Transpiration

1. Stomata and cuticle.

2. Potometer (Ganong's potometer).

3. During the day (in the presence of sunlight).

4. Lenticels.

5. Cuticle.

1. Leaf spines and reduced transpiration (protection).

2. Lenticels and lenticular transpiration.

3. Thick cuticle and reduced transpiration.

4. Stomata and stomatal transpiration (gaseous exchange).

1. Transpiration is the loss of water from the aerial parts.

2. Major type of transpiration is stomatal.

3. Transpiration rate is the highest during summer.

Excretion of minerals

Reason — Transpiration helps in cooling the leaves and creates a suction force that assists the uptake of water from the soil. Excretion of minerals is not a function of transpiration.

Age of the plant

Reason — The rate of transpiration is affected by external factors such as light, humidity, wind and temperature, and by internal features of the leaf. The age of the plant is not a factor that affects the rate of transpiration.

Abscisic acid

Reason — Abscisic acid (ABA) is a chemical substance used to reduce the rate of transpiration, hence it is an antitranspirant. Auxin, gibberellin and cytokinin are growth hormones.

Increased leaf surface area

Reason — Desert plants have small or fewer leaves to reduce water loss. Increasing leaf surface area would cause more water to evaporate, so they do not have this adaptation.

Guttation

Reason — Guttation is the loss of water in the form of liquid droplets from the margins of leaves through special pores called hydathodes.

more from the lower surface of a dicot leaf.

Reason — In a dicot leaf, the stomata are more numerous on the lower (abaxial) surface than on the upper surface, so more transpiration occurs from the lower surface.

Cobalt chloride paper

Reason — Cobalt chloride paper turns from blue to pink when exposed to moisture. Hence, it is used to demonstrate unequal transpiration in a dicot leaf.

Greater the thickness of cuticle, lesser is the transpiration.

Reason — The cuticle is a waxy layer that prevents evaporation of water from the leaf surface. The thicker the cuticle, the lesser is the loss of water by transpiration.

When blue cobalt chloride paper turns pink with moisture

Reason — Dry cobalt chloride paper is blue. When water vapour lost during transpiration reaches it, the paper absorbs the moisture and turns pink.

Both (A) and (R) are true but (R) is not the correct explanation of (A)

Explanation — Guttation occurs through hydathodes present at the margins of the leaf, and bleeding occurs through the cut or injured parts of a plant. Both statements are true, but bleeding does not explain why guttation occurs, so (R) is not the correct explanation of (A).

Old stem

Reason — Lenticels are special openings that develop on the older woody stem in place of stomata and allow lenticular transpiration.

A, C and D

Reason — Light intensity, wind velocity and temperature are all directly proportional to the rate of transpiration. An increase in carbon dioxide concentration causes stomata to close and thus decreases transpiration, so it is inversely related.

Phenyl Mercuric Acetate

Reason — PMA stands for Phenyl Mercuric Acetate, a chemical substance used to reduce the rate of transpiration.

NAA

Reason — PMA, ABA and aspirin are used to reduce the rate of transpiration and are antitranspirants. NAA (naphthalene acetic acid) is a growth-regulating auxin and is not an antitranspirant.

Lenticels

Reason — Lenticels are the openings on the bark of old woody stems. They have no mechanism to open and close, so they remain permanently open.

Creates a suction force for absorption of water from the soil

Reason — Transpiration develops a suction force (transpirational pull) which helps in the ascent of sap and in the absorption of water from the soil through the roots.

It is the loss of water in the form of water vapour from the aerial parts of the plant.

Reason — Transpiration is the loss of water in the form of water vapour (not droplets) from the aerial parts of the plant, mainly the leaves.

Around the stoma

Reason — Each stoma is bordered by two kidney-shaped guard cells which regulate the opening and closing of the stoma.

Reduces transpiration

Reason — The cuticle is a waxy layer secreted by the epidermis. It prevents evaporation of water from the leaf surface and thus reduces transpiration.

Guttation

Reason — Hydathodes are special marginal pores of the leaf through which excess water oozes out as droplets during guttation.

High temperature and low humidity

Reason — High temperature and low humidity both increase the rate of transpiration, so more water is taken up and the air bubble moves rapidly.

The paper turns pink faster around the plant in sunlight due to higher transpiration.

Reason — Sunlight (light and higher temperature) increases the rate of transpiration, so more water vapour is released. Hence the cobalt chloride paper turns pink faster around the plant kept in sunlight.

P- capillary; Q- water uptake

Reason — In a potometer, the air bubble is introduced into the graduated capillary tube. As transpiration proceeds, a suction force pulls water and the bubble moves, and this movement reflects the rate of water uptake.

(i) Guard cells.

(ii) Hydrilla.

(iii) Nerium.

(iv) Lenticels.

(v) Bleeding.

(vi) Potometer (Ganong's potometer).

(vii) Cuticle.

(i) True.

(ii) True.

(iii) False.
Corrected statement — Moist cobalt chloride paper turns pink in colour.

(iv) False.
Corrected statement — The wall of guard cells towards stomata is thick.

(v) True.

(vi) False.
Corrected statement — Potometer is an instrument used to measure the rate of transpiration.

Transpiration is the loss of water in the form of water vapour from the surface of the aerial parts (leaves and stem) of a plant.

Guttation is the loss of water in the form of liquid droplets from the margins of leaves through hydathodes, when the root pressure is high and transpiration is low.

Bleeding is the exudation (oozing out) of cell sap from the cut or injured parts of a plant due to root pressure.

Root pressure is the positive pressure that develops in the roots due to the active absorption of water, which helps to push water upward into the stem.

Differences between guttation and bleeding in plants are:

Differences between Transpiration and evaporation are:

Differences between Transpiration and guttation are:

Differences between Stomatal transpiration and lenticular transpiration are:

Differences between Stoma and Stroma are:

(i) Stomata — allow transpiration and exchange of gases.

(ii) Hydathodes — allow guttation (exudation of water droplets).

(iii) Sunken stomata — reduce the rate of transpiration.

(iv) Leaf spines — reduce the surface area and thus reduce transpiration (also give protection).

(v) Lenticels — allow lenticular transpiration and exchange of gases.

In hot weather the high temperature makes the water molecules evaporate faster, and in dry weather the low humidity keeps a steep water potential gradient between the leaf and the air. Both conditions increase the rate of transpiration.

The water lost by transpiration from the leaves sets up a suction force (transpirational pull) that draws water up through the xylem from the roots. Thus almost the same amount of water that is transpired is absorbed by the roots.

Leaves are modified into spines to reduce the surface area exposed to the atmosphere. This greatly reduces the loss of water by transpiration in dry conditions.

Antitranspirants reduce the rate of transpiration. With less water lost, the cut flowers remain turgid and stay fresh for a longer time.

At midday the rate of transpiration exceeds the rate of absorption of water by the roots. As a result the cells lose their turgidity and the plant wilts, even when the soil has enough water. It recovers in the evening when transpiration slows down.

On a bright sunny day the leaves roll up to reduce the exposed surface area of the leaf. This reduces the loss of water by transpiration.

Pieces of dried (a) cobalt chloride paper are fixed to the two surfaces of a leaf. The leaf should remain attached to the plant. The plant is exposed to bright sunlight for a few hours. The colour of the paper on the lower surface of the leaf turns (b) pink faster than the one on the upper surface. This proves that more (c) transpiration occurs from the (d) lower surface of the leaf as it has a greater number of (e) stomata.

(a) Absorption of water

(b) Transportation of water

(c) Transpiration

The three types of transpiration are:

1. Stomatal transpiration (through the stomata of leaves).
2. Cuticular transpiration (through the cuticle of the leaf).
3. Lenticular transpiration (through the lenticels of old stems).

Stomatal transpiration accounts for the maximum transpiration in the plant (about 90% of the total water loss).

According to the potassium ion exchange theory, the opening of stomata during the day takes place as follows:

1. During the day, due to rapid photosynthesis, the concentration of CO₂ in the guard cells decreases, so their pH increases.
2. At this higher pH, the starch stored in the guard cells is converted into organic acid.
3. The organic acid dissociates into H⁺ ions and malate ions in the guard cells.
4. The H⁺ ions are actively transported (using ATP) into the neighbouring subsidiary cells in exchange for K⁺ ions.
5. The uptake of K⁺ ions is balanced by the uptake of Cl^- ions.
6. The increased concentration of K⁺, malate and Cl^- ions raises the osmotic pressure of the guard cells, so water enters them from the subsidiary cells by endosmosis.
7. The guard cells become turgid, their outer thin walls bulge out and the inner thick walls are pulled apart, so the stoma opens and transpiration takes place.

An antitranspirant is a chemical substance that is applied to a plant to reduce the rate of transpiration. Some antitranspirants are phenyl mercuric acetate (PMA), abscisic acid (ABA) and silicon emulsions.

Hydathodes are located at the margins (tips) of the leaves, at the ends of the veins.

Two adaptations in plants to reduce transpiration are:

1. Presence of sunken stomata covered with hair.
2. Presence of a thick, waxy cuticle over the epidermis.

Transpiration occurs through stomata, lenticels and cuticle, while guttation occurs through hydathodes.

Transpiration from the leaves creates a suction force called transpirational pull in the xylem. This pull draws water upwards through the xylem and thereby helps in the absorption and uptake of water from the soil by the roots.

Wilting is the drooping and withering of the leaves and soft parts of a plant due to loss of turgidity when the water lost exceeds the water absorbed.

Some plants show wilting at noon even in well watered soil because at noon the rate of transpiration is very high and exceeds the rate of absorption of water by the roots. The cells therefore lose their turgidity and the leaves wilt. In the evening, when transpiration slows down, the water is made up by absorption and the leaves regain their turgidity.

(i) High temperature — the rate of transpiration increases, because a higher temperature makes the water molecules evaporate faster.

(ii) High humidity — the rate of transpiration decreases, because the air is already moist and the water potential gradient between the leaf and the air is reduced.

Four disadvantages of transpiration to the plant are:

1. Wilting — when water loss exceeds absorption, the cells become flaccid and the plant wilts.
2. Stunted growth — excessive transpiration results in stunted growth of the plant.
3. Abscission — shortage of water causes premature fall of leaves and delayed flowering.
4. Energy expenditure — most of the absorbed water is lost in transpiration, so a lot of the energy spent by the plant is wasted.

(i) The structure shown is a hydathode (water stoma).

(ii) The parts labelled 1 to 4 are:

1. Guard cell (water pore)
2. Intercellular space
3. Tracheids
4. Chlorenchyma

(iii) Part labelled 1 is the guard cell bordering the water pore of the hydathode. This pore stays open and allows the excess water, along with dissolved mineral salts, to ooze out as droplets during guttation.

(i) Guttation

(ii) Guttation is the process by which water droplets are exuded from the edges or tips of leaves through special pores called hydathodes, usually during the early morning or at night.

(iii) Guttation occurs when root pressure is high and transpiration is low, especially at night, pushing out extra water through special pores called hydathodes.

(i) A → Guard cell, B → Stoma (stomatal pore).

(ii) The biological term for the above structure is stomata (stoma).

(iii) The guard cell (A) regulates the opening and closing of the stoma.

(iv) Two structural features of A that help in this function are:

1. The guard cells are kidney (bean) shaped.
2. Their inner wall (bordering the stoma) is thick and inelastic, while the outer wall is thin and elastic, so a change in turgidity makes them bend and open or close the pore.

(v) This structure is found mainly on the lower (abaxial) surface of a dicot leaf, and on both surfaces of a monocot leaf.

(vi) The structure helps the plant by allowing transpiration (loss of water vapour) and the exchange of gases — intake of CO₂ for photosynthesis and O₂ for respiration.

(vii) As seen in the diagram, five epidermal (subsidiary) cells surround this structure.

(i) Transpiration.

(ii) Transpiration is the process in which water is lost in the form of vapour from the aerial parts of the plant.

(iii) The weight of test tube A is greater before the experiment than after the experiment. This is because the leafy shoot absorbs water from test tube A and loses it as water vapour by transpiration.
The weight of test tube B remains the same before and after the experiment because there is no plant to transpire, and the oil layer prevents direct evaporation of water.

(iv) It is a control experiment where the purpose of using test tube B is to compare the level of water in both test tubes.

(i) To prove that more transpiration occurs from the lower surface of a dicot leaf.

Reason — In most dicot leaves, the lower surface has more stomata than the upper surface. Therefore, more water vapour is lost from the lower surface.
The upper surface has fewer stomata, so transpiration is less. Stomata are unevenly distributed in dicot leaves. Transpiration occurs continuously through the stomata and cuticle.

(ii) Blue.

Reason — Dry cobalt chloride paper is blue and turns pink when it absorbs moisture.
Cobalt chloride paper becomes pink only after absorbing water. brown is not the colour of cobalt chloride paper. Cobalt chloride paper is blue when dry, not white.

(iii) Upper surface – less pink, Lower surface – more pink.

Reason — The lower surface has more stomata, so it loses more water vapour. Hence, the cobalt chloride paper on the lower surface turns pink more rapidly.
The upper surface has fewer stomata and therefore loses less water. Cobalt chloride paper changes from blue to pink, not white.

(iv) Narrow leaves, Fewer stomata.

Reason — Narrow leaves reduce the exposed surface area, and fewer stomata reduce the number of openings for water loss, thereby decreasing transpiration.
Although narrow leaves reduce transpiration, a thin cuticle increases water loss. Broad leaf has a larger surface area, increasing transpiration. Sunken stomata reduce transpiration, but a thin cuticle increases water loss.

(v) High humidity in the air and low temperature.

Reason — High humidity reduces the diffusion gradient for water vapour, and low temperature slows evaporation. Both conditions decrease transpiration.
Low humidity increases transpiration by increasing the water vapour gradient. Bright sunlight and high temperature increase the rate of transpiration. Low light reduces transpiration, strong wind removes moist air around the leaf and increases transpiration.

(i) Ganong's Potometer

(ii) Transpiration

(iii) It is the loss of water as water vapour from the aerial parts of the plant.

(iv) Two limitations of using Ganong's Potometer are:

1. Introducing the air bubble is not very easy.
2. The twig may not remain fully alive for a long time.

(v) The movement of the air bubble indicates the volume of water taken up by the cut shoot in a given time. This water uptake is used to estimate the rate of transpiration.

(vi) Stomata, Lenticels, Cuticle.

(i) Cuticle

Reason — The cuticle is a thin, waxy layer covering the epidermis of leaves. It reduces water loss by transpiration and protects the leaf.
Spongy parenchyma is a tissue inside the leaf that helps in gaseous exchange and photosynthesis. Cambium is a meristematic tissue responsible for secondary growth in stems and roots. hPloem is a vascular tissue that transports food.

(ii) Hydrilla

Reason — Hydrilla is a submerged aquatic plant. Since it remains completely under water, gaseous exchange occurs directly through its surface, and stomata are absent.
Nerium is a xerophyte with sunken stomata to reduce transpiration. Neem is a terrestrial plant with stomata on its leaves. Lotus is a floating aquatic plant with stomata on the upper surface of its leaves.

(iii) Nerium

Reason — Nerium is a xerophytic plant. Its sunken stomata reduce water loss by transpiration.
Neem has normal stomata and is not a xerophyte with sunken stomata. Banyan is a mesophytic plant and does not possess sunken stomata. Croton has ordinary stomata and is not known for having sunken stomata.

(iv) Lenticels

Reason — Lenticels are permanently open pores present on old woody stems. They allow a small amount of transpiration and gaseous exchange.
Stomata are mainly present on leaves and young green stems. Hydathodes occur at the tips and margins of leaves and are involved in guttation. Root hairs absorb water and minerals from the soil.

(v) Guttation

Reason — Guttation is the loss of liquid water droplets from the tips or margins of leaves through hydathodes, usually when root pressure is high and transpiration is low.
Bleeding is the oozing of sap from a cut or injured part of a plant. Transpiration is the loss of water vapour, mainly through stomata, not liquid water droplets. Evaporation is the conversion of liquid water into water vapour from any exposed surface.

(i) Transpiration process is depicted in the diagram. Oil was added to the water in order to prevent the loss of water by evaporation.

(ii) When the setup is placed in bright sunlight for four hours, its final weight will be less than its initial weight because water is lost from the aerial parts of the plant by transpiration.

(iii) The level of water is affected as follows:

1. Humid conditions — Slight or no drop in water level.
2. Windy conditions — Drop in level of water in the jar as rate of transpiration increases.

(iv) Three adaptations found in plants to overcome transpiration are:

1. Sunken stomata
2. Narrow leaves
3. Reduced exposed surface by rolling or folding of leaves.

(v) Loss of water in the form of droplets from the margins (hydathodes) of leaves is called guttation.

(i) The process being studied is Transpiration. Transpiration is the process during which water is lost in the form of water vapour through aerial parts of the plant.

(ii) The pot was enclosed in a rubber sheet to prevent evaporation of water from the soil.

(iii) Two external factors which can accelerate Transpiration are:

1. Bright sunlight
2. Low humidity

(iv) Two adaptations in plants to reduce the above process are:

1. Sunken stomata
2. Narrow leaves

(v) Stomatal apparatus is shown in the labelled diagram below: