Transpiration | ICSE Class 10 Biology Notes

This note covers transpiration in flowering plants: what transpiration is, its three kinds, the structure and distribution of stomata, how stomata open and close according to the potassium ion exchange theory, the transpiration pull with cohesion and adhesion, the importance of transpiration, the factors that change its rate, the experiments that demonstrate it, including Ganong's potometer, the adaptations that reduce water loss, and guttation and bleeding.

What is transpiration?

A plant takes up water from the soil through its roots, but most of that water does not stay inside the plant. It escapes from the leaves and other aerial parts as water vapour.

Definition: Transpiration is the loss of water in the form of vapour from the aerial parts of a plant.

Most of this water escapes through stomata, the tiny pores present on the surface of leaves. The same pores let in the carbon dioxide needed for photosynthesis, so large amounts of water can be lost through them while the leaf takes in carbon dioxide.

Provided the plant has an adequate supply of water, the water lost through the stomata is replaced by water from the xylem vessels in the leaf. Evaporation of water molecules from the cells of the leaf creates a suction that pulls water up from the xylem of the roots.

Only a small part of the water a plant absorbs is used in photosynthesis and its other life processes. Most of it passes through the plant and is transpired.

A simple activity

1. Take two small pots of about the same size with the same amount of soil. Grow a plant in one, and place a stick of the same height as the plant in the other.
2. Cover the soil in both pots with a plastic sheet so that moisture cannot escape from the soil by evaporation.
3. Cover the plant and the stick with plastic sheets.
4. Place both sets in bright sunlight for half an hour and compare the insides of the two sheets.

Drops of water collect on the inside of the sheet over the plant but not on the sheet over the stick. Since the soil was covered, this water must have come out of the plant.

A tree is drawn with its branching roots spread out in the soil. Arrows run along the roots towards the base of the trunk and up through the trunk into the branches. More arrows point outwards from the leafy crown, and one of them is labelled "Water vapour".

What are the kinds of transpiration?

Transpiration is grouped into three kinds according to the part of the plant through which the water vapour escapes.

Stomatal transpiration

Stomatal transpiration is the loss of water vapour through the stomata of leaves and young green stems. It accounts for most of the water a plant loses.

Because the guard cells can open and close each pore, this is the kind of transpiration a plant can regulate. It takes place mainly by day, when the stomata are open.

Cuticular transpiration

The epidermis of leaves and young stems is covered with a waxy layer of cutin called the cuticle. Some water evaporates directly from the epidermal cells and passes through the cuticle. This is cuticular transpiration.

The thicker the cuticle, the less water escapes this way. In some plants of very dry habitats, the epidermis is covered by a thick cuticle that reduces water loss. The plant cannot open or close the cuticle, so this loss continues by day and by night.

Lenticular transpiration

Woody stems have openings in the bark called lenticels, which allow the living cells inside to exchange gases with the air. A very small amount of water vapour also escapes through them. This is lenticular transpiration.

What are stomata, and where are they found on a leaf?

Stomata are structures present in the epidermis of leaves. They regulate the process of transpiration and gaseous exchange.

Structure of a stoma

• Each stoma is made of two bean-shaped guard cells that enclose a stomatal pore. In grasses, the guard cells are dumb-bell shaped.
• The outer walls of the guard cells, away from the pore, are thin. The inner walls, towards the pore, are highly thickened.
• The guard cells possess chloroplasts and regulate the opening and closing of the stoma.
• Sometimes, a few epidermal cells near the guard cells become specialised in shape and size. These are the subsidiary cells.
• The stomatal aperture, the guard cells and the surrounding subsidiary cells together form the stomatal apparatus.

Part (a) shows a stoma with two bean-shaped guard cells, each dotted with chloroplasts, curving around a lens-shaped stomatal pore. Around them lie the subsidiary cells and then the ordinary epidermal cells, all labelled.

Part (b) shows a stoma of the grass type, with two narrow, dumb-bell shaped guard cells lying side by side between two subsidiary cells.

Distribution of stomata

In a dorsiventral (dicot) leaf, the lower or abaxial epidermis generally bears more stomata than the upper or adaxial epidermis, which may even lack stomata. In an isobilateral (monocot) leaf, stomata are present on both surfaces.

The section shows the adaxial epidermis at the top and tall, closely packed palisade mesophyll cells below it. Beneath these are loosely packed spongy mesophyll cells with air cavities between them.

A vascular bundle with xylem and phloem, ringed by a bundle sheath, sits in the mesophyll. In the abaxial epidermis at the bottom, a stoma opens into a sub-stomatal cavity.

The path of water vapour out of a leaf

1. Water reaches the leaf through the xylem of the veins and passes into the mesophyll cells.
2. Water evaporates from the moist walls of the mesophyll cells into the air spaces between them.
3. The water vapour collects in the air cavities and the sub-stomatal cavity below each stoma.
4. When the stoma is open, the vapour diffuses out through the pore into the drier air outside.

How do stomata open and close?

The opening and closing of the pore is a function of the guard cells. The guard cells swell when water flows into them, and the stomatal pore opens. The pore closes when the guard cells shrink.

So a stoma opens when its guard cells are turgid and closes when they are flaccid. The potassium ion exchange theory explains what makes water move into or out of the guard cells.

Why a turgid guard cell opens the pore

The thin outer wall of each guard cell stretches more easily than the thick inner wall next to the pore. When a guard cell fills with water, its thin outer wall bulges outwards and the cell curves. The thick inner walls of the two guard cells are pulled apart, and the pore opens.

Opening in light

1. In light, energy in the form of ATP becomes available in the guard cells, which contain chloroplasts and can photosynthesise.
2. Using this energy, hydrogen ions (H⁺) are pumped out of the guard cells, and potassium ions (K⁺) move in from the neighbouring subsidiary and epidermal cells. This exchange gives the theory its name.
3. Negative ions such as chloride also enter, and starch in the guard cells is changed into malate. These balance the positive charge of the K⁺ ions.
4. The concentration of solutes in the cell sap of the guard cells rises above that of the neighbouring cells.
5. Water enters the guard cells from the neighbouring cells by endosmosis, and the guard cells become turgid.
6. The turgid guard cells curve outwards and the stomatal pore opens.

Closing in darkness or water stress

1. In darkness, K⁺ ions move out of the guard cells into the neighbouring cells, together with the negative ions.
2. The solute concentration of the guard cells falls below that of the neighbouring cells.
3. Water leaves the guard cells by exosmosis, and they become flaccid.
4. The guard cells straighten, and the stomatal pore closes.

Water shortage also closes stomata. Water stress causes the stomata to close, which reduces the loss of water but also cuts down the supply of carbon dioxide. The plant hormone abscisic acid (ABA) stimulates the closure of stomata and increases the tolerance of plants to stress.

In (a), two plump, bean-shaped guard cells bow away from each other, leaving a wide, lens-shaped stomatal pore between them. The guard cells, the stomatal pore and the chloroplasts inside the guard cells are labelled.

In (b), the guard cells are narrower and straighter and lie pressed together, so the pore is closed. Epidermal cells surround the stoma in both drawings.

How does transpiration help to pull water up the plant?

In the xylem, the vessels and tracheids of the roots, stems and leaves are joined to form a continuous system of water-conducting channels that reaches all parts of the plant. Two forces move water up these channels.

Root pressure

Root cells in contact with the soil actively take up ions. This creates a difference in ion concentration between the root and the soil, so water moves into the root. There is a steady movement of water into the root xylem, and a column of water is pushed upwards. This push is called root pressure.

Root pressure by itself is unlikely to be enough to move water to the heights seen in tall plants. Its effect is more important at night.

Transpiration pull

During the day, when the stomata are open, the transpiration pull becomes the major driving force for the movement of water in the xylem.

1. Water evaporates from the mesophyll cells of the leaf and diffuses out through the stomata.
2. The mesophyll cells, having lost water, draw water from the neighbouring cells and from the xylem of the leaf veins.
3. This creates a suction, or tension, in the xylem that pulls on the whole column of water in the stem.
4. The column is pulled upwards without breaking, because water molecules cling to one another and to the walls of the xylem.
5. Water from the soil enters the root to replace the water drawn up.

Cohesion and adhesion

• Cohesion is the force of attraction between water molecules. It keeps the column of water in the narrow xylem channels continuous.
• Adhesion is the force of attraction between water molecules and the walls of the xylem vessels and tracheids. It helps the column hold on to the walls against gravity.

Together, cohesion, adhesion and transpiration pull lift an unbroken column of water from the roots to the leaves, carrying minerals dissolved in it.

Why is transpiration important to plants?

Transpiration uses up a great deal of water, but it also does useful work for the plant.

Advantages of transpiration

• Absorption and ascent of water: the suction created by transpiration helps the absorption and upward movement of water from the roots to the leaves.
• Movement of minerals: minerals dissolved in the water are carried up with it from the roots to the leaves and other parts.
• Temperature regulation: water takes up heat as it evaporates from the leaf, so transpiration helps to keep leaves from overheating in sunlight.
• Removal of excess water: plants can get rid of excess water by transpiration, and it also helps in the elimination of wastes.
• Moisture for the air: the moisture in the atmosphere comes from water bodies by evaporation and from plants by transpiration, so transpiration is part of the water cycle.

The effect is large enough to shape local climate. Rural areas and forests, with more vegetation, stay cooler through shade and plant transpiration than cities do. Clearing forests reduces transpiration, which can lead to a decline in local rainfall.

Transpiration as a necessary evil

Transpiration is often called a necessary evil. It is necessary because the stomata must be open to take in carbon dioxide for photosynthesis, and water vapour is bound to escape through open stomata.

It is an evil because, when water is lost faster than the roots can absorb it, the cells lose turgor and the plant wilts. Wilting reduces the surface area of the leaves and their metabolic activity, and stomata closing under water stress reduce the carbon dioxide available for photosynthesis.

What factors affect the rate of transpiration?

The rate of transpiration depends on conditions in the environment (external factors) and on features of the plant itself (internal factors).

External factors

Internal factors

• Number of stomata: the more stomata a leaf has, and the more of them are open, the higher the rate.
• Leaf area: a larger leaf surface loses more water than a smaller one.
• Cuticle: a thick cuticle reduces the loss of water from the epidermis.
• Hairs: epidermal hairs, or trichomes, help in preventing water loss due to transpiration.

How can transpiration be demonstrated with a bell jar?

The bell jar experiment shows that the aerial parts of a plant give out water vapour, which collects where it can be seen and tested.

1. Take a healthy, well-watered potted plant.
2. Cover the pot and the soil with a polythene or rubber sheet tied around the base of the stem, so that no water can evaporate from the soil or the pot.
3. Place the plant on a glass plate and cover it with a dry bell jar.
4. Set up a control in the same way, with a covered pot that has no plant in it (or a stick of the same height in place of the plant).
5. Keep both sets in sunlight for a few hours.

Observation and conclusion

Droplets of water collect on the inner surface of the bell jar over the plant. No droplets form in the control. The liquid can be shown to be water because it turns white anhydrous copper sulphate blue, or turns dry blue cobalt chloride paper pink.

The soil was covered, so the water must have come out of the leaves and stem as vapour and condensed on the cool glass. This shows that plants lose water by transpiration.

Precaution: the pot and soil must be covered completely. Otherwise water evaporating from the soil or the pot would also collect on the glass.

Draw a potted plant standing on a glass plate under a bell jar, with the pot and soil wrapped in a polythene sheet tied round the stem. Show droplets on the inner wall of the jar. Beside it, draw the control: a similar covered pot with no plant, under a bell jar with no droplets. Label the bell jar, polythene sheet, glass plate and water droplets.

How does loss in weight show transpiration?

A transpiring plant loses water, and so loses weight. Two simple set-ups measure this loss with a balance.

Loss in weight of a potted plant

1. Water a potted plant well and let the extra water drain away.
2. Cover the pot and the soil with a polythene sheet tied around the base of the stem, so that water can escape only from the leaves and stem.
3. Weigh the whole set-up and note the weight.
4. Keep it in sunlight for a few hours and weigh it again.

The second weight is less than the first. The loss is due to water given out by the plant. A control, a covered pot of watered soil with no plant, shows no noticeable loss.

Loss in weight of a leafy shoot in a test tube

1. Fill a test tube with water and place a freshly cut leafy shoot in it.
2. Pour a thin layer of oil on the surface of the water to stop water evaporating directly from the tube.
3. Mark the water level and weigh the test tube with the shoot.
4. Leave the set-up in sunlight for a few hours, then note the water level and weigh it again.

The water level falls and the set-up weighs less. The oil layer blocks evaporation from the water surface, so the water that is lost must have been absorbed by the shoot and transpired from its leaves. A similar test tube with water and oil but no shoot serves as the control.

How does cobalt chloride paper show unequal transpiration in a dorsiventral leaf?

Cobalt chloride paper is filter paper soaked in cobalt chloride solution and dried. When dry, it is blue. When it absorbs moisture, it turns pink. How quickly it turns pink shows how much water vapour is reaching it.

Procedure

1. Choose a healthy dorsiventral leaf on a potted plant that has been kept in light.
2. Dry two pieces of cobalt chloride paper until they are blue, and handle them only with forceps.
3. Place one piece on the upper surface of the leaf and the other directly below it on the lower surface.
4. Cover each piece with a glass slide and hold the slides in place with a clip, so that the paper is not exposed to the outside air.
5. Note the time each piece takes to turn pink.

Observation and explanation

The paper on the lower surface turns pink sooner than the paper on the upper surface. More water vapour is therefore given out from the lower surface.

This happens because in a dorsiventral leaf the abaxial (lower) epidermis generally bears more stomata than the adaxial (upper) epidermis, which may have very few or none.

If the experiment is repeated with an isobilateral leaf, which has stomata on both surfaces, both papers change colour at about the same rate.

Draw a leaf still attached to its plant, with a square of cobalt chloride paper on the upper surface and another on the lower surface. Show a glass slide over each paper, held by a clip. Label the upper and lower surfaces, the papers, the slides and the clip, and note that the lower paper turns pink first.

How is Ganong's potometer used, and what are its limitations?

A potometer measures the rate at which a leafy shoot takes up water. Since most of the water a shoot absorbs is lost by transpiration, the rate of uptake is taken as roughly equal to the rate of transpiration. Ganong's potometer is a commonly used form.

A leafy shoot is fitted into the top of a vertical arm on the left. Beside it, a reservoir with a stopcock joins the apparatus from above. A long, horizontal graduated capillary tube with an air bubble in it leads away to the right.

The free end of the capillary bends down into a beaker of coloured water. The apparatus rests on a stand. The leaf shoot, reservoir, air bubble, graduated capillary tube and coloured water are labelled.

Reference: CISCE ICSE Class X practice question 56

Procedure

1. Fill the whole apparatus with water so that no air is trapped inside.
2. Cut a leafy shoot under water and fit it into the vertical arm through a cork. Seal the joint with vaseline so that it is airtight.
3. Dip the free end of the capillary tube into a beaker of coloured water.
4. Lift the end of the capillary out of the water for a moment and dip it back, so that a single air bubble enters the tube.
5. As the shoot absorbs water, the bubble moves along the capillary towards the shoot. Note the distance it moves on the scale in a fixed time.
6. Open the stopcock of the reservoir to push the bubble back to the starting point, and repeat the reading.

The faster the bubble moves, the faster the shoot is taking up water. Readings taken in sunlight and shade, or in still air and in front of a fan, compare the effect of each factor.

Limitations

• It measures the uptake of water, not the actual loss. A small part of the absorbed water is used by the shoot in photosynthesis and in keeping its cells turgid.
• It is difficult to introduce a single air bubble of the right size.
• A cut shoot does not stay fresh for long, and it behaves differently from a shoot on a whole plant with its roots.
• The apparatus must be completely airtight; any leak gives a wrong reading.
• Changes in temperature can make the air bubble expand or contract and so disturb the reading.

How are plants adapted to reduce transpiration?

Plants in dry places risk losing water faster than they can absorb it. Many have features of the leaf and stem that cut down transpiration.

Bulliform cells and leaf rolling

In grasses, some upper epidermal cells along the veins are large, empty and colourless. These are bulliform cells. When they are turgid, the leaf surface is exposed. When they are flaccid due to water stress, they make the leaves curl inwards to minimise water loss.

Physiological adaptations

Under water stress, stomata close, and abscisic acid stimulates this closure. Some desert plants take up carbon dioxide at night, so their stomata need not be open in the heat of the day.

What are guttation and bleeding?

Water can also leave a plant in liquid form. Guttation and bleeding are two such losses, and both are linked to root pressure.

Guttation

Definition: Guttation is the loss of water in liquid form, as drops, from the tips and margins of leaves.

Guttation takes place through special openings near the vein endings called hydathodes, or water pores. Hydathodes do not open and close like stomata.

It happens at night and early in the morning, when the soil is moist, the air is humid and transpiration is low. Root pressure keeps pushing water into the leaves, and the excess water collects as droplets around the hydathodes. It is seen on the tips of grass blades and on the leaves of many herbaceous plants, such as tomato and Colocasia.

Guttation water is not pure. It carries dissolved salts, which may be left behind as a crust when the droplets dry.

Bleeding

Bleeding is the oozing of sap from a cut or injured part of a plant. It is usually caused by root pressure.

If a small, soft-stemmed plant is cut near the base of the stem with a sharp blade early in the morning, when there is plenty of moisture in the air, drops of sap soon ooze out of the cut stem. This shows positive root pressure.

Glossary

• Transpiration — the loss of water in the form of vapour from the aerial parts of a plant, mostly through the stomata of the leaves.
• Stoma — a structure in the epidermis of a leaf, made of two guard cells that enclose a stomatal pore, through which gases are exchanged and water vapour escapes.
• Guard cells — the two bean-shaped cells, dumb-bell shaped in grasses, that enclose a stomatal pore, contain chloroplasts and regulate its opening and closing.
• Cuticle — a waxy layer of cutin over the epidermis that reduces the loss of water; it is absent in roots.
• Transpiration pull — the suction created by the evaporation of water from leaf cells, which pulls a continuous column of water up the xylem.
• Cohesion — the force of attraction between water molecules, which keeps the column of water in the xylem unbroken.
• Adhesion — the force of attraction between water molecules and the walls of the xylem vessels and tracheids.
• Root pressure — the push on the water column produced as root cells take up ions and water follows them into the root xylem.
• Potometer — an apparatus, such as Ganong's potometer, that measures the rate of water uptake by a leafy shoot as a measure of transpiration.
• Guttation — the loss of water in liquid form, as drops, from the tips and margins of leaves through hydathodes.
• Hydathodes — water pores near the vein endings at the tips and margins of leaves, through which guttation takes place.
• Bleeding — the oozing of sap from a cut or injured part of a plant, usually caused by root pressure.

Common errors and misconceptions

• Misconception: Transpiration is just evaporation. Correct: Evaporation is a physical process from any wet surface. Transpiration is the loss of water vapour from a living plant, regulated by the opening and closing of stomata.
• Misconception: A potometer measures transpiration directly. Correct: It measures the rate of water uptake by the cut shoot. A small part of the absorbed water is used by the shoot, so uptake is only close to the rate of transpiration.
• Misconception: The stomatal pore opens when the guard cells lose water. Correct: The pore opens when the guard cells take in water and become turgid, and closes when they lose water and become flaccid.
• Misconception: Both surfaces of every leaf transpire equally. Correct: In a dorsiventral leaf the lower surface generally bears more stomata and loses more water. An isobilateral leaf has stomata on both surfaces.
• Misconception: Guttation is a kind of transpiration. Correct: Guttation is the loss of liquid water through hydathodes when transpiration is low. Transpiration is the loss of water vapour.
• Misconception: Root pressure alone lifts water to the top of tall trees. Correct: Root pressure is unlikely to be enough for such heights. During the day, transpiration pull is the major force moving water up the xylem.

Exam-style questions with model answers

Q1. Define transpiration. Name its three kinds. [2 marks]

1. Transpiration is the loss of water in the form of vapour from the aerial parts of a plant.
2. Its three kinds are stomatal transpiration, through the stomata; cuticular transpiration, through the cuticle; and lenticular transpiration, through the lenticels in the bark of woody stems.

Q2. How is the rate of transpiration affected by (a) high temperature, (b) low humidity and (c) low intensity of sunlight? Give a reason for each. [3 marks]

1. (a) High temperature increases the rate, because water evaporates faster from the leaf cells and warm air can hold more water vapour.
2. (b) Low humidity increases the rate, because dry outside air makes the difference in water vapour between the leaf and the air larger, so vapour diffuses out faster.
3. (c) Low intensity of sunlight decreases the rate, because stomata open in light, so in dim light fewer are open and less vapour escapes.

Q3. An apparatus has a leafy shoot fitted at one end, a reservoir with a stopcock, a horizontal graduated capillary tube with an air bubble, and its free end dipping into coloured water. (a) Identify the apparatus. (b) What does it measure? (c) Give one limitation. [3 marks]

1. (a) It is Ganong's potometer.
2. (b) It measures the rate of water uptake by the cut leafy shoot, shown by how fast the air bubble moves along the graduated tube. This is taken as roughly equal to the rate of transpiration.
3. (c) It is difficult to introduce a single air bubble. Also, it measures uptake rather than actual loss, because a small part of the absorbed water is used by the shoot.

Q4. Describe an experiment using cobalt chloride paper to show that the two surfaces of a dorsiventral leaf transpire at unequal rates. [4 marks]

1. Dry two pieces of cobalt chloride paper until they are blue, handling them with forceps.
2. Place one piece on the upper surface and one on the lower surface of a dorsiventral leaf of a potted plant kept in light.
3. Cover each piece with a glass slide and clip the slides in place so that the papers are not exposed to the outside air.
4. Observation: the paper on the lower surface turns pink sooner than the paper on the upper surface.
5. Conclusion: more water vapour is lost from the lower surface, because the lower (abaxial) epidermis of a dorsiventral leaf generally bears more stomata than the upper (adaxial) epidermis.

Q5. Explain the opening and closing of stomata on the basis of the potassium ion exchange theory. [5 marks]

1. A stoma opens when its guard cells become turgid and closes when they become flaccid. The inner walls of the guard cells, next to the pore, are thick, and the outer walls are thin.
2. In light, energy in the form of ATP becomes available in the guard cells. Using this energy, H⁺ ions are pumped out of the guard cells and K⁺ ions enter from the neighbouring subsidiary and epidermal cells.
3. Chloride ions enter and starch is changed into malate, balancing the charge. The solute concentration of the guard cells rises.
4. Water enters the guard cells from the neighbouring cells by endosmosis, and the guard cells become turgid.
5. Their thin outer walls bulge outwards, the thick inner walls are pulled apart, and the pore opens.
6. In darkness, K⁺ ions and the negative ions move out of the guard cells into the neighbouring cells. Water leaves by exosmosis, the guard cells become flaccid and straighten, and the pore closes.

Q6. Describe the bell jar experiment to demonstrate transpiration. State the control, one precaution and a test for the liquid collected. [5 marks]

1. Take a well-watered potted plant and cover the pot and soil with a polythene sheet tied around the base of the stem.
2. Place the plant on a glass plate, cover it with a dry bell jar and keep it in sunlight for a few hours.
3. Control: set up an identical covered pot with no plant, or with a stick in place of the plant, under a second bell jar.
4. Observation: droplets of water collect on the inner surface of the bell jar over the plant, but not in the control.
5. Test: the liquid turns white anhydrous copper sulphate blue, or dry blue cobalt chloride paper pink, showing that it is water.
6. Precaution: the pot and soil must be completely covered, so that no water evaporates from them.

Q7. Give four differences between transpiration and guttation. [4 marks]

1. In transpiration water is lost as vapour; in guttation it is lost as liquid drops.
2. Transpiration takes place through stomata, the cuticle and lenticels; guttation takes place through hydathodes at the tips and margins of leaves.
3. Transpiration takes place mainly by day; guttation takes place at night and early in the morning, when transpiration is low.
4. Transpiration is regulated by the opening and closing of stomata; guttation is not regulated, as hydathodes do not open and close.
5. Transpired water is pure vapour; guttation water contains dissolved salts.

Q8. "Transpiration is a necessary evil." Explain this statement, giving the advantages and disadvantages of transpiration. [5 marks]

1. Transpiration is necessary because the stomata must be open to take in carbon dioxide for photosynthesis, and water vapour escapes whenever they are open.
2. Advantage: it creates the transpiration pull, which helps the absorption and upward movement of water, with dissolved minerals, from the roots to the leaves.
3. Advantage: evaporation of water cools the leaves and helps to regulate their temperature.
4. Advantage: it removes excess water and adds moisture to the air as part of the water cycle.
5. Disadvantage: when water is lost faster than the roots absorb it, the cells lose turgor and the plant wilts, which reduces leaf surface area and metabolic activity.
6. Disadvantage: under water stress the stomata close, which reduces the carbon dioxide available for photosynthesis.

Key takeaways

• Transpiration is the loss of water in the form of vapour from the aerial parts of a plant, mostly through the stomata of the leaves.
• In light, K⁺ ions enter the guard cells in exchange for H⁺ ions, water follows by endosmosis and the stoma opens; in darkness the K⁺ ions leave and the stoma closes.
• Transpiration pull, helped by cohesion and adhesion, is the major force lifting water up the xylem by day, while root pressure matters more at night.
• Transpiration moves water and minerals upwards, cools the leaves, removes excess water and adds moisture to the air, but excess loss causes wilting.
• High temperature, low humidity, bright light, wind and low atmospheric pressure increase the rate of transpiration, while a shortage of soil water decreases it.
• Thick cuticles, hairs, sunken stomata, rolled leaves, spines and leaf fall reduce transpiration in plants of dry places.
• Guttation is the loss of liquid water through hydathodes, and bleeding is the oozing of sap from cut parts; both are linked to root pressure.

Test yourself

Through which openings does most transpiration take place?

Most transpiration takes place through the stomata, the tiny pores in the epidermis of leaves.

What colour change is seen in cobalt chloride paper when it absorbs moisture?

Dry cobalt chloride paper is blue, and it turns pink when it absorbs moisture.

Which surface of a dorsiventral leaf loses more water, and why?

The lower (abaxial) surface loses more water, because it generally bears more stomata than the upper (adaxial) surface, which may even lack them.

What does the movement of the air bubble in Ganong's potometer show?

It shows the rate at which the cut shoot takes up water, which is taken as roughly equal to its rate of transpiration.

When and through which structures does guttation occur?

Guttation occurs at night and early in the morning, when transpiration is low and root pressure is high, through hydathodes at the tips and margins of leaves.

How do bulliform cells reduce water loss in grasses?

Under water stress the bulliform cells become flaccid, making the leaves curl inwards and reducing water loss.