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Chapter 5: Transpiration

In the 1970s a puzzle hung over the Amazon basin in South America. Rain falls there almost every day, thousands of kilometres inland from the Atlantic Ocean. Air blowing in from the sea should have dropped most of its moisture near the coast and arrived deep inland dry. Yet the rain kept falling, month after month.

A Brazilian team led by Enéas Salati set out to trace where that rain came from. Water molecules are not all identical. Some carry slightly heavier oxygen atoms than others, and water that has evaporated from an ocean carries a different mixture of them from water that has passed through a plant. The team collected rainwater at stations spread right across the basin and compared these mixtures.

The pattern was clear. Rain falling near the coast carried the ocean’s signature. Rain falling far inland carried the signature of water that had already been inside leaves. The forest was not simply receiving rain — it was handing water back to the sky, over and over. The team’s estimate was that roughly half the rain falling on the basin was water the forest itself had returned to the air.

Every one of those molecules had been drawn out of the soil by a root, carried up a trunk, and released from a leaf as invisible vapour. That quiet, unending release is what this chapter is about.

Diagram

Fig. 5.0-1 — Water Returned To The Air By A Forest Simple schematic side view of a stretch of forest with a cloud and rain above it; upward arrows rise from the leaf canopy to the cloud, rain arrows fall from the cloud back onto the canopy, and a single tree at the front is cut away to show water moving from soil into roots, up the trunk and out of the leaves. Label soil water, root, water carried up the stem, water vapour released from leaves, cloud, rainfall. Line drawing, no map or territory shown.

NoteCuriosity Corner

Q1. Roots pull in water all day long, but the plant uses only a small part of it. Where does the rest of it go? Q2. Water vapour cannot be seen. How could you show that a plant is giving it out? Q3. Most cacti have no broad leaves at all. How does that help a plant living where water is scarce? Q4. Why do drops of water sometimes sit along the edges of leaves early in the morning, when there has been no rain? Q5. How can a forest add to the rain that falls on it?

5.1 The Process Of Transpiration

A plant’s roots draw in water from the soil all through the day and night. That water does not stay in the root. It is carried upwards through the stem and delivered to every aerial part of the plant — the parts that stand above the ground, such as the stem, the branches, the buds, the flowers and, above all, the leaves.

Very little of this water is actually spent. Only about \(2\%\) of the water a plant absorbs is used up in photosynthesis and in the other activities going on inside its cells. The rest — very nearly all of it — passes out of the plant into the air as water vapour. Fig. 5.1-1 sets out these two very unequal shares.

Diagram

Fig. 5.1-1 — What Happens To The Water A Plant Absorbs Outline of a leafy plant with its roots in soil; a broad upward arrow runs from the roots through the stem to the leaves; at the leaves, one very thin arrow turns inward into the plant and a very broad arrow passes outward into the air. Label water absorbed from soil, water carried up the stem, about 2 per cent used in photosynthesis and other activities, the rest lost as water vapour.

This escape of water is not a leak from a damaged part. It happens from healthy living surfaces, mainly from the leaves, and mostly through tiny pores in the leaf surface. The process has its own name: transpiration — the loss of water as vapour from the living aerial parts of a plant.

Notice the words ‘as vapour’. The water leaves the plant in the invisible gaseous form, not as drops of liquid. This is why you can stand beside a large tree on a hot afternoon, while it is losing litres of water every hour, and see nothing at all.

NoteDefinition

Transpiration The loss of water in the form of water vapour from the leaves and other aerial parts of a plant.

Q1. Roots pull in water all day long, but the plant uses only a small part of it. Where does the rest of it go?

A. It leaves the plant as invisible water vapour, mainly through the leaves. Only about 2 per cent of the absorbed water is used by the plant; the remainder is transpired into the air.

Why The Loss Is Not Simply Waste

Losing \(98\%\) of what you take in sounds like poor housekeeping. For a plant, though, this steady loss does two jobs that are hard to do any other way.

First, as water evaporates away at the top, it pulls on the water still standing in the stem below, rather as sucking at one end of a straw pulls liquid along the whole length of it. This suction keeps water and dissolved mineral nutrients moving up from the roots.

Second, evaporation carries heat away with it. A leaf sitting in full sunlight would otherwise grow dangerously hot, and the enzymes that run its chemical reactions would be damaged by that heat. Transpiration cools the leaf as it goes.

These two benefits, and one more, are taken up properly in section 5.7. For the moment it is enough to hold on to the idea that a plant does not merely tolerate this loss — it depends on it.

Where The Word Comes From ‘Transpiration’ is built from the Latin trans, meaning across, and spirare, meaning to breathe. The word therefore says something like ‘breathing across a surface’ — a fair description of vapour passing outward through the skin of a leaf.

Transpiration And Respiration Transpiration is the loss of water vapour from a plant’s aerial parts. Respiration is the release of energy from food inside living cells. Both involve gases moving through the leaf, but only one of them is about water.

5.2 Demonstrating Transpiration

Water vapour is colourless and invisible, so a plant losing it gives nothing away to the eye. To show that transpiration is happening at all, you have to catch the vapour and turn it into something you can see or test.

Two simple experiments do exactly this. Both work on the same idea: trap the air around a living plant, then look for the water that appears in it.

5.2.1 Enclosing A Plant In A Polythene Bag

Take a medium-sized potted plant that has been well watered. Cover the whole plant with a transparent polythene bag and tie the mouth of the bag tightly around the base of the stem, as in Fig. 5.2-1. Leave the set-up in sunlight for an hour or two.

Diagram

Fig. 5.2-1 — A Potted Plant Enclosed In A Polythene Bag A well-watered potted plant with a transparent polythene bag drawn over its shoot and tied at the base of the stem so that the soil and pot stay outside the bag; small water droplets shown on the inner surface of the bag. Label polythene bag, water droplets on inner surface, string tied at base of stem, soil left outside the bag.

Drops of water soon appear on the inner side of the bag. They form because the leaves keep giving out water vapour into a closed space, until the air inside can hold no more. FC 5.1 follows that chain from leaf to droplet.

Flowchart

FC 5.1 — How Droplets Appear Inside The Bag Leaves give out water vapour → vapour collects in the closed bag → air inside becomes saturated → vapour meets the cooler inner surface of the bag → vapour condenses → drops of liquid water appear on the inside

The last step deserves a word of caution. Vapour turns back into liquid only where it touches a surface cooler than itself, so the droplets show up clearly when the air outside the bag is cool enough. On a very hot day the vapour may stay in the vapour form and the bag can look almost dry.

Q. The bag is tied at the base of the stem, leaving the pot and soil outside it. Why not simply cover the whole pot as well?

A. Wet soil evaporates water too. Keeping the soil outside the bag makes certain that whatever collects inside has come from the plant and from nothing else.

The Need For A Control

An experiment proves nothing on its own. A second polythene bag, empty, with its mouth tied and kept in the same sunlight, shows no drops at all. This second set-up is the control — an identical arrangement with the one factor under test removed, kept so that the two results can be compared.

Because the only difference between the two bags is the presence of a plant, the drops in the first bag can have come only from the plant.

Finally, the drops themselves can be checked. Touched with dry cobalt chloride paper — a paper that is blue when dry and turns pink when it meets moisture — they turn the blue paper pink, confirming that the liquid is water.

5.2.2 Cobalt Chloride Paper As A Test For Moisture

Cobalt chloride paper is ordinary filter paper soaked in a solution of cobalt chloride and then dried. Dry, it is blue. As soon as water reaches it, it turns pink.

Four things make it well suited to work on transpiration.

  • The colour change is sharp and easy to read, so no measuring instrument is needed.
  • It responds to water vapour, not only to liquid water, which is what an experiment on transpiration requires.
  • It is quick, giving its answer in minutes.
  • The change can be reversed by drying the paper again, so the same paper can be prepared and used afresh.

One rule governs its use: the paper must be thoroughly dry, and therefore blue, before the experiment begins. A paper that has already picked up moisture from damp air has nothing left to tell you.

Q2. Water vapour cannot be seen. How could you show that a plant is giving it out?

A. Enclose the shoot of a well-watered plant in a transparent polythene bag and leave it in sunlight. Water vapour from the leaves saturates the trapped air and condenses as visible drops on the inside of the bag, and dry blue cobalt chloride paper turns pink to confirm that those drops are water.

Why The Colour Changes Cobalt chloride is blue when it has no water attached to its particles. When water molecules attach to them, the substance becomes pink. The paper is therefore not detecting water indirectly — the water itself becomes part of the coloured substance.

5.2.3 The Three Bell Jar Experiment

The polythene bag experiment shows the water. This second experiment shows the water and tests it at the same time, so that a single set-up carries two independent proofs. Three bell jars are arranged side by side, as in Fig. 5.2-2.

Diagram

Fig. 5.2-2 — Three Bell Jars Set Up To Demonstrate Transpiration Three bell jars standing side by side on a bench. Jar A: a small potted plant with broad leaves, its pot fully enclosed in a polythene bag tied at the base of the stem, water droplets on the inner walls of the jar. Jar B: an identical plant and pot arrangement, with a strip of blue cobalt chloride paper pinned to a wooden stick beside the plant, droplets on the walls and the paper turned pink. Jar C: no plant, only a strip of blue cobalt chloride paper on a stick, walls clear. Label bell jar A, bell jar B, bell jar C (control), cobalt chloride paper, polythene bag around pot.

Setting Up The Three Jars

Set-up A. Take a small well-watered potted plant, preferably one with broad leaves. Enclose the pot completely in a polythene bag and tie the mouth of the bag firmly around the base of the stem, so that no vapour can escape from the soil. Now cover the whole plant with a bell jar.

Set-up B. Arrange a second plant in exactly the same way under a second bell jar. This time also place a strip of dry cobalt chloride paper beside the plant inside the jar, pinned to a wooden stick or to a strip of cork sheet so that it stands clear of the leaves.

Set-up C. Take a third bell jar with no plant under it at all, containing only a similar strip of cobalt chloride paper. Keep all three jars together in the sun.

Reading The Results

After about half an hour, the three jars are compared.

  • Bell jar A shows water vapour condensing on its inner walls.
  • Bell jar B shows the same condensation, and the cobalt chloride paper inside it, blue at the start, has turned pink.
  • Bell jar C shows nothing. Its walls stay clear and its paper stays blue.

Jar C is the control of this experiment. Its unchanged paper proves that the air itself had no moisture to give; the moisture in the other two jars was put there by the plants.

Taken together, the three jars give a double visual proof of transpiration: water vapour condensing into droplets, and cobalt chloride paper changing colour.

Breathe out slowly onto a cool windowpane or a mirror and watch the misty patch form, then fade. The vapour in your breath was invisible until a cool surface condensed it — the same reason droplets appear on the inside of a bell jar and not in open air.

5.3 Measuring Transpiration

Showing that a plant transpires is one thing. Finding out how fast it does so is another, and it is the more useful question, because the speed of the process changes with the weather, the time of day and the plant itself.

What we want is the rate of transpiration — the amount of water a plant loses in a given time. Since the water leaves as invisible vapour, nobody can measure it as it goes. Every method therefore measures something else that stands in for it: either the weight the plant loses, or the water it draws in to make good the loss.

5.3.1 The Weighing Method

Water has weight. If a plant gives water away to the air, the whole set-up must become lighter, and a balance can detect that.

Take a small, light potted plant and weigh it. Leave it for a fixed period, then weigh it again. The difference between the two readings is the water lost during that time.

One precaution decides whether the reading means anything. Water can evaporate from wet soil and from the porous walls of an earthen pot just as readily as it can leave a leaf. The soil surface and the pot are therefore covered completely — usually with a polythene sheet tied around the base of the stem — so that the only exposed living surface left is the plant itself.

Cover the soil of a small potted plant with a polythene sheet tied at the stem, and weigh the pot on a kitchen balance in the morning and again in the evening. Keep the plant well watered the day before. The weight it has lost is water that left through its leaves.

Measuring Volume As Well As Weight

The method can be improved so that it gives a volume reading as well. The plant stands in a glass bottle of water joined by a rubber tube to a graduated side tube, also filled with water, as in Fig. 5.3-1. As the shoot draws water out of the bottle, the level in the graduated side tube falls, and the fall can be read off directly as a volume.

Diagram

Fig. 5.3-1 — Measuring Water Loss By Volume And By Weight Part A: a leafy shoot standing in a stoppered glass bottle of water, with a rubber tube leading from the bottle to a vertical graduated side tube filled with water; mark the starting and final water levels in the side tube. Part B: the same bottle and shoot placed on the pan of a weighing machine showing a reading. Label graduated side tube, water level, rubber tube, glass bottle, weighing machine.

The two readings can then be checked against each other, because \(1\) cubic centimetre (cc) of water has a mass of about \(1\) gram. A fall of 5 cc in the side tube should show up as a loss of about 5 g on the weighing machine.

A Cut Shoot In A Test Tube

A whole potted plant is not always convenient. The same idea works with a cut shoot, as shown in Fig. 5.3-2.

Fill a test tube with water and insert a leafy shoot, one with no roots on it. Pour a thin layer of oil over the water surface. Oil floats, seals the surface and stops water evaporating from the test tube itself, so that any water lost must have gone out through the leaves.

Diagram

Fig. 5.3-2 — A Leafy Shoot In A Test Tube Sealed With Oil A test tube filled with water holding a leafy shoot without roots; a thin dark layer of oil floats on the water surface around the stem; the test tube stands upright in a small beaker. Label leafy shoot, layer of oil, water, test tube, beaker.

Stand the test tube in a small beaker and weigh the two together. Remove the test tube without disturbing its contents, keep it upright in a test tube stand for a few hours, then return it to the same beaker and weigh again. The difference in weight is the water the shoot has lost by transpiration.

Expect a small figure here. A cut shoot has no roots actively pulling in water, so it cannot keep its leaves fully supplied, and the water loss is much less than a rooted plant of the same size would show.

5.3.2 The Potometer Method

The second family of methods turns the problem around. Instead of following the water that leaves, they follow the water that comes in to replace it.

A potometer is an apparatus that measures the rate at which a plant takes in water. Since a plant draws in water largely because it is losing water at the top, the water taken in is very nearly equal to the water transpired, and the intake reading can stand in for the loss.

Several potometers exist, designed by different workers for different purposes.

  • Farmer’s potometer and Ganong’s potometer measure the rate of water intake by a plant.
  • Darwin’s potometer demonstrates the suction force created by transpiration.
  • Garreau’s potometer demonstrates the unequal transpiration from the two surfaces of a dorsiventral leaf — a leaf with a clearly different upper and lower surface, as most broad leaves have.

Where The Name Comes From The word comes from the Greek potos, meaning a drink, and metron, meaning a measure. A potometer is literally a drink-meter: it records how much the cut shoot is drinking.

5.3.3 Ganong’s Potometer

Of these, Ganong’s potometer is the one usually set up in a school laboratory. Its parts are shown in Fig. 5.3-3.

Diagram

Fig. 5.3-3 — Ganong’s Potometer Horizontal graduated capillary tube with a scale, one end bent downwards and dipping into a beaker of coloured water, the other end joining a wide tube that carries a leafy twig sealed into its mouth; a water-filled reservoir with a stop-cock is fixed above the junction; a single air bubble sits in the graduated capillary tube. Label leafy twig, reservoir, stop-cock, air bubble, graduated capillary tube, beaker of coloured water.

Setting It Up

Cut a twig of a suitable plant, such as Coleus, with a sharp knife and fix it into the mouth of the apparatus. Fill the whole apparatus with water so that no air spaces are left anywhere inside it.

A single air bubble is then introduced into the horizontal graduated capillary tube. This is done by lifting the bent end of the capillary tube out of the coloured water for a moment, so that a little air is sucked in, and then dipping it back into the water. The bubble is the marker whose movement will be read.

How It Works

As the twig transpires, water is lost from its leaves. That loss sets up a suction force which pulls water along the twig from the apparatus, and the water column shifts to follow it — carrying the air bubble along the scale. FC 5.2 sets out the chain.

Flowchart

FC 5.2 — How The Bubble Moves In A Potometer Twig loses water vapour by transpiration → suction force is set up in the twig → water is pulled from the capillary tube and the beaker → water column moves towards the twig → air bubble travels along the graduated scale → reading gives the volume of water taken up in that time

Q. If the bubble travels a longer distance in the same ten minutes, what has changed in the plant?

A. The plant is transpiring faster. The bubble tracks the water being drawn in, and water is drawn in only as fast as it is being lost above.

To take a fresh reading, the bubble must be returned to its starting mark. Opening the stop-cock lets a little water down from the reservoir into the capillary tube, which pushes the bubble back to its original position, and the experiment can be repeated.

What The Reading Actually Measures

Read carefully, a potometer does not tell you how much water was transpired. It tells you how much water the cut shoot took up. Those two quantities are close but not identical, because a small share of the water taken in is kept back by the cells and used for other work — the manufacture of food being the largest such use. The potometer reading is therefore a very good stand-in for transpiration, not a direct measure of it.

5.3.4 Precautions In The Use Of A Potometer

Two precautions decide whether the reading can be trusted.

  1. The apparatus must be made completely water-tight. A leak anywhere lets water escape or air enter, and the bubble then moves for reasons that have nothing to do with the plant.
  2. The twig must be cut obliquely, and cut under water. The slanting cut exposes a larger surface for water intake. Cutting under water prevents an air bubble from being sucked into the cut end, which would block the xylem and stop water being absorbed into the twig altogether.

A Precaution And A Limitation A precaution is something you do while setting up, to keep the reading fair. A limitation is a weakness of the apparatus that careful working cannot remove. The first is in your hands; the second is not.

5.3.5 Limitations In The Use Of A Potometer

Even a carefully set up potometer has shortcomings that stay with it.

  1. Introducing the air bubble is not easy, and a poorly formed bubble breaks up or sticks.
  2. The twig may not remain fully alive for very long, and a dying twig no longer transpires normally.
  3. Any change in the temperature of the surrounding air alters the position of the air bubble in the capillary tube, because air expands on warming and contracts on cooling. The bubble then moves without the plant having done anything.

5.4 Kinds Of Transpiration

Water vapour does not leave a plant from one place only. Transpiration from the aerial parts occurs from three different regions, and the three are named after the openings or surfaces the vapour passes through.

  1. From the leaves through the stomata — stomatal transpiration.
  2. Directly from the surface of the leaves and stems — cuticular transpiration.
  3. From the lenticels, which are minute openings on the surface of old woody stems — lenticular transpiration.

These three shares are nothing like equal. The major part of transpiration occurs through the stomata; the other two together contribute very little. FC 5.3 sets the three side by side.

Flowchart

FC 5.3 — The Three Kinds Of Transpiration Transpiration from aerial parts → branches into three: (a) stomatal transpiration → through stomata of leaves → major part of the total; (b) cuticular transpiration → directly through the cuticle of leaves and stems → very small; (c) lenticular transpiration → through lenticels of old woody stems → very small

5.4.1 Stomatal Transpiration

5.4.1.1 The Structure Of A Stoma

The outermost layer of cells covering a leaf is the epidermis. Scattered through it are minute openings called stomata (singular: stoma), and each of these is a genuine hole through the leaf surface.

A stoma is not a gap left by accident. It is surrounded by two bean-shaped guard cells, which stand on either side of the pore like a pair of curved lips, as Fig. 5.4-1 shows. Unlike the other epidermal cells around them, guard cells contain chloroplasts.

Diagram

Fig. 5.4-1 — Surface View Of A Single Stoma Surface view of a portion of leaf epidermis showing one stoma; two bean-shaped guard cells enclose a central slit-like pore and are surrounded by ordinary epidermal cells with wavy outlines. Label guard cell, stoma (pore), chloroplasts, nucleus, cell wall, epidermal cell. Lead lines straight and non-crossing.

Stomata are not scarce. Their number may range from about \(1{,}000\) to \(10{,}000\) per square centimetre of leaf surface, so an ordinary leaf carries them by the hundred thousand.

During the daytime the stomata stand wide open, and it is worth being clear about why. They are open primarily so that carbon dioxide can diffuse into the leaf for photosynthesis. Water vapour escaping outward through the same open pores is the price the plant pays for that intake.

Guard Cells Two curved cells enclose the pore → when they swell with water they bow apart and the pore opens → when they lose water they straighten and the pore closes → the plant can open and shut this doorway as its needs change.

The Stoma And Its Guard Cells The stoma is the pore itself — an opening, not a cell. The guard cells are the two living cells that surround it and control its size. A stoma cannot open or close; the guard cells do that to it.

5.4.1.2 The Escape Of Water Vapour From The Leaf

Water absorbed by the roots rises through the stem and reaches the tissues of the leaf through the veins. Once inside the leaf, it must cross the green tissue between the two epidermal layers — the mesophyll — before it can leave.

A large number of spongy mesophyll cells have their surfaces exposed to the intercellular spaces, the small air-filled gaps between cells. These exposed surfaces give out some of their water as a thin film.

Water evaporates from this film, and the vapour formed saturates the air in the intercellular spaces. The vapour then diffuses into the other connecting spaces and finally reaches the sub-stomatal space, the small chamber lying just inside each stoma, from where it escapes through the pore.

The whole of this journey, from the surface of a cell to the outside atmosphere, happens by diffusion, shown in Fig. 5.4-2. Molecules of water vapour, like those of any gas, move from a region of their higher concentration to a region of their lower concentration — and the air inside a leaf is far more heavily loaded with vapour than the air outside it.

Diagram

Fig. 5.4-2 — Vertical Section Of A Leaf Showing The Escape Of Water Vapour Vertical section through a part of a leaf blade showing cuticle, upper epidermis, palisade mesophyll, spongy mesophyll with intercellular spaces, lower epidermis, a stoma with its guard cells and the sub-stomatal space beneath it; an arrow traces water vapour from the intercellular spaces through the sub-stomatal space and out of the stoma into the outside air. Label cuticle, upper epidermis, palisade mesophyll, spongy mesophyll, intercellular space, sub-stomatal space, lower epidermis, stoma, water vapour.

Q. Does water leave the mesophyll cell as liquid or as vapour?

A. As liquid. It seeps out as a film onto the cell surface, and only then evaporates into the air space. The change of state happens outside the cell, not inside it.

5.4.1.3 How Water Reaches The Evaporating Surfaces

Something has to keep pushing water out to those wet cell surfaces, and something has to keep replacing what evaporates away. Fig. 5.4-3 shows both movements.

The cell sap inside each leaf cell presses outward on the cell wall. This outward push of a full cell against its wall is its turgor pressure, and it forces some water out of the cell wall into the air space between the cells. There the water evaporates, and the vapour diffuses through the air spaces into the sub-stomatal space and finally out through the stomata.

Cells that lose water in this way replace it at once by drawing more from the nearest vein. Most of that replacement water travels along the cell walls by imbibition — soaking through the wall material much as water creeps along a strip of blotting paper — and only a small quantity actually enters the cells by osmosis.

Diagram

Fig. 5.4-3 — Movement Of Water Through The Cells Of A Leaf Part A: section through a leaf blade showing a xylem vessel in a vein, mesophyll cells, intercellular spaces, the sub-stomatal space and a stoma, with arrows showing water moving by osmosis from cell to cell and by evaporation into the air spaces, and water vapour leaving through the stoma. Part B: an enlarged group of three or four leaf cells beside a xylem vessel, with bold arrows along the cell walls and thin arrows entering the cells. Label xylem vessel, movement by osmosis, movement by evaporation, intercellular space, sub-stomatal space, stoma, water vapour, most water travels along the cell walls by imbibition, a small proportion of water enters the cells by osmosis, diffusion to the outside.

One Water Molecule Leaving A Leaf 1. It arrives in the leaf inside a xylem vessel of a vein, pulled up from the stem. 2. It moves out of the vein into the wall of a nearby mesophyll cell, mostly by imbibition along the walls. 3. Turgor pressure inside the cell forces it out of the wall onto the wet surface facing an air space. 4. It evaporates there, changing from liquid to vapour, and joins the saturated air of the intercellular space. 5. It diffuses from one intercellular space to the next, always towards lower concentration, and reaches the sub-stomatal space. 6. It slips out through the open stoma into the drier air outside the leaf, and is carried away.

Flowchart

FC 5.4 — Path Of Water Vapour Out Of A Leaf Xylem vessel of the vein → cell walls of mesophyll cells → film of water on the cell surface evaporates → intercellular space → sub-stomatal space → stoma → outside atmosphere

5.4.1.4 Transpirational Pull

Now multiply that single molecule by the thousands of leaf cells evaporating water at the same moment. Each cell that loses water draws more from its neighbour, and the whole leaf ends up tugging on the water standing in the xylem vessels below.

Water is drawn up as a continuous column, because water molecules cling to one another and so a pull applied at the top is felt all the way down. This pull created by evaporation at the leaves is called the transpirational pull.

It is a remarkably strong effect. The transpirational pull can draw water up to about 50 metres or more in tall trees, without the plant spending any energy of its own on the lifting. Evaporation at the top does the work.

5.4.1.5 Unequal Transpiration From The Two Leaf Surfaces

The two faces of a broad leaf do not transpire equally. In a dicot leaf, more transpiration occurs from the under surface, and the reason is simply that there are far more stomatal openings there.

This can be proved with an experiment you can set up on a plant growing in the school garden, shown in Fig. 5.4-4. Attach a piece of dry cobalt chloride paper to each of the two surfaces of a leaf, and hold each in position with a glass slide, the two slides tied on either side with elastic bands or held with clips. The leaf must remain attached to its own plant throughout.

Diagram

Fig. 5.4-4 — Comparing Transpiration From The Two Surfaces Of A Dicot Leaf A broad dicot leaf still attached to its plant, with a strip of cobalt chloride paper pressed against its upper surface and another against its lower surface, each held by a glass slide and metal clips. Show the paper on the upper surface still blue and the paper on the lower surface turned pink. Label upper (adaxial) surface, lower (abaxial) surface, cobalt chloride paper remaining blue, cobalt chloride paper turned pink, glass slide, clip.

After a while, the two papers tell different stories. The one facing the upper surface either does not turn pink at all, or takes a much longer time to do so. The one facing the lower surface turns pink much faster. More water vapour is clearly leaving from below.

Q. Why must the leaf stay joined to its plant instead of being plucked first?

A. A plucked leaf is cut off from its water supply. Its cells soon lose water, its stomata close, and transpiration stops — so the experiment would be comparing two surfaces of a leaf that is barely transpiring at all.

5.4.1.6 Stomatal Regulation Of Transpiration

Stomata occur in large numbers on the lower epidermis of a leaf, and they are the plant’s one adjustable outlet. Transpiration goes on as long as they are open, and it stops when they are closed.

The opening and closing is regulated by the amount of water and solutes present in the guard cells. As the stomata open during the daytime, gases diffuse in and out freely, meeting the leaf’s need for photosynthesis and allowing transpiration at the same time.

If for any reason the water content of the leaf begins to fall short, the guard cells cannot stay turgid. They turn flaccid — that is, they lose their turgidity and go limp — and as they slacken, the stomatal opening closes and transpiration stops. The detailed account of how the guard cells manage this is taken up in the next chapter.

This is worth stating plainly, because it separates the three kinds of transpiration sharply. Stomatal transpiration is controlled by the plant, by adjusting the size of the stoma. Cuticular and lenticular transpiration are not controlled at all.

Wilting At Midday

You may have noticed a garden plant looking limp in the early afternoon and standing perfectly upright again by evening. The leaves of some plants, balsam (Impatiens) among them, wilt during midday even though there is plenty of water in the soil.

The cause is a mismatch of rates. At midday the rate of transpiration exceeds the rate at which the roots can absorb water, so the cells lose turgidity faster than it can be restored and the leaves droop.

In the evening and during the night the stomata are constricted and the temperature is no longer high. There is now no loss of water through transpiration, the leaves re-acquire their turgidity, and they stand out erect again.

5.4.2 Cuticular Transpiration

The cuticle is a waxy layer secreted by the epidermis over the two surfaces of a leaf. Its thickness varies a good deal from plant to plant.

The cuticle exists primarily to prevent evaporation of water from the leaf surfaces — it is a waterproof coat. Even so, some evaporation does take place through it, and this is cuticular transpiration.

The relation between the two is straightforward: the greater the thickness of the cuticle, the lesser the evaporation, and therefore the lesser the transpiration. Plants of dry climates tend to have thicker cuticles for exactly this reason.

5.4.3 Lenticular Transpiration

Lenticels are special openings that develop on the bark of older stems. Their main business is to allow the diffusion of gases for respiration, since a woody stem covered in bark has no other easy way of exchanging gases with the air.

Lenticels differ from stomata in one decisive respect: they never close. They remain open all the time, because there are no guard cells to shut them. Water from the cell surfaces directly facing a lenticel evaporates and contributes to transpiration.

Look closely at the bark of a young woody twig — guava and mango twigs show this well. The tiny raised dots or short slits scattered over the surface are lenticels. Feel the bark: it is dry and corky, yet gases and water vapour are passing through those dots all day and all night.

5.4.4 Stomata Compared With Lenticels

Both stomata and lenticels are openings, both let gases through, and both contribute to transpiration — which is exactly why they are easy to confuse. Table 5.1 separates them by structure, location and function.

Table 5.1 — Stomata Compared With Lenticels | Feature | Stomata | Lenticels | |—|—|—| | Structure | Small openings surrounded by two bean-shaped guard cells; they can be opened and closed | Special openings not surrounded by guard cells; they remain open all the time | | Location | Mostly found on the abaxial (lower) surface of leaves | Found on the surface of older woody stems | | Function | Responsible for the diffusion of gases during photosynthesis and respiration; they also help in transpiration | Mainly responsible for the diffusion of gases during respiration; they also help in transpiration |

One last point of proportion. The amount of transpiration through the cuticle is certainly much less than stomatal transpiration, but it is more than lenticular transpiration. Ranked from most to least, the three kinds run: stomatal, then cuticular, then lenticular.

5.5 Factors That Affect The Rate Of Transpiration

A plant does not transpire at a fixed speed. The same plant may lose water quickly on a hot windy afternoon and hardly at all on a still, damp night.

The reason is that transpiration is really two physical events joined together: water evaporating inside the leaf, and vapour diffusing out of it. Anything that speeds up or slows down either event changes the rate of transpiration.

The conditions that do this fall into two groups. External factors act on the plant from outside — they are features of the surrounding air. The internal factor is a condition of the plant itself.

External And Internal Factors An external factor belongs to the surroundings: sunlight, temperature, wind, humidity. An internal factor belongs to the plant: how much water its leaves are holding at that moment. Change the weather and you change the first; change the plant’s water supply and you change the second.

5.5.1 External Factors

  1. Intensity of sunlight. During the day the stomata are open to let carbon dioxide diffuse inwards for photosynthesis, and open stomata also let vapour out. At night they are closed, so more transpiration occurs during the day. When the day is cloudy, the stomata are only partially open and transpiration is reduced.

  2. Temperature. If the outside temperature is higher, there is more evaporation from the leaves and therefore more transpiration. A rise in temperature allows more water to evaporate, and a fall in temperature reduces evaporation. Warm air can also hold more water than cold air, so it keeps accepting vapour from the leaf instead of turning it away.

  3. Velocity of wind. Transpiration increases with the velocity of the wind. Still air sits against the leaf surface and slowly fills with vapour, but if the wind blows faster, the water vapour released is swept away faster and the air just outside the leaf never becomes saturated. Diffusion outward therefore continues at full speed.

  4. Humidity. Transpiration is reduced if the air outside is humid. Humidity is the quantity of water vapour already present in the air, and air that is heavily loaded with vapour leaves only a small difference in concentration between the inside of the leaf and the outside. That small difference slows the outward diffusion of vapour across the stomata, and so slows transpiration.

  5. Carbon dioxide. Ordinary air contains only a very small share of carbon dioxide — about \(0.04\%\) by volume, roughly 420 parts per million as measured in 2024. If the carbon dioxide level of the outside air rises above this normal value, the stomata close, and transpiration decreases as a result.

  6. Atmospheric pressure. The rate of transpiration increases as the atmospheric pressure decreases. Thinner air offers less resistance to molecules moving through it, so a fall in pressure enhances the diffusion of water vapour away from the leaf.

Table 5.2 gathers these six conditions into the shape in which they are most often needed: what raises the rate, and what lowers it.

Table 5.2 — Conditions That Raise And Lower The Rate Of Transpiration | Factor | Rate rises when | Rate falls when | |—|—|—| | Intensity of sunlight | Bright daylight keeps the stomata open | Night or cloud keeps the stomata closed or partly closed | | Temperature | The air is warm | The air is cool | | Velocity of wind | The wind is brisk and carries vapour away | The air is still and becomes saturated near the leaf | | Humidity | The air outside is dry | The air outside is humid | | Carbon dioxide | The level stays at its normal value and stomata stay open | The level rises above normal and the stomata close | | Atmospheric pressure | The pressure is low | The pressure is high |

Wet two similar handkerchiefs equally. Hang one in still air indoors and the other in front of a running fan, and note which dries first. The moving air carries vapour away and stops the layer next to the cloth becoming saturated — the same reason wind speeds up transpiration.

Windbreaks Around Fields Farmers in dry and windy districts often plant rows of tall trees or shrubs along the edges of their fields. These shelterbelts slow the wind before it reaches the crop, and slower wind means the air near the leaves stays a little more saturated. The crop then loses less water by transpiration and needs less irrigation to stay turgid. The same reasoning explains why a nursery keeps newly transplanted saplings under a shade net: their roots have been disturbed and cannot yet absorb much water, so their water loss must be cut down until the roots recover.

5.5.2 The Internal Factor: Water Content Of The Leaves

The one factor that lies inside the plant is the water content of its leaves.

If the water content of the leaves decreases, because the roots are not absorbing enough to keep pace, the leaves wilt and transpiration is reduced. The reduction is not direct. It comes about because the guard cells, short of water themselves, allow the stomata to close, and closed stomata cut off the main outlet.

Seen this way, wilting is not simply damage. It is a natural mechanism by which the plant conserves the water still left within it, shutting its own doors when the supply runs low.

Humidity Through The Year In most of India the air is dry through the summer months and heavily humid during the monsoon. A garden plant therefore transpires far more freely in May than it does in July, even when the July day is warm — the vapour simply has nowhere to go.

5.6 Adaptations In Plants To Reduce Excessive Transpiration

Closing the stomata is a plant’s short-term answer to water loss, and it is a costly one, because a closed stoma also shuts out the carbon dioxide needed for photosynthesis.

Plants growing where water is scarce cannot afford to keep making that choice every day. Many of them, and especially those of dry climates, have evolved permanent adaptations that curtail transpiration — features built into the structure of the plant, present all the time, and inherited rather than switched on for the afternoon.

Six such adaptations are common.

  1. Sunken stomata. The stomata may lie in pits below the general level of the leaf surface, or be covered over by hairs, as in Nerium (oleander). Fig. 5.6-1 shows both arrangements together.

Diagram

Fig. 5.6-1 — Sunken Stomata Covered By Hairs In Nerium Vertical section through a small portion of a Nerium leaf showing a deep pit in the lower surface, several stomata with their guard cells lying at the floor of the pit, and branching hairs arising from the pit walls and partly covering its mouth. Label lower epidermis, trichome (hair), guard cell, stomata, pit.

A Sunken Stoma The pore lies at the bottom of a pit, sheltered from moving air → vapour collects in the pit and the air there stays humid → the concentration difference across the pore stays small → water vapour diffuses out slowly.

  1. Fewer stomata. The number of stomata on the leaf surface may simply be reduced. Fewer outlets mean less vapour can leave in a given time.

  2. Narrow leaves. The leaves may become narrower, which reduces the surface area exposed to the air. Nerium again shows this: its leaves are long, slender and leathery rather than broad and thin.

  3. Reduced exposed surfaces. In some cases the leaves become wavy, rolled or folded, so that a good part of the surface — usually the surface bearing most of the stomata — is turned inward and no longer faces open air.

  4. Loss of leaves. In some plants the leaves are dropped, or are absent altogether, or are changed into spines, as in most cacti. Fig. 5.6-2 shows what that leaves behind: a swollen green stem which stores water and takes over the work of photosynthesis, with the spines exposing almost no surface at all.

Diagram

Fig. 5.6-2 — Adaptations Of A Cactus To Reduce Water Loss Whole cactus plant with a thick ribbed green stem bearing clusters of spines, drawn with its root system below soil level; roots shown spreading widely near the surface and one root running deep. Label green stem carries out photosynthesis, leaves reduced to spines to reduce surface area for transpiration, swollen stem stores water, stomata sunken in grooves, shallow roots absorbing water from light rainfall, deep roots reaching a low water table.

  1. Thick cuticle. The leaves may be covered by a thick cuticle, as in the banyan (Ficus benghalensis) and most evergreen trees. As section 5.4.2 established, a thicker waxy layer means less evaporation through the leaf surface.

Q3. Most cacti have no broad leaves at all. How does that help a plant living where water is scarce?

A. A broad leaf is mostly surface, and surface is where water is lost. By reducing its leaves to spines, a cactus removes almost all the surface from which transpiration could occur; its swollen green stem, with sunken stomata in its grooves, then carries out photosynthesis and stores water instead.

A Name For Such Plants Plants structurally adapted to live where water is scarce are called xerophytes, from the Greek xeros, meaning dry. Cacti and oleander are xerophytes. The features listed in this section are what the word describes.

Look at a patch of lawn grass on a hot, dry afternoon and again after sunset. Many grasses roll their leaf blades inwards as the day heats up, hiding the stomata-bearing surface inside the roll, and unroll them again when the air cools. Adaptation number 4 is happening in front of you.

5.7 The Significance Of Transpiration

A process that throws away almost all the water a plant works to collect looks, at first sight, like a design fault. It is not. Transpiration has great significance for plants, and its main advantages are three: cooling, creating a suction force, and distributing water.

Cooling The Leaf

Evaporation always takes heat away with it. The fastest-moving water molecules are the ones that escape, and what is left behind is cooler than before, so evaporation from a leaf reduces the temperature of the leaf surface.

This matters most on exactly the days when transpiration is fastest. A leaf in strong sunlight absorbs a great deal of heat, and the enzymes that run its chemical reactions are damaged by high temperatures. By carrying heat away as it goes, transpiration keeps the leaf working on hot sunny days.

The Same Trick, Elsewhere Sweat cools you for this reason: the water evaporating off your skin takes heat with it. Water kept in an unglazed earthen pot stays cool for the same reason — a little of it seeps through the clay and evaporates from the outside surface.

Creating A Suction Force

Transpiration helps in the ascent of sap — the upward movement of water and dissolved minerals from the roots to the top of the plant — by producing a suction force that acts from the top downwards.

The chain begins in the leaf cells. As water evaporates from them, their cell sap becomes more concentrated and their osmotic pressure rises. Such a cell pulls water out of the cell next to it, which then pulls from the cell below that, in a sequential manner, until the pull reaches the xylem and finally favours the absorption of water from the soil by the roots.

Fig. 5.7-1 shows this pull working over the height of a whole tree. What it represents is the transpiration stream — the continuous flow of water from the soil, into the roots, up the stem, through the leaves and out into the air as vapour.

Diagram

Fig. 5.7-1 — The Transpiration Stream Outline of a leafy tree with its root system in the soil. Short outward arrows leave the leaf surfaces at the top; a long upward arrow runs through the trunk; inward arrows enter the root hairs from the surrounding soil. Label evaporation into the atmosphere from the leaf surface (transpiration), water passes up the trunk (ascent of sap), water absorbed by the roots from the soil (endosmosis).

As the water evaporates from the leaves, a suction force is produced at the top of the plant. It draws more water up through the stem, and in doing so it makes the roots absorb more water from the soil. The plant lifts water to its topmost leaf without spending energy on the lifting.

Distributing Water And Minerals

Leaves are present at the tips of all the branches and twigs of a plant — that is, at the far ends of every pathway water can take. Transpiration from their surfaces tends to draw water towards them.

Because the pull comes from every extremity at once, water is delivered throughout the plant body rather than up one favoured route. And since minerals travel dissolved in that water, the higher the rate of transpiration, the greater the rate of absorption of water and solutes from the soil.

Transpiration Is Not Excretion

Students often describe transpiration as the plant getting rid of ‘excess water’, which makes it sound like an excretory process. It is not appropriate to think of it that way.

Excretion, wherever it occurs, is a deliberate active process, carried out by the organism to rid itself of unwanted metabolic waste substances, particularly nitrogenous ones. Transpiration removes neither a waste product nor a nitrogenous substance. It is the unavoidable physical escape of water through openings the plant keeps open for another purpose altogether.

5.7.1 Transpiration And Climate

It has been shown experimentally that plants give out large quantities of water during transpiration. Three measured examples give a sense of the scale.

  • A full grown single sunflower plant is estimated to lose about half a litre of water per day as water vapour.
  • A single maize plant loses about 2 litres of water per day.
  • A large apple tree may lose about 30 litres of water per day.

Reading The Three Figures Compare the three plants first. The sunflower and the maize plant are both single herbaceous plants of roughly a season’s growth, yet the maize plant loses about four times as much. The apple tree, a large woody plant carrying thousands of leaves, loses about sixty times what the sunflower does. The pattern is that water loss follows the total leaf surface a plant carries — more leaf, more stomata, more vapour. Now take the figures out of the single plant and into a field. Suppose a field carried 50,000 maize plants, which is a realistic number for one hectare. At 2 litres each, that field would return about 100,000 litres of water to the air in a single day. Nothing in this calculation is new biology; it is the same 2 litres, multiplied by the number of plants standing in the field.

These figures give an idea of the huge quantities of water released into the atmosphere by vast stretches of fields, and particularly by forests. Transpiration increases the moisture in the atmosphere, and moist air rising and cooling is what brings rain. In this way transpiration from plants affects climate.

Q5. How can a forest add to the rain that falls on it?

A. Every tree in the forest is returning water to the air as vapour, day after day. Over a large forest this adds an enormous quantity of moisture to the atmosphere above it, and that moisture condenses and falls again as rain — so the forest is not only receiving rainfall but supplying part of it.

Where This Sits On The Ladder Zoom in and transpiration is a film of water evaporating from the wall of one mesophyll cell. Zoom out and it is a whole ecosystem handing water back to the atmosphere. The same process reads as cell biology at one scale and as climate at another.

5.7.2 Transpiration Compared With Evaporation

Transpiration involves evaporation, and the two words are often used as though they meant the same thing. They do not. Evaporation is a purely physical change that will happen to any exposed water surface; transpiration is that change taking place inside a living organ, under the organ’s partial control. Table 5.3 sets out the differences.

Table 5.3 — Evaporation Compared With Transpiration | Feature | Evaporation | Transpiration | |—|—|—| | What is losing water | Loss of water from the surface of water bodies, in the form of vapour | Loss of water in the form of vapour from the aerial parts of a plant | | Nature of the process | A physical change, controlled by the temperature and humidity of the atmosphere | A vital and partly physical process, controlled by both internal and external factors | | Speed | A fast process | A slow process |

The middle row is the one that matters most. A puddle has no say in how fast it dries; a plant, by opening and closing its stomata, has a say in how fast it transpires.

5.8 Guttation And Bleeding

Everything so far has concerned water leaving a plant as vapour. But some plants lose water, or other fluids along with dissolved substances, directly in liquid form.

This kind of loss is called exudation — to exude means to ooze or sweat out — and the fluid given out is known as an exudate. It occurs in two ways: guttation and bleeding.

5.8.1 Guttation

Walk through a garden very early on a warm, damp morning and you may find leaves carrying a neat row of water drops along their edges, as in Fig. 5.8-1. This is guttation, the loss of water as liquid droplets from the margins of the leaves of certain plants. The name comes from gutta, meaning a drop.

Diagram

Fig. 5.8-1 — Droplets Of Water Along The Margin Of A Leaf Close photograph of a leaf of a herbaceous plant in the early morning, showing distinct droplets of water standing at regular intervals along the toothed margin of the blade, with the leaf surface itself dry. Label droplets of water, leaf margin.

Why The Water Is Pushed Out

Guttation happens particularly in plants growing in warm, humid conditions, and the reason lies in a mismatch that the last two sections have already prepared you for.

A humid environment hampers transpiration, because vapour cannot diffuse away into air that is already loaded with it. The roots, meanwhile, take no notice of the humidity and continue to absorb water from the soil. Water keeps coming in at the bottom while the usual outlet at the top is blocked.

The result is a big hydrostatic pressure built up inside the plant — the pressure of water pushed into a space it cannot easily leave. That pressure forces the excess water out directly from the tips of the veins in the leaf. FC 5.5 follows the sequence.

Flowchart

FC 5.5 — How Guttation Comes About Warm humid air outside → transpiration is hampered → roots continue absorbing water from the soil → water accumulates and hydrostatic pressure builds up inside the plant → excess water is forced out at the vein endings → droplets appear along the leaf margins

Water cannot force its way out through the cuticle, so there must be an opening, and there is. Special pore-bearing structures called hydathodes sit on the margins of the leaf, at the ends of the veins, and allow this exudation. Fig. 5.8-2 shows where one lies.

Diagram

Fig. 5.8-2 — A Hydathode At The Ending Of A Vein Vertical section through the margin of a leaf showing a xylem vein ending in a group of loosely packed thin-walled cells, above which lies an open pore in the epidermis with a droplet of water standing on it. Label xylem of vein ending, loosely packed cells, hydathode pore, droplet of water, epidermis.

Guttation can be observed in many plants and is quite common in banana, nasturtium and strawberry.

A Hydathode An open pore lying directly above the end of a vein → water pushed along the vein by pressure from below reaches it with nothing in the way → the water escapes as liquid, not vapour → the plant sheds surplus water on a day when evaporation cannot do the job.

Guttation Drops And Dew Dew is moisture from the air condensing onto a cold leaf surface, so it settles anywhere on the blade. Guttation drops come from inside the plant, pushed out through hydathodes, so they stand only at the margins or the tip.

Q4. Why do drops of water sometimes sit along the edges of leaves early in the morning, when there has been no rain?

A. Because on a humid night transpiration nearly stops while the roots go on absorbing water. The pressure that builds up inside the plant forces the surplus water out in liquid form through the hydathodes at the vein endings, which lie along the leaf margins. The drops came out of the leaf, not out of the sky.

Go out to a lawn or a field just after sunrise, before the sun is strong, and look at the tips of the grass blades. Many of them will carry a single clear drop. Check whether the rest of the blade is wet too — if only the tip carries water, you are looking at guttation and not at dew.

5.8.2 Bleeding

The second kind of exudation needs no special pore, because it uses a wound. Bleeding is the escape of plant sap from a cut or ruptured surface of a plant, and it happens only as a result of injury.

Snap a stem or cut through a young branch, and sap oozes from the exposed surface within moments. The plant is not choosing to release it. The sap is already under pressure inside the intact plant, and cutting simply gives it a way out.

Part of that pressure comes from the roots. Root pressure — the pressure developed in the root as water is absorbed, which pushes water upward into the stem — assists in bleeding, driving sap out of the cut surface.

Tapping A Tree For Its Sap Several industries live on bleeding made deliberate. In rubber cultivation, a shallow cut is made in the bark of the rubber tree, Hevea brasiliensis, and the milky latex that flows from the wound is collected in a cup fixed below it; the cut is renewed each day. In the cold regions of North America, holes are bored into the trunks of sugar maple trees, Acer saccharum, in early spring, and the sap that runs out under pressure is boiled down to make maple syrup. In both cases a tree that has been injured is doing exactly what section 5.8.2 describes — and it is done in thin cuts, at intervals, so that the tree recovers and can be tapped again for years.

5.8.3 Telling Guttation, Bleeding And Transpiration Apart

Three processes now describe water leaving a plant, and they are easiest to hold apart in pairs. Table 5.4 separates the two kinds of exudation.

Table 5.4 — Guttation Compared With Bleeding | Feature | Guttation | Bleeding | |—|—|—| | Where it occurs | From the edges of leaves, through hydathodes, in uninjured plants | From any cut or injured part of a plant | | What the exudate is | Mainly water with some dissolved mineral salts | Mainly plant sap and sugars | | When it occurs | During early mornings or late nights | At the time of injury | | Which plants show it | Certain plants only, such as banana, nasturtium and strawberry | All plants that have been cut or injured |

Table 5.5 sets guttation against transpiration, the pair most often confused, since both are ways of losing water from a leaf.

Table 5.5 — Transpiration Compared With Guttation | Feature | Transpiration | Guttation | |—|—|—| | Form in which water is lost | As water vapour | As water droplets | | Purity of the water lost | Pure water only changes into vapour | The water lost contains mineral salts in solution | | Route out of the plant | Through stomata, lenticels and the cuticle | Through pores called hydathodes, found at the ends of the veins | | Effect on the leaf | May lead to loss of turgidity in leaf cells, causing wilting | Has no effect on the turgidity of the leaf | | Control | Stomatal transpiration is regulated by the guard cells | The opening of hydathodes cannot be regulated | | Time of occurrence | In the presence of sunlight | At night or in the early morning | | Effect on temperature | Cools the plant body | Has no cooling effect on the plant | | Conditions favouring it | Occurs during dry conditions | Occurs in humid conditions |

Read the last row of each table together and the logic of the whole section falls into place. Transpiration is what a plant does when the air is dry; guttation is what it does when the air is too wet for transpiration to work; bleeding is what happens to it when it is cut.

5.9 The Complete Path Of Water Through A Plant

Each part of this chapter has dealt with one stretch of a single journey. It is worth setting that journey down end to end, from the film of water around a soil particle to the vapour drifting away above a leaf.

FC 5.6 gives the route in order. Every stage in it has been met somewhere in this chapter or the last.

Flowchart

FC 5.6 — From Soil Water To The Atmosphere Water in the soil → root hair → epidermal and cortical cells of the root → xylem vessels of the root → xylem vessels of the stem and branches → xylem of the leaf veins → palisade and spongy parenchyma of the leaf → sub-stomatal space → stomata → water vapour lost to the atmosphere (transpiration)

The Forces Along The Way

No single force carries water the whole distance. Different stretches of the path are worked by different agents, and they act together.

At the root, water enters by imbibition into the cell walls and by osmosis into the cells themselves. Through the root, stem and leaf, water then moves as a continuous mass, held together by cohesion — the tendency of water molecules to cling to one another — and helped along by root pressure pushing from below and by capillarity, the rise of water in very narrow tubes. At the top, evaporation from the leaf cells maintains the transpirational pull that keeps the whole column moving.

Fig. 5.9-1 shows all of this on one plant, with nine numbered phenomena marked on it.

Diagram

Fig. 5.9-1 — The Path Of Water Through A Plant And The Forces That Move It Diagrammatic whole-plant section arranged in three bands. Root band: soil level, soil water, a root hair, cortex and xylem, marked 1 for imbibition and 2 for osmosis. Stem band: xylem vessels and tracheids with medullary ray and cortex, upward arrows marked 3, 4 and 5 for the mass movement of water by cohesion, root pressure and capillarity. Leaf band: mid-rib, xylem, palisade parenchyma, spongy parenchyma, intercellular spaces, lower epidermis, with outward arrows marked 6 for stomatal transpiration, 7 for guttation at the leaf margin and 8 for cuticular transpiration; a side branch of the stem marked 9 for lenticular transpiration. Label root, stem, leaf and each of the nine numbered phenomena; straight, non-crossing lead lines.

The nine marks stand for the following.

  1. Imbibition of water into the walls of the root cells.
  2. Osmosis, carrying water into the root cells themselves.
  3. Mass movement of water through the root.
  4. Mass movement of water up the stem.
  5. Mass movement of water into and through the leaf.
  6. Stomatal transpiration.
  7. Guttation from the leaf margin.
  8. Cuticular transpiration.
  9. Lenticular transpiration from the woody stem.

Transpirational Pull And Root Pressure Transpirational pull acts at the top: evaporation from the leaves drags the water column upward. Root pressure acts at the bottom: absorption in the root pushes water up the stem. One pulls, the other pushes, and both move water the same way.

Seen whole, the plant is not a store of water but a channel for it. Water enters at one end, travels as an unbroken column, and leaves at the other — and the loss at the top is what keeps the whole stream in motion.

Stand a white flower with a long stalk, or a stick of celery, in a glass of water coloured with red or blue ink. Leave it in a bright, airy place. Within a few hours the colour reaches the petals or the upper stalk, tracing the very path FC 5.6 sets out. Cut the stalk across afterwards and look for the coloured dots — those are the xylem vessels.

5.10 Summary

  1. Transpiration is the loss of water in the form of water vapour from the leaves and other aerial parts of a plant.

  2. Of the water absorbed by the roots, only about 2 per cent is used in photosynthesis and other activities; almost all the rest is transpired into the air.

  3. Transpiration can be demonstrated by enclosing a well-watered potted plant in a transparent polythene bag tied at the base of the stem. Water vapour from the leaves saturates the trapped air and condenses as droplets inside the bag, while a similar empty bag, kept as the control, stays dry.

  4. Cobalt chloride paper is blue when dry and turns pink on meeting moisture. It is used to confirm that the liquid collected is water.

  5. In the three bell jar experiment, jars A and B, each containing a plant with its pot sealed in polythene, show condensation, and the cobalt chloride paper in B turns pink. Jar C, the control with no plant, shows no change. The set-up gives a double proof of transpiration: condensed droplets and a colour change.

  6. The rate of transpiration is the amount of water a plant loses in a given time. It is measured either by weighing methods or by a potometer.

  7. In the weighing method, a small potted plant with its soil and pot fully covered is weighed before and after a fixed period, and the loss in weight is the water transpired. Volume can be read at the same time from a graduated side tube, 1 cc of water having a mass of about 1 g.

  8. A leafy shoot standing in a test tube of water sealed with a layer of oil loses much less water, because it has no roots to absorb water actively.

  9. A potometer measures the rate of water intake by a plant, which is very nearly equal to the water lost in transpiration but is not the same thing, since some water is used by the cells for other work.

  10. Farmer’s and Ganong’s potometers measure the rate of water intake. Darwin’s potometer demonstrates the suction force created by transpiration, and Garreau’s potometer demonstrates unequal transpiration from the two surfaces of a dorsiventral leaf.

  11. In Ganong’s potometer, transpiration by the twig sets up a suction force that draws water along the apparatus, moving an air bubble down a graduated capillary tube. The distance travelled gives the volume of water taken up. Opening the stop-cock lets water from the reservoir return the bubble to its starting mark.

  12. Precautions with a potometer: the apparatus must be water-tight, and the twig must be cut obliquely and under water. Its limitations are that the bubble is difficult to introduce, the twig may not stay alive long, and changes in air temperature shift the bubble.

  13. There are three kinds of transpiration — stomatal through the stomata of leaves, cuticular through the cuticle of leaves and stems, and lenticular through the lenticels of old woody stems. Most transpiration is stomatal; cuticular transpiration is much less than stomatal but more than lenticular.

  14. A stoma is a minute pore in the epidermis, surrounded by two bean-shaped guard cells. Stomata number about 1,000 to 10,000 per square centimetre and stand open by day primarily to admit carbon dioxide for photosynthesis.

  15. In stomatal transpiration the water vapour travels from the xylem of the veins to the walls of the mesophyll cells, evaporates into the intercellular spaces, diffuses to the sub-stomatal space and escapes through the stoma. The whole movement takes place by diffusion, from higher to lower concentration.

  16. Turgor pressure forces water out of the cell walls into the air spaces. Most of the water replacing it travels along the cell walls by imbibition, and only a small quantity enters the cells by osmosis.

  17. The transpirational pull created by thousands of evaporating leaf cells can draw water up to about 50 metres or more in tall trees.

  18. More transpiration occurs from the under surface of a dicot leaf, because it carries more stomata. This is shown by fixing dry cobalt chloride paper on both surfaces of a leaf that remains attached to its plant: the paper on the lower surface turns pink much faster.

  19. Turgid guard cells keep the stoma open; flaccid guard cells close it and transpiration stops. Only stomatal transpiration is controlled by the plant — cuticular and lenticular transpiration are not.

  20. The leaves of some plants, such as balsam, wilt at midday because the rate of transpiration then exceeds the rate of absorption by the roots. They recover their turgidity in the evening.

  21. The thicker the cuticle, the less the water lost through it. Lenticels, having no guard cells, never close.

  22. The external factors affecting the rate of transpiration are the intensity of sunlight, temperature, velocity of wind, humidity, carbon dioxide level and atmospheric pressure. The rate rises in bright sunlight, high temperature, brisk wind, dry air and low atmospheric pressure. It falls at night, in humid air, and when the carbon dioxide level rises above normal and closes the stomata.

  23. The internal factor is the water content of the leaves. When it falls, the stomata close, transpiration is reduced, and water is conserved within the plant.

  24. Plants of dry climates reduce transpiration by permanent adaptations: sunken stomata, fewer stomata, narrow leaves, reduced exposed surfaces by waviness, rolling or folding, loss of leaves or their change into spines, and a thick cuticle.

  25. Transpiration is significant for the plant in three ways: it cools the leaf surface, it creates the suction force responsible for the ascent of sap, and it distributes water and mineral salts throughout the plant body.

  26. Transpiration is not an excretory process. Excretion is a deliberate active removal of unwanted nitrogenous metabolic wastes, which transpiration does not carry out.

  27. Plants release large quantities of water: about half a litre a day from a full grown sunflower plant, about 2 litres from a maize plant, and about 30 litres from a large apple tree. Over fields and forests, this moisture increases the water vapour in the atmosphere and brings rain.

  28. Evaporation is a fast, purely physical loss of water from the surface of water bodies, governed by temperature and humidity. Transpiration is a slow, vital and partly physical process from the aerial parts of a plant, governed by both internal and external factors.

  29. Exudation is the loss of water or fluid in liquid form, and it occurs in two ways. Guttation is the loss of water as droplets from the margins of leaves through hydathodes, in humid conditions, in plants such as banana, nasturtium and strawberry. Bleeding is the escape of plant sap from a cut or injured surface, assisted by root pressure.

  30. Water follows one continuous path through the plant: soil water, root hair, root cells, xylem of the root, stem and leaf, mesophyll, sub-stomatal space, stoma, atmosphere. It is moved by imbibition and osmosis at the root, by cohesion, root pressure and capillarity along the way, and by transpirational pull at the top.

5.11 Key Terms

  1. Ascent of sap — the upward movement of water and dissolved minerals from the roots to the top of the plant.

  2. Bleeding — the escape of plant sap from a cut or ruptured surface of a plant, caused by injury.

  3. Cobalt chloride paper — filter paper treated with cobalt chloride, which is blue when dry and turns pink when it meets moisture.

  4. Control — an identical set-up with the one factor being tested removed, kept so that the two results can be compared.

  5. Cuticular transpiration — the loss of water vapour directly through the cuticle covering the leaves and stems.

  6. Exudate — the fluid given out by a plant during exudation.

  7. Exudation — the loss of water or other fluids from a plant directly in liquid form rather than as vapour.

  8. Guard cells — the two bean-shaped cells surrounding a stoma, which open and close the pore.

  9. Guttation — the loss of water as liquid droplets from the margins of the leaves of certain plants, through hydathodes.

  10. Hydathodes — pore-bearing structures at the endings of veins on a leaf margin, through which guttation takes place.

  11. Lenticels — permanently open pores on the bark of older stems, allowing the diffusion of gases and some water loss.

  12. Lenticular transpiration — the loss of water vapour through the lenticels of old woody stems.

  13. Potometer — an apparatus that measures the rate at which a plant takes in water.

  14. Rate of transpiration — the amount of water a plant loses by transpiration in a given time.

  15. Stoma (plural: stomata) — a minute pore in the epidermis of a leaf, bounded by two guard cells.

  16. Stomatal transpiration — the loss of water vapour from the leaves through the stomata.

  17. Transpiration — the loss of water in the form of water vapour from the leaves and other aerial parts of a plant.

  18. Transpiration stream — the continuous flow of water from the soil, through the roots and stem to the leaves, and out into the air as vapour.

  19. Transpirational pull — the upward pull on the water column in the xylem, created by evaporation of water from the leaf cells.

5.12 Points To Ponder

  1. A plant opens its stomata to let carbon dioxide in, and loses water through the very same pores. Suppose a plant appeared whose stomata admitted carbon dioxide but let no water vapour out. What would it gain — and what three things established in this chapter would it lose?

  2. A potometer measures the water a shoot takes in, while what we really want to know is the water it gives out. Under what conditions would the difference between those two quantities be at its largest, and under what conditions would the potometer reading be closest to the truth?

  3. A dicot leaf lies flat, with its lower surface shaded, and carries far more stomata below than above. A grass leaf stands almost upright, so that both its surfaces meet sunlight and moving air about equally. What would you predict about the distribution of stomata on a grass leaf, and what observation would test your prediction?

  4. Transpirational pull can raise water about 50 metres or more without the plant spending any energy on the lifting. Since the pull costs nothing, what do you think sets the upper limit to the height of a tree — the leaf, the stem, or the water column itself?

  5. Guttation is often treated as a curiosity. But consider a plant growing in warm humid air whose roots keep absorbing water while its stomata are useless. What might begin to go wrong inside such a plant if it had no hydathodes at all?

  6. A forest returns to the air a large part of the water that later falls on it as rain. If a wide stretch of that forest were cleared, what would you expect to happen to rainfall over the forest that remains — and what kind of measurements, taken over how long, would be needed before anyone could claim to have settled the question?

5.13 Objective Questions

Multiple Choice
  1. Most of the water absorbed by a plant from the soil is: (1 mark)
    1. used up in photosynthesis
    2. stored in the cells of the stem
    3. lost to the air as water vapour from the aerial parts
    4. converted into cell sap
  2. Dry cobalt chloride paper is fixed to both surfaces of a leaf that is still attached to its plant. The paper on the lower surface turns pink much sooner because that surface has: (1 mark)
    1. a thicker cuticle
    2. a greater number of stomata
    3. more chloroplasts
    4. a larger number of lenticels
  3. The distance travelled by the air bubble in Ganong’s potometer is a measure of: (1 mark)
    1. the water lost by the twig in transpiration only
    2. the water taken up by the cut shoot
    3. the water used by the twig in photosynthesis
    4. the rate of respiration of the twig
  4. Which one of the following conditions will lower the rate of transpiration? (1 mark)
    1. A brisk, dry wind
    2. A rise in the humidity of the surrounding air
    3. A rise in the temperature of the air
    4. A fall in atmospheric pressure
  5. Lenticular transpiration cannot be regulated by the plant because lenticels: (1 mark)
    1. occur only on leaves
    2. are lined with chloroplasts
    3. have no guard cells and stay open all the time
    4. lose water only in the form of droplets
Assertion And Reason

The following questions are assertion–reason based. Choose the answer from the codes given below.

  1. A is true and R is false
  2. A is false and R is true
  3. Both A and R are true
  4. Both A and R are false
  1. Assertion (A): More transpiration occurs from the under surface of a dicot leaf. Reason (R): The under surface of a dicot leaf bears a larger number of stomata. (1 mark)
  2. Assertion (A): The twig used in a potometer is cut under water. Reason (R): An air bubble drawn into the cut end would block the xylem and stop the absorption of water. (1 mark)
  3. Assertion (A): Guttation is commonly seen in plants growing in warm, humid surroundings. Reason (R): High humidity in the air raises the rate of transpiration from the leaves. (1 mark)
  4. Assertion (A): Desert plants commonly have a thick cuticle over their leaves. Reason (R): The greater the thickness of the cuticle, the lesser the evaporation from the leaf surface. (1 mark)
Match The Following
  1. Match the terms in Column A with the descriptions in Column B. (4 marks)
Column A Column B
a. Guard cell i. Waxy layer secreted by the epidermis
b. Hydathode ii. Opening that can never be closed
c. Lenticel iii. Pore lying at the ending of a leaf vein
d. Cuticle iv. Controls the size of the stomatal pore
Fill In The Blanks
  1. Fill in each blank with one suitable word. (4 marks)
    1. The pull that draws water up the xylem of a tall tree is called the ______.
    2. A potometer measures the rate of ______ by a cut shoot.
    3. Water lost as droplets from the margin of a leaf passes out through pores called ______.
    4. Dry cobalt chloride paper changes from blue to ______ in the presence of moisture.
True Or False
  1. State whether each statement is true or false. (4 marks)
    1. Lenticular transpiration accounts for the largest share of the water lost by a plant.
    2. A leaf must be plucked from its plant before cobalt chloride paper is fixed to its two surfaces.
    3. Evaporation of water from a leaf surface lowers the temperature of that surface.
    4. The stomata of most plants remain closed at night.
Name The Following
  1. Name the following. (4 marks)
    1. The waxy layer secreted by the epidermis over the surface of a leaf.
    2. The apparatus used to measure the rate of water intake by a cut shoot.
    3. The air-filled space lying immediately inside a stoma.
    4. The fluid that oozes out of a cut or injured part of a plant.
Odd One Out
  1. Pick out the odd term in each group and give a reason for your choice. (4 marks)
    1. Stoma, lenticel, hydathode, chloroplast
    2. Humidity, velocity of wind, number of stomata, atmospheric pressure

5.14 Check Your Concepts

Two-Mark Questions

  1. Define transpiration and state the form in which water leaves the plant during it. (2 marks)

  2. Give a reason why the soil surface and the pot must be covered before a potted plant is weighed to measure its water loss. (2 marks)

  3. State any two limitations in the use of a potometer. (2 marks)

  4. Why does a leafy shoot standing in a test tube of water lose far less water than a rooted plant of the same size? (2 marks)

Three-Mark Questions

  1. Differentiate between stomata and lenticels, stating the basis of difference in each case. (3 marks)

  2. Explain why the leaves of a balsam plant droop at midday and stand erect again by the evening. (3 marks)

  3. Name the three kinds of transpiration and arrange them in order of the quantity of water they account for, beginning with the greatest. (3 marks)

  4. Why is transpiration not regarded as an excretory process? (3 marks)

Four-Mark Questions

  1. Describe, step by step, how water reaching a leaf vein finally leaves the plant as water vapour. (4 marks)

  2. Give a reason for each of the following. (4 marks)

    1. Transpiration increases with the velocity of the wind.
    2. A plant with sunken stomata loses less water than one with stomata level with the surface.
    3. Transpiration falls when the surrounding air is humid.
    4. Stomatal transpiration can be controlled by the plant, while cuticular transpiration cannot.

  3. Differentiate between transpiration and evaporation on the basis of the surface involved, the control exercised over the process, and its speed. (4 marks)

Five-Mark Questions

  1. ‘A plant depends on the very water it loses.’ Justify this statement by explaining the three main advantages of transpiration to a plant. (5 marks)

  2. A well-watered potted plant is kept inside a closed glass chamber in which the air has been made fully saturated with water vapour. Explain what would happen to each of the following, giving a reason in each case. (5 marks)

    1. The rate of transpiration from the leaves
    2. The absorption of water by the roots
    3. The turgidity of the leaf cells
    4. The chance of droplets appearing along the leaf margins

  3. Describe any five permanent adaptations by which plants of dry climates reduce excessive transpiration, explaining how each one works. (5 marks)

5.15 Figure-Based Questions

  1. Study Fig. 5.4-1, the surface view of a single stoma, and answer the following. (4 marks)
    1. Name the two cells that enclose the pore.
    2. State one way in which these two cells differ from the other epidermal cells around them.
    3. What happens to the pore when these two cells lose water?
    4. Name the layer of the leaf in which this structure is found.
  2. Fig. 5.3-3 shows Ganong’s potometer. (4 marks)
    1. Name the parts labelled reservoir and stop-cock, and state the use of each.
    2. What does the movement of the air bubble along the graduated tube indicate?
    3. State one precaution to be taken while fixing the twig into the apparatus.
    4. Why does the reading of this apparatus not give the exact quantity of water transpired?
  3. Fig. 5.4-4 shows a leaf with cobalt chloride paper fixed to both its surfaces. (3 marks)
    1. Name the process being investigated.
    2. Which of the two papers turns pink faster, and why?
    3. Why must the leaf remain attached to its plant throughout the experiment?
  4. Fig. 5.6-1 shows a portion of a Nerium leaf in section. (3 marks)
    1. Name the adaptation shown by the position of the stomata.
    2. Explain how this position reduces the loss of water.
    3. Name one other feature visible in the figure that also reduces water loss.
  5. Redraw a neat, labelled vertical section of a leaf, based on Fig. 5.4-2, to show the escape of water vapour from the leaf. Label the upper epidermis, spongy mesophyll, intercellular space, sub-stomatal space and stoma, using straight, non-crossing lead lines. (5 marks)