuole through neighbouring cells, across membranes and tonoplast	Moves down a water potential gradient by osmosis

Steps in the root

Water enters the root hairs and epidermal cells by osmosis, down the gradient of water potential.
Water moves inward through the cortex by the apoplast (walls), symplast (cytoplasm) and vacuolar routes, often together.
At the endodermis the apoplast is blocked by the Casparian strip (suberin and lignin band), so water and minerals must enter the cytoplasm of the endodermal cells.
The cells of the endodermis regulate what passes, and water then enters the pericycle and the xylem vessels and tracheids.

Cross-section of a root showing the apoplast (a), symplast (b) and vacuolar (c) pathways, stopped or controlled at the endodermis.

72025 Annual I, Set D, Q.6

Explain the movement of water in xylem through TACT mechanism.

Solution

The TACT theory

The ascent of sap in tall plants needs a large force, because water must move against gravity. The TACT theory states that four forces, transpiration pull, adhesion, cohesion and tension, together raise the water column in the xylem.

The four components

Component	What it is	Role in ascent
Transpiration pull	Evaporation of water from the leaf mesophyll cells through the stomata	Pulls water from the xylem of the leaf, creating a pull that is transmitted down the column
Adhesion	Attraction between water molecules and the cellulose walls of the xylem	Holds the water column to the walls and counters gravity
Cohesion	Attraction between water molecules by hydrogen bonds	Keeps the water in the xylem as one continuous column
Tension	The pull created in the column by the hydrogen bonds	Makes the water column tense, so it is pulled up as one piece

Steps

Stomata open and water evaporates from the mesophyll cells into the air spaces (transpiration).
Water is pulled from the xylem of the leaf, and the column of water is put under tension.
Cohesion holds the column together, and adhesion holds it to the walls, so the whole column moves up as one unit.
Water is replaced at the root by osmosis, so the column is continuous from soil to leaf.

The TACT forces in a plant: transpiration pull at the leaf, cohesion holding the water column, adhesion to the xylem walls, and tension along the column.

82026 Annual I, Set B, Q.5 OR2026 Annual I, Set D, Q.5

Describe the mechanisms involved in the opening and closing of stomata.

Solution

Turgor and the guard cells

Each stoma is bounded by two guard cells. The stoma opens when the guard cells become turgid, and closes when they become flaccid. Two theories explain the change in turgor.

Starch-sugar interconversion theory

In the morning, light starts photosynthesis in the guard cells, which contain chloroplasts.
Sugar (glucose) builds up, so the solute potential and water potential of the guard cells fall.
Water moves into the guard cells from the surrounding epidermal cells by osmosis. The guard cells become turgid and the stoma opens.
In the evening, photosynthesis stops, some sugar is used in respiration, some is converted to sucrose for transport, and the rest is stored as starch.
The sugar falls, the water potential of the guard cells rises, water leaves by osmosis, and the cells become flaccid. The stoma closes.

Potassium ion (K⁺) influx and efflux theory

In the morning, photosynthesis uses CO₂ in the guard cells, so their CO₂ falls.
Low CO₂ and blue light stimulate active transport of K⁺ into the guard cells from the surrounding epidermal cells.
Malic acid is ionised into malate⁻ and H⁺. The H⁺ are pumped out, and the K⁺ combine with malate⁻ to form potassium malate, which lowers the solute potential.
Water moves in by osmosis, the guard cells become turgid and the stoma opens.
In the evening, CO₂ rises in the guard cells and K⁺ leaves by active transport. Malate combines with H⁺ again, the solute potential rises, water leaves, and the stoma closes.

Feature	Starch-sugar theory	K⁺ influx and efflux theory
Solute that changes	Glucose and sucrose	Potassium ions and malate
Trigger	Photosynthesis in the guard cells	Low CO₂ and blue light
Transport	Osmosis only	Active transport of K⁺ and H⁺

Guard cells open (turgid) with K⁺ and water entering, and closed (flaccid) with the same ions and water leaving.

Explain the movement of sugars within plants.

Solution

Translocation

The movement of prepared food (organic solutes, mainly sucrose) through the phloem from the places where it is made or stored to the places where it is used is called translocation.

Source and sink

A source is an area that supplies sugar, such as a mature photosynthesising leaf or a storage organ that is releasing its food.
A sink is an area that uses or stores sugar, such as a growing fruit, a root or a storage organ.

Pressure-flow (mass-flow) theory

At the source, sucrose is actively loaded into the sieve tube elements by the companion cells. This needs ATP.
The sugar concentration in the phloem rises, so the water potential falls and water enters by osmosis from the nearby xylem.
The inflow of water raises the hydrostatic pressure in the sieve tubes at the source, and the sap is pushed towards the sink.
At the sink, sucrose is actively unloaded into the surrounding cells, the sap's solute concentration falls, and water leaves the sieve tube by osmosis.
The hydrostatic pressure is low at the sink, so the pressure gradient moves the sap from source to sink. Water from the sink returns to the xylem.

Why the sap moves in any direction

Because sources and sinks can be above or below one another, translocation can go upward or downward in the plant. The direction is set by the source and sink, not by gravity, so the flow is described as source to sink.

Pressure-flow mechanism: sucrose is loaded at the source leaf and unloaded at the sink root, and water moves in and out by osmosis.

Explain movement of water into or out of cell in isotonic, hypotonic and hypertonic conditions.

Solution

Definitions

The direction of water movement depends on the water potential of the medium compared with the cell sap. A hypotonic medium has higher water potential and lower solute concentration than the cell sap. A hypertonic medium has lower water potential and higher solute concentration. An isotonic medium has the same water potential and solute concentration as the cell sap.

Condition	Water potential of medium	Water movement	State of the plant cell
Hypotonic	Higher than cell sap	Water enters by endosmosis	Turgid (swollen), with high pressure potential
Isotonic	Same as cell sap	No net movement	No change in volume; the cell stays as it is
Hypertonic	Lower than cell sap	Water leaves by exosmosis	Flaccid, and the protoplast shrinks from the wall (plasmolysis)

Steps

Identify the water potential of the medium relative to the cell sap.
Water moves down the gradient of water potential, into the cell in a hypotonic medium and out of it in a hypertonic medium.
In a hypotonic medium the vacuole swells and the cell becomes turgid, pressing against the wall.
In a hypertonic medium the vacuole shrinks, the protoplast pulls away from the wall, and the cell becomes plasmolysed.

Plant cells in hypotonic, isotonic and hypertonic media: water enters, does not move, or leaves.

112025 Annual I, Set D, Q.2 (ii)

Describe osmotic adjustments in hydrophytic (marine and freshwater), xerophytic and mesophytic plants.

Solution

Plants are grouped by the availability of water in their habitat. Each group faces a different osmotic condition and therefore shows different adjustments.

Feature	Hydrophytes	Mesophytes	Xerophytes
Habitat	Aquatic	Terrestrial (moderate water)	Terrestrial (severely dry)
Osmotic condition of medium	Hypotonic	Nearly isotonic	Strongly hypertonic
Problem faced	Flooding (excess water)	No drastic problem	Severe dehydration
Rate of transpiration	Very high	Medium	Very low
Stomatal distribution	Upper epidermis	Lower epidermis	Sunken stomata
Stomatal opening	Open all the time	Open in day, closed at night	Closed in day, open at night
Cuticle	Very thin or none	Less thick	Very thick
Storage of water	None	Little storage	Succulent

Hydrophytes

Freshwater hydrophytes live in hypotonic conditions and so face the problem of flooding, which is an excess of water. They show the highest transpiration rate because their broad leaves have a large surface area and stomata on the upper epidermis that stay open day and night. They store no water and have a very thin or no cuticle.
The book's hydrophyte examples are freshwater plants. It does not describe marine hydrophytes separately; the saline-soil adaptations of section 9.4.2 are the relevant ones for plants in salt water.

Mesophytes

Mesophytes such as wheat, rice and corn face nearly isotonic conditions in moderate water. They have a moderate transpiration rate and stomata on the lower epidermis, which open in the day and close at night. They store little water and have a thin cuticle.

Xerophytes

Xerophytes (cactus, opuntia) live in strongly hypertonic conditions, so they lose water easily by exosmosis.
They reduce transpiration with narrow, needle-like leaves or spines, a very thick cuticle, and stomata sunk in depressions that stay closed in the day.
They store a large amount of water in their succulent tissues.

Note. The question asks about marine hydrophytes. The book's hydrophyte section gives freshwater examples only, so the answer uses that description and notes the gap.

Explain the osmotic adjustments in plants in saline soils.

Solution

The problem

A saline soil has a very low water potential because of its high salt concentration. Plants cannot take in water even when it is present, which is a physiological drought. Plants that live in these conditions are halophytes.

Strategy 1: accumulating ions in vacuoles

Halophytes take up sodium (Na⁺) and chloride (Cl⁻) from the soil at high rates.
The ions are stored in the vacuoles of the leaf cells.
This keeps the salt concentration in the cytoplasm and organelles low, so the enzymes and metabolism are not disturbed.
The stored salt lowers the solute potential of the vacuole, so water can enter the cell from the saline soil by osmosis.

Strategy 2: compatible solutes

Some halophytes do not take up salt from the soil.
Instead they make organic compounds, mainly nitrogenous compounds such as glycine and, in some plants, sugar alcohols such as sorbitol.
These compatible solutes accumulate in the cytoplasm and lower its water potential without interfering with enzymes and metabolism.

Osmotic adjustment in a halophyte cell: salt ions are stored in the vacuole, and compatible solutes are kept in the cytoplasm.

List the adaptations in plants to cope with low and high temperature.

Solution

Adaptations to low temperature

Unsaturated fatty acids: plants increase the proportion of unsaturated fatty acids in membranes, which have a low freezing point and keep the membrane fluid.
Changes in solute composition: plants native to cold regions (oaks, maples, roses) change the solutes in their cells so that the cytosol supercools without forming ice, although ice may form in the cell wall.
Gradual acclimation: the adaptations take time, so a rapid chill is more stressful than a gradual fall in air temperature.

Adaptations to high temperature

Shiny cuticle: reflects much of the incident light, which reduces heat absorption.
Transpiration: numerous stomata allow water loss, which cools the plant.
Closing stomata: above about 45 °C most plants close their stomata to prevent water loss.
Heat shock proteins: large quantities of heat-tolerant proteins are made, which protect enzymes and other proteins from denaturation.

Explain the turgor pressure and explain its significance in providing support to herbaceous plants.

Solution

Turgor pressure

In plants, parenchyma cells have large central vacuoles filled with water. When the cell takes in water, the protoplast presses the cell wall outward and the wall pushes back. This pressure on the walls is called turgor pressure. It makes the cell firm and stiff.

Steps of turgor

Water enters the cell by osmosis, because the cell sap has a lower water potential than the surroundings.
The vacuole swells and the protoplast presses against the cell wall.
The wall resists stretching, so the cell wall pushes back: this is the turgor pressure.
The turgid cells are stiff, so the tissue they make up is firm.

Significance in herbaceous plants

Herbaceous plants lack the woody, lignified tissues of trees and shrubs. Their stems and leaves stay upright because turgid parenchyma cells press their walls against one another, which gives the whole organ rigidity.

When water is lost, the turgor falls and the plant wilts or collapses, which shows that turgor is its main support.

A turgid parenchyma cell presses its wall outward; a flaccid cell with little water has no turgor, so the tissue wilts.

Describe the structure of supporting tissue in plants.

Solution

Collenchyma

Living cells with thin primary walls that are thickened at the corners by extra cellulose deposits.
They are flexible and can grow and stretch with the young organ, so they support young stems, petioles and leaves.

Sclerenchyma

Dead cells with very thick secondary walls impregnated with lignin, which gives rigidity and great tensile strength.
Fibres are long, tapering cells with pointed ends that interlock without air spaces. They are found in the pericycle of stems, and in xylem and phloem.
Sclereids are roughly spherical or irregular cells with thick walls, common in fruit walls and seed coats.

Feature	Collenchyma	Sclerenchyma
Cell status	Living	Dead at maturity
Wall	Thin, thickened at corners	Very thick, lignified all round
Position	Peripheral in young stems, petioles and leaves	Pericycle, xylem, phloem, fruit and seed coats
Growth	Can stretch	Cannot stretch

Collenchyma cells with thickened corners (left) and sclerenchyma fibres with thick lignified walls and a narrow lumen (right).

Define growth and explain primary and secondary growth in plants.

Solution

Definition

Growth is an irreversible increase in the size and mass of an organism. In plants it occurs through four phases: cell division, cell elongation, cell maturation and differentiation.

Primary growth

Primary growth is the increase in length of the root and shoot. It begins after germination and continues through the life of the plant (indeterminate growth). It is carried out by cell division in the apical meristem, and the tissues it makes are called primary tissues.

Secondary growth

Secondary growth is the increase in thickness and diameter of stems and roots of dicots and gymnosperms. It is carried out by the lateral meristems (vascular cambium and cork cambium), and the tissues it makes are called secondary tissues.

Feature	Primary growth	Secondary growth
Result	Increase in length	Increase in thickness and diameter
Meristem	Apical meristem at the tips	Lateral meristems (vascular and cork cambium)
Plants	All plants with roots and shoots	Dicots and gymnosperms only
Tissues made	Primary tissues	Secondary tissues (wood, bark)

Primary growth lengthens the shoot from the apex (left); secondary growth thickens the stem from the cambium ring (right).

Describe the role of apical meristem and lateral meristem in primary and secondary growth.

Solution

Apical meristem

Found at the tips of roots and shoots, it is also called the primary meristem because it is present from the embryonic stage.
Its cells divide repeatedly and the new cells (derivatives) enlarge and differentiate, lengthening the root and shoot.
It gives rise to the primary xylem, primary phloem and primary tissues, and to new leaves, branches and buds.

Lateral meristem

Cylinders of dividing cells on the lateral sides of stems and roots of dicots and gymnosperms; also called the secondary meristem.
The vascular cambium lies between primary xylem and phloem. It divides faster than the cork cambium and adds secondary xylem on the inside and secondary phloem on the outside.
The cork cambium (phellogen) lies in the outer primary cortex. It adds secondary cortex inwards and cork outwards, which thickens the stem's outer covering.

Meristem	Position	Growth it produces	Tissues made
Apical	Tips of roots and shoots	Primary growth (length)	Primary xylem, phloem, leaves, buds
Vascular cambium	Between primary xylem and phloem	Secondary growth (thickness)	Secondary xylem (inside) and phloem (outside)
Cork cambium	In the outer primary cortex	Secondary growth (thickness)	Cork (outside) and secondary cortex (inside)

Where the meristems are: apical meristems at the shoot apex and root tip, and lateral meristems (vascular and cork cambium) in the stem.

Explain how annual rings are formed in plants.

Solution

Formation

Annual rings (growth rings) are formed by the vascular cambium, which produces secondary xylem every year. Its rate of production changes with the seasons, so each year leaves a distinct band of wood.

Steps

In spring and summer, water is plentiful and the vascular cambium produces large xylem cells with wide cavities. This is early wood, which is light in colour.
In autumn and winter, water is scarce, so the cambium produces smaller xylem cells with thick walls. This is late wood, which is dark.
The change from large to small cells gives a sharp boundary between two years.
One pair of light and dark bands, from early wood to late wood, is one ring and represents one year's growth.
Counting the rings in a stem cross-section gives the age of the plant.

Notes

The rings are clearest in plants with distinct seasons; a tree that grows at a steady rate all year has no clear rings (section 9.6.3).
Wood that is inner to the cambium is secondary xylem. The inner heartwood is dark because it is blocked by waste, and the sapwood outside it carries water.

Cross-section of a stem with annual rings: each ring is one year, with large cells in early wood and small cells in late wood.

Explain influence of apical meristem on the growth of lateral shoot.

Solution

Apical dominance

Apical dominance is a negative growth correlation in which the growing apical bud inhibits the growth of the lateral buds below it. The same effect occurs in roots, where the main root inhibits the growth of lateral roots.

Mechanism

The apical bud produces auxin (IAA), which moves down the stem.
Auxin inhibits the growth of the lateral buds, so they remain small or dormant.
Lateral buds produce cytokinin, which works against auxin and promotes lateral bud growth.
Strong apical dominance means more auxin and less cytokinin reach the lateral buds, so few branches form. Weak apical dominance means less auxin and more cytokinin, so many branches grow.

Examples

Tall plants with strong apical dominance have short side branches, and their form is upright and narrow.
Plants with weak apical dominance grow short with many lateral branches, forming a dense bush.

Strong apical dominance (left) gives a tall plant with few branches; weak apical dominance (right) gives a bushy plant.

Explain the role of important plant growth regulators.

Solution

Plant growth regulators (plant hormones) are chemicals that move from their site of synthesis to their site of action. Five major groups are recognised. Auxins, gibberellins and cytokinins are growth promoters; abscisic acid and ethene are growth inhibitors.

A comparison of the effects of growth regulators on plant parts (book table 10.4, summarised)
Regulator	Type	Main roles
Auxin (IAA)	Promoter	Promotes cell elongation behind the shoot apex; causes apical dominance; promotes fruit ripening; delays leaf senescence; promotes bud initiation; in low concentration promotes root growth
Gibberellins	Promoter	Promotes stem elongation by cell division and elongation; breaks seed dormancy; promotes bud initiation and flowering in long-day plants; promotes ripening
Cytokinins	Promoter	Promotes cell division in apical meristem and cambium; promotes lateral bud growth; breaks bud dormancy; delays leaf senescence; promotes lateral root growth
Abscisic acid (ABA)	Inhibitor	Inhibits growth during stress (drought, salinity, waterlogging); promotes seed dormancy; promotes leaf senescence and abscission; closes stomata
Ethene	Inhibitor	Promotes ripening of fruit; inhibits root growth; promotes flowering in pineapple; drives abscission according to the text

Explain types of movement in plants in response to light, gravity, touch and chemicals.

Solution

Plants cannot move from place to place, but their organs show growth movements called tropisms when the rate of growth differs on the two sides of an organ. A tropism is positive if the organ grows towards the stimulus and negative if it grows away.

Stimulus	Tropism	Response	Example
Light	Phototropism	Shoots bend towards light (positive); roots grow away from it (negative)	A shoot growing towards a window
Gravity	Geotropism (gravitropism)	Shoots grow upward (negative); roots grow downward (positive)	Roots growing into the soil
Touch	Thigmotropism	Organ coils round a support	Climbing plants that coil round a wall, stick or rope
Chemicals	Chemotropism	Organ grows towards the chemical	Fungal hyphae growing towards nutrients

Steps of a tropism

The organ receives the stimulus on one side.
A growth regulator (mainly auxin) moves to the shaded or lower side.
Cells on that side elongate more, so the organ bends towards or away from the stimulus.

Four tropisms: a shoot bending to light, a root growing down under gravity, a tendril coiling round a rod, and hyphae growing to a nutrient.

Classify plants on the basis of photoperiodism and give examples.

Solution

Photoperiodism is the response of a plant to the total duration of light in the whole day (the photoperiod), which controls the change from vegetative to flowering growth. Plants are classified into three groups.

Group	When they flower	Examples
Short-day plants	When the photoperiod is shorter than a critical length (in fact, when the night is longer than a critical length)	Maryland mammoth, cocklebur (critical photoperiod 15.5 h), chrysanthemum, tobacco (14 h)
Long-day plants	When the photoperiod exceeds a critical length (in fact, when the night is shorter than a critical length)	Spinach, sugar beet, henbane (critical 11 h), cabbage, spring wheat
Day-neutral plants	After a considerable vegetative growth, in any photoperiod	Tomato, pansy, bean, sweet pea, rose

Notes from the book

The critical photoperiod is different for each plant. Short-day plants are really long-night plants, and long-day plants are short-night plants, because flowering depends on the uninterrupted dark period.
Flowering is controlled by the dark period: a night interruption by a brief light flash changes the response, and red and far-red light have opposite effects (section 9.7.3).

232025 Annual I, Set B, Q.6

Explain the mechanism of photoperiodism with reference to the mode of action of phytochrome.

Solution

Phytochrome

Phytochrome is a blue-green photoreceptor pigment. It is a protein with an iron-containing non-protein part, and it exists in two forms that can be converted into each other: **P_R (absorbs red light at 660 nm) and P_{FR}** (absorbs far-red light at 730 nm).

Steps of the mechanism

In daylight, red light converts P_R into P_{FR}. P_{FR} is the active form.
Far-red light converts P_{FR} back into P_R, so the two forms are in equilibrium according to the light.
In darkness, P_{FR} slowly converts back to P_R. This slow conversion gives the plant a biological clock that measures the length of the night.
At the end of the night the ratio of P_{FR} to P_R determines the response. The ratio is high after a short night and low after a long one.
A short-day plant flowers when the ratio is low (a long, uninterrupted night). A long-day plant flowers when the ratio is high (a short night).
The ratio triggers the production of florigen, which changes vegetative buds into floral buds.

Effect of night interruption

A brief red flash in the middle of a long night converts P_R to P_{FR}, which prevents flowering in a short-day plant such as cocklebur. A far-red flash after the red reverses this, so the last light treatment decides the response.

Interconversion of the two forms of phytochrome: red light makes P_{FR}, far-red light makes P_R, and P_{FR} slowly reverts in darkness.

Define vernalization and explain the role of low temperature treatment on flower production.

Solution

Definition

Vernalization is the promotion of flowering by exposure to low temperature. The low-temperature stimulus is received by the shoot apex of a mature plant or by the embryo, not by the leaves as in photoperiodism.

Role of low temperature

Most vernalization temperatures lie between 0 °C and 10 °C, and about 4 °C is the most effective.
Biennial plants complete their life cycle in two calendar years. Winter wheat sown in October or November is harvested in June or July of the next year.
If winter wheat is sown in February or March like spring wheat, it shows vegetative growth only, but it can flower if its seeds receive several weeks at about 4 °C first.
In some plants a low-temperature period is absolute: they do not flower without vernalization.

Steps

Seeds or the shoot apex of the plant receive several weeks of low temperature.
Low temperature stimulates the production of vernalin, which the book identifies as gibberellin.
Vernalin transforms vegetative buds into floral buds.
The plant then flowers in the appropriate season.

The process of vernalization: low temperature at the shoot apex produces vernalin (gibberellin), which turns vegetative buds into floral buds and leads to flowering.

FBISE 2025 and 2026 questions

Questions from the FBISE HSSC-I 2026 (1st Annual) and FBISE HSSC-I 2025 (1st Annual) papers that belong to this unit, answered the way an examiner expects.

2026 Annual I, Set B, MCQ 71 mark

Which plant traps insects using sticky leaves?

ANepenthesAnswer
BSaltwort
CSunflower
DPine

Why

Of the four plants, only Nepenthes (the pitcher plant) is a carnivorous plant that traps insects in modified leaves. Saltwort is a halophyte of saline soils, and sunflower and pine are ordinary photosynthetic plants that do not trap insects, so B, C and D are wrong.

Note. The plant that really traps insects on sticky leaves is the sundew (Drosera), which is not among the options. The pitcher plant is the only carnivorous plant listed, so A is the best key.

2025 Annual I, Set B, MCQ 121 mark

Bark is produced by:

ACork cambiumAnswer
BLateral meristem
CApical meristem
DIntercalary meristem

Why

The cork cambium (phellogen) is the meristem that adds cork to the outside and secondary cortex to the inside. The cork cells die and, with the epidermis, form the rough outer bark of a mature tree, so the bark is produced by the cork cambium.

Lateral meristem (B) is tempting because the cork cambium is a lateral meristem, but the vascular cambium is also a lateral meristem and it makes wood, so the general name does not identify the bark-producing tissue. Apical meristem (C) gives primary growth in length and intercalary meristem (D) gives growth at the nodes or bases of leaves, not bark.

Note. The book defines bark as everything outside the vascular cambium, which includes secondary phloem made by the vascular cambium. The question asks for the layer that gives the outer bark (cork), so the key is A.

2026 Annual I, Set B, Q.2 (v)1x3 = 3 marks

Briefly explain three types of plant movements.

Solution

Plants cannot move from place to place, but their organs move by differential growth in response to an external stimulus. These growth movements are called tropisms (tropic movements). Three of them are:

1. Phototropism (1 mark)

Movement of a plant part towards (positive) or away from (negative) light. Shoots grow towards light (positive); roots grow away from light (negative).

2. Geotropism (1 mark)

Movement of a plant part in response to gravity. Shoots grow upward, away from gravity (negative); roots grow downward, towards gravity (positive).

3. Thigmotropism (1 mark)

Movement in response to touch. Climbing plants coil round a wall, rod or rope that supports them.

(Chemotropism, the growth of fungal hyphae towards nutrients, may be written instead of any one of these.)

2025 Annual I, Set B, Q.2 (vii)3 marks

Compare apoplast and symplast pathways involved in uptake of water by roots in plants.

Solution

Water entering the root hairs reaches the xylem by the apoplast, symplast or vacuolar pathway. The first two compare as follows:

Point	Apoplast pathway	Symplast pathway
Made of	The system of adjacent cell walls (and the spaces in them), continuous through the plant	The system of interconnected protoplasts (cytoplasm) of the cells
Linked by	Spaces in the cell walls	Plasmodesmata between neighbouring cells, helped by cytoplasmic streaming
Membranes crossed	None until the endodermis	None after the first cell (water moves from cytoplasm to cytoplasm)
At the endodermis	Water is stopped by the Casparian strips (suberin and lignin) and must enter the cytoplasm	Water passes on through the cytoplasm to the xylem

Figure 9.4: Pathways of water in the root. The three pathways are in the accent colour, each with its own line style.

2026 Annual I, Set B, Q.5 OR3+3 = 6 marks

Describe the mechanism involved in opening and closing of stomata by \mathrm{K^+} ion influx / efflux theory.

Solution

Stomata open when their guard cells are turgid and close when the guard cells are flaccid. According to the \ce{K+} theory the movement of potassium ions in and out of the guard cells changes their water potential.

Opening of stomata in the morning (3 marks)

Light starts photosynthesis, so the \ce{CO2} level in the guard cells falls.
Low \ce{CO2} stimulates the influx of \ce{K+} ions into the guard cells from the surrounding epidermal cells by active transport.
Blue light ionises malic acid into malate and \ce{H+} ions; the \ce{H+} ions are pumped out to the epidermal cells to keep the pH steady.
\ce{K+} combines with malate to form soluble potassium malate, which lowers the solute and water potential of the guard cells.
Water enters from the epidermal cells by osmosis, the guard cells become turgid and the stoma opens.

Closing of stomata in the evening (3 marks)

Photosynthesis stops and the \ce{CO2} level in the guard cells rises.
High \ce{CO2} stimulates the efflux of \ce{K+} ions from the guard cells to the epidermal cells by active transport.
Without blue light, malate and \ce{H+} recombine into malic acid, so no potassium malate remains.
The solute and water potential of the guard cells rise, water moves out to the epidermal cells, the guard cells become flaccid and the stoma closes.

2025 Annual I, Set B, Q.61+1+2+2 = 6 marks

Explain the mechanism of photoperiodism.

Solution

Photoperiodism (1 mark)

The effect of the photoperiod (the total duration of light in the day) on the development of flowers is photoperiodism. Short-day plants flower when the day is shorter than a critical length, long-day plants when it is longer, and day-neutral plants flower in any day length.

Dark period is the real stimulus (1 mark)

Night-interruption experiments show that the uninterrupted dark period is critical. A short-day plant (cocklebur) will not flower if its long night is interrupted by light, so short-day plants are really long-night plants. A long-day plant (henbane) behaves in the opposite way: it is a short-night plant. Red light (660 nm) is the effective light and far-red light (730 nm) reverses it; the last treatment decides the response.

Phytochrome and its two forms (2 marks)

The mechanism depends on the phytochromes, blue-green protein pigments that exist in two interconvertible forms: Pr (\mathrm{P_{660}}), which absorbs red light and changes into Pfr (\mathrm{P_{730}}), and Pfr, which absorbs far-red light and reverts to Pr. Pr changes into Pfr in the day; Pfr changes slowly back into Pr during the night, and this rate of conversion works as a biological clock that measures the length of the night.

Figure 9.19: Interconversion of the two forms of phytochrome. Pfr is the active form (accent outline); it leads to a physiological response such as seed germination, stomatal opening or flowering.

Group	Ratio of Pfr to Pr at end of night	Reason
Short-day plant	Low	A long enough dark period (cocklebur: 8.5 hours or more) converts all Pfr to Pr
Long-day plant	High	A short dark period (henbane: 13 hours or less) cannot convert all Pfr to Pr

Florigen (2 marks)

The night length is measured by the conversion of Pfr to Pr.
The right ratio of Pfr to Pr at the end of the night is reached.
This ratio triggers the production of the hormone florigen.
Florigen changes the vegetative buds into floral buds, so flowers are produced.

2026 Annual I, Set D, MCQ 71 mark

Venus flytrap responds to:

ATouch stimulusAnswer
BHeat stimulus
CLight stimulus
DGravity stimulus

Why

The Venus flytrap (Dionaea muscipula) has modified leaves that act as traps. When an insect touches the sensitive hairs on the leaf, the trap closes, so it responds to a touch stimulus.

Heat (B), light (C) and gravity (D) are not the stimulus that closes the trap. Light and gravity are the stimuli for phototropism and geotropism of shoots and roots.

2025 Annual I, Set D, MCQ 101 mark

Growth movements shown by climbing plants with a supporting structure is:

APhototropism
BGeotropism
CThigmotropismAnswer
DChemotropism

Why

Thigmotropism is the growth movement in response to touch. Climbing plants coil round a wall, rod or rope that supports them because they touch it.

Phototropism (A) is a response to light, geotropism (B) to gravity and chemotropism (D) to a chemical such as nutrients, so none of them is the response to a supporting structure.

2025 Annual I, Set D, MCQ 161 mark

In TS of a dicot stem, which substances are expected to pass through labelled parts P and Q respectively?

Transverse section of a dicot stem: vascular bundles in a ring round the pith; in each bundle the phloem (P) is outside and the xylem (Q, solid) is inside.

AWater and sucrose
BSucrose and amino acids
CSucrose and waterAnswer
DWater and amino acids

Why

In the vascular bundle of a dicot stem the phloem lies on the outer side and the xylem on the inner side (towards the pith). P is the outer part, the phloem, which carries organic food such as sucrose and amino acids. Q is the inner part, the xylem, which carries water and minerals.

So the order is sucrose then water (C). Option A reverses the two tissues. Options B and D are wrong because B names no water-carrying tissue and D names no sucrose-carrying tissue.

2025 Annual I, Set D, Q.2 (ii)3 marks

Compare any three adaptations for osmotic adjustments in hydrophytes and mesophytes.

Solution

Hydrophytes live in water (hypotonic conditions, risk of flooding); mesophytes live where water is moderate (nearly isotonic). Their adaptations differ as follows:

Adaptation	Hydrophytes	Mesophytes
Rate of transpiration	Very high (broad leaves, large surface area)	Moderate (medium-sized leaves)
Position of stomata	On the upper epidermis	On the lower epidermis
Stomatal opening	Open all the time, day and night	Open in the day and close at night
Cuticle	Very thin or absent	Less thick
Storage of water	None	Very little

Any three rows give the three marks.

2026 Annual I, Set D, Q.2 (v)1x3 = 3 marks

State three stimuli affecting plant movement.

Solution

Plant organs move by differential growth in response to external stimuli. Three important stimuli are:

Stimulus	Movement it causes	Example (as in the book)
Light	Phototropism	Shoots grow towards light (positive); roots grow away from it (negative)
Gravity	Geotropism	Shoots grow upward (negative); roots grow downward (positive)
Touch	Thigmotropism	Climbing plants grow over a wall, rod or rope

A fourth stimulus is a chemical (nutrients), which causes chemotropism in fungal hyphae.

2026 Annual I, Set D, Q.53+3 = 6 marks

Describe the mechanism involved in opening and closing of stomata by starch sugar inter conversion theory.

Solution

Stomata open when the guard cells are turgid and close when they are flaccid. The theory says that the guard cells are the only photosynthesising cells of the leaf epidermis (they have more chlorophyll than the neighbouring epidermal cells), and that the change of sugar and starch alters their water potential.

Opening of stomata in the morning (3 marks)

In the light, photosynthesis starts in the guard cells.
The concentration of glucose rises, so the solute potential and water potential of the guard cells become lower (more negative).
The surrounding epidermal cells have a higher water potential, so water moves into the guard cells by osmosis.
The guard cells become turgid and the stomata open.

Closing of stomata in the evening (3 marks)

Photosynthesis stops, so the glucose concentration falls. Some glucose is used in respiration, some is changed to sucrose for transport, and the rest is converted into insoluble starch.
The solute potential and water potential of the guard cells rise.
Water moves out of the guard cells into the surrounding epidermal cells.
The guard cells become flaccid and the stomata close.

Starch sugar interconversion theory: the sequence in the morning (opening) and in the evening (closing).

2025 Annual I, Set D, Q.64+2 = 6 marks

Describe the components of theory explaining the ascent of sap in plants. Also draw the diagram.

Solution

The upward movement of water and minerals from the roots to the leaves through the xylem, against gravity, is the ascent of sap. It is explained by the TACT theory: Transpiration pull, Adhesion, Cohesion and Tension together make one collective force.

Components of the TACT theory (4 marks)

Component	Meaning	Role
Transpiration pull	Pulling force produced by evaporation of water from the leaf	Open stomata let water vapour out; the dry air spaces pull water from the cells, and these pull it from the xylem, so the sap rises
Adhesion	Attraction between water molecules and the polar cellulose walls of the xylem	Water sticks to the walls of the narrow xylem tubes, helping it rise and holding it when transpiration stops
Cohesion	Attraction among water molecules by hydrogen bonds	Keeps the water as a continuous, unbroken column, as strong as a steel wire of the same diameter
Tension	The hydrogen bonds between water molecules, with the column kept taut by the pull from above	Keeps the string of water tense so it is pulled up as one unit

Mechanism: transpiration at the leaf pulls on the column, cohesion and adhesion stop it breaking, and fresh water enters the root hairs by osmosis. The energy comes from the sun, which drives evaporation.

Diagram (2 marks)

Exam focus

Describe the mechanisms of opening and closing of stomata (K+ influx/efflux and starch sugar interconversion), with a labelled figure. (long)
Asked in both 2026 papers: Set B Q.5 OR (K+ theory) and Set D Q.5 (starch sugar theory), 6 marks each. Learn both theories as morning and evening steps.
Go to answer
Explain the ascent of sap by the TACT theory and draw the diagram. (long)
2025 Set D Q.6 (6 marks, 4+2) and book long question 7. Name and define the four components and label the diagram.
Go to answer
Explain the mechanism of photoperiodism: dark period, phytochrome (Pr and Pfr) and florigen. (long)
2025 Set B Q.6 (6 marks) and book long question 23; also classify plants into short-day, long-day and day-neutral.
Go to answer
Compare apoplast and symplast pathways of water in the root. (short)
2025 Set B Q.2 (vii) (3 marks) and book short question 10; give three points including the Casparian strip.
Go to answer
State and explain the types of plant movement (phototropism, geotropism, thigmotropism, chemotropism) and their stimuli. (short)
2026 Set B Q.2 (v), 2026 Set D Q.2 (v), 2025 Set D MCQ 10 and 2026 Set D MCQ 7 (Venus flytrap touch response).
Go to answer
Compare the osmotic adjustments of hydrophytes, mesophytes and xerophytes (three points). (short)
2025 Set D Q.2 (ii) (3 marks) and book long question 11; Table 9.3 gives the points.
Go to answer
Identify xylem and phloem in a stem section and the substances they carry. (mcq)
2025 Set D MCQ 16 (labelled dicot stem section): phloem outside carries sucrose, xylem inside carries water.
Go to answer
Cork cambium and vascular cambium: tissues each adds, bark, wood and annual rings. (short)
2025 Set B MCQ 12 (bark is produced by cork cambium) and book questions on annual rings and secondary growth.
Go to answer
Carnivorous plants: examples and why they trap insects. (short)
2026 Set B MCQ 7 and Set D MCQ 7 both used carnivorous plants (pitcher plant, Venus flytrap); book short question 1.
Go to answer

9.1Nutrition in plants

9.1.1Types of plant nutrientsOften examined

Role of plant nutrients and effect of their deficiencies

Science, Technology and Society connections: symptoms of mineral deficiency

9.1.2Nutrition of carnivorous plants

9.2Gaseous exchange in plants

Gas exchange during the day and the night

9.2.1Role of palisade and spongy mesophyll in exchange of gasesOften examined

Types of mesophylls in dicot and monocot leaves

9.2.2Relationship of stomata and transpiration with exchange of gasesOften examined

Putting the three together

9.3Transport in plants

9.3.1Movement of water between plant cells and their environmentOften examined

Water potential (Ψw​)

Solute potential (Ψs​)

Pressure potential (Ψp​)

Book example: two adjacent vacuolated cells

9.3.2Uptake of water by roots and pathwaysOften examined

Apoplast pathway

Symplast pathway

Vacuolar pathway

9.3.3Structure and function of xylem and phloemOften examined

Components of xylem and their functions

Components of phloem and their functions

9.3.4Ascent of sapOften examined

TACT theory

1. Transpiration pull

2. Adhesion

3. Cohesion and tension

Mechanism of TACT force

9.3.5Opening and closing of stomataOften examined

Starch sugar interconversion theory

\ce{K+} ions influx/efflux theory

9.3.6Translocation of organic solutesOften examined

Pattern or direction of translocation

Composition of translocating fluid (phloem sap)

Mechanism of translocation: pressure flow (mass flow) theory

9.3.6STEAM activity: investigating plant vascular systems

Materials required

Procedure

Conclusion and extension

9.4Homeostasis in plants

9.4.1Osmotic adjustment (osmoregulation)Often examined

Movement of water in different osmotic conditions

Osmotic adjustments in plants of different environments

Hydrophytes

Mesophytes

Xerophytes

9.4.2Osmotic adjustments of plants (halophytes) in saline soils

9.4.3Thermoregulation in plantsOften examined

Adaptations in plants to cope with low temperature stress

Adaptations in plants to cope with high temperature stress

9.5Support in plants

9.5.1Role of turgor mechanism in supportOften examined

9.5.2Role of mechanical tissues in supportOften examined

Collenchyma

Sclerenchyma

9.5.3Arrangement of vascular bundles

9.6Growth and development in plants

9.6.1Meristematic tissues (meristems)Often examined

Apical meristems

Lateral meristems

9.6.2Types of plant growthOften examined

Primary growth

Secondary growth

9.6.3Role of lateral meristems in secondary growthOften examined

Role of vascular cambium

Formation of annual (growth) rings

Role of cork cambium (phellogen)

Wood and bark

9.6.4Growth correlationsOften examined

Apical dominance

9.7.1Plant growth regulatorsOften examined

Auxins

Gibberellins

Cytokinins

Abscisic acid

Ethylene or ethene

9.7.2Growth movements in plants (tropic movements)Often examined

Phototropism

Water potential ( $Ψ_{w}$ )

Solute potential ( $Ψ_{s}$ )

Pressure potential ( $Ψ_{p}$ )