Transpiration Class 10 ICSE — Selina Chapter 5

Transpiration class 10 ICSE appears in Chapter 5 of Selina Concise Biology. This chapter explains how plants lose water as vapour from their leaves and stems. It covers the demonstration experiments, the potometer, the three kinds of transpiration, the factors that affect it, and its significance. The chapter also covers guttation and bleeding as direct water losses.

Download Transpiration Class 10 ICSE Biology Chapter 5 PDF

Download the Transpiration Class 10 ICSE Biology chapter PDF from the Selina Concise textbook to revise offline or print for practice. It is the full chapter as printed in the book — 12 pages, 10 sections, 45 questions.

What is inside Chapter 5

12 pages · 10 sections · 45 exercise questions

• PROGRESS CHECK: 3 questions
• PROGRESS CHECK: 4 questions
• PROGRESS CHECK: 2 questions
• PROGRESS CHECK: 36 questions

5.1 TRANSPIRATION

Transpiration is the evaporative loss of water as water vapour from the aerial parts of a plant. This ICSE chapter states the definition in two wordings. The first says it is the evaporative loss from the leaves and stem. The second says it is the loss of water vapour from the leaves and aerial parts. Both describe the same process.

Plants absorb water through their roots continuously. This water is conducted upwards through the stem. It is distributed to all aerial parts, including the leaves. Only a small quantity, about 2%, is used by the plant. The rest is almost entirely lost to the atmosphere as water vapour during transpiration.

The 2% that stays supports photosynthesis and other activities. The remaining 98% escapes as vapour. This makes transpiration the dominant route for water leaving a plant. It also creates the suction force that pulls water upward. That link between root absorption and transpiration is explained in the chapter on absorption by roots in this book.

5.2 DEMONSTRATION OF TRANSPIRATION

Two classroom experiments demonstrate transpiration. Both give visual proof that plants release water vapour.

Experiment 1 uses a well-watered potted plant. Cover it with a transparent polythene bag. Tie the mouth around the base of the stem. Leave it in sunlight for an hour or two. Drops of water appear on the inner side of the bag. The vapour from the leaves saturates the air inside. It condenses when the outside temperature is cool enough. An empty tied bag shows no drops. This control proves the water comes from the plant.

Test the drops with dry cobalt chloride paper. The paper turns from blue to pink. This confirms the drops are water. Cobalt chloride paper is an indicator of moisture. It is blue when dry and pink when exposed to moisture.

Experiment 2 uses three bell jars. Jar A holds a plant with the pot sealed. Jar B holds a plant plus dry cobalt chloride paper. Jar C is empty except for the paper. After half an hour in the sun, jars A and B show condensation. The paper in B turns pink. The paper in C stays blue. Jar C is the control. It proves no moisture came from the air itself.

The double proof is condensation plus the colour change. Together they confirm transpiration beyond doubt.

5.3 MEASUREMENT OF TRANSPIRATION

Two main methods measure transpiration. This ICSE chapter covers both. The weighing method measures weight loss. The potometer method measures water uptake.

In the weighing method, weigh a small potted plant. Cover the soil and pot first. This stops evaporation from surfaces other than the plant. Weigh again after a set time. The loss in weight equals water lost by transpiration. One cubic centimetre of water weighs one gram. So weight loss converts directly to volume loss.

An improved set-up uses a glass bottle with a graduated side tube. A leafy shoot stands in water. An oil layer on top stops evaporation. The falling water level shows the volume lost.

The potometer measures water intake. The word comes from “poton”, meaning drink, and “meter”, meaning measure. Several types exist. Farmer’s and Ganong’s potometers measure water intake. Darwin’s potometer shows suction force. Garreau’s potometer shows unequal transpiration from the two leaf surfaces.

Ganong’s potometer is the common one. A twig is cut obliquely and under water. The apparatus is filled so no air spaces remain. An air bubble sits in a graduated capillary tube. As the twig transpires, suction pulls water from the beaker. The bubble moves along the tube. The reading gives the water taken up.

Worked example: the bubble moves 6 cm in one hour. The capillary tube has a cross-section of 0.1 cm². The volume of water taken up is:

\text{Volume} = 6\ \text{cm} \times 0.1\ \text{cm}^2 = 0.6\ \text{cm}^3 = 0.6\ \text{mL}

So the shoot took up 0.6 mL in one hour. A potometer measures uptake, not loss. Some water is used by the cells. It does not all leave as vapour. Three limitations apply. Introducing the bubble is not easy. The twig may not stay alive long. Temperature changes shift the bubble.

5.4 KINDS OF TRANSPIRATION

Transpiration from the aerial parts of a plant occurs from three different regions. Each region contributes a different share of the total water loss.

The first region is the leaves, through the stomata. This is called stomatal transpiration. The second region is the direct surface of leaves and stems. This is called cuticular transpiration. The third region is the lenticels. These are minute openings on the surface of old stems. This is called lenticular transpiration.

The major part of transpiration occurs through the stomata. The other two types contribute very little. This ordering matters for experiments. It explains why stomata get the most attention in this chapter. It also explains why the lower surface of a dicot leaf loses more water. That surface carries more stomata. The next three subsections examine each kind in turn.

5.4.1 Mechanism of stomatal transpiration

Stomata are minute openings in the epidermal layer of leaves. Each stoma is surrounded by two bean-shaped guard cells. Their number ranges from 1,000 to 10,000 per square centimetre. During the day they open wide. This allows carbon dioxide in for photosynthesis. It also allows water vapour out.

The path of water vapour follows a set route. Water reaches the leaf tissues through veins. Spongy mesophyll cells have surfaces exposed to intercellular spaces. These surfaces give out a thin film of water. The film evaporates and saturates the air in the spaces. The vapour diffuses through connecting spaces. It reaches the sub-stomatal space. From there it escapes through the stoma.

Diffusion drives this movement. Water vapour moves from higher concentration to lower concentration. The inside of the leaf has a higher concentration than the outside air.

Most water travels along the cell walls. This movement is by imbibition. Only a small quantity enters cells by osmosis. The transpiration pull can draw water up 50 metres or more in tall trees.

More transpiration occurs from the lower surface of a dicot leaf. That surface has more stomata. An experiment proves this. Attach dry cobalt chloride paper to both surfaces of a leaf. Hold it with glass slides. The leaf stays attached to its plant. The paper on the lower surface turns pink faster.

Guard cells regulate the stomata. Turgid guard cells keep them open. Flaccid guard cells close them. Some plants wilt at midday. Balsam is one example. Transpiration then exceeds absorption. Cells lose turgidity. In the evening, stomata constrict and the leaves recover. The link between stomatal opening and photosynthesis is covered in the next chapter.

5.4.2 Cuticular transpiration

The cuticle is a waxy layer. The epidermis secretes it on both surfaces of the leaf. Its thickness varies from plant to plant.

The cuticle’s primary job is to prevent evaporation. It covers the leaf surfaces. Some evaporation still occurs through it. The rule is simple: the greater the thickness of the cuticle, the lesser the transpiration. This is why desert plants have thicker cuticles. They need to cut down water loss in dry conditions.

Cuticular transpiration is therefore small. It is a passive loss through a protective layer. The plant cannot regulate it. Unlike stomatal transpiration, it has no guard cells to control it. It continues as long as water is available in the leaf.

5.4.3 Lenticular transpiration

Lenticels are special openings on older stems. They develop in place of stomata. They allow diffusion of gases. This supports respiration and photosynthesis. Lenticels never close. They remain open all the time.

Water from cell surfaces facing a lenticel evaporates. This water is lost as lenticular transpiration. The amount is more than cuticular transpiration. It is much less than stomatal transpiration.

Lenticels are visible on many tree trunks. They appear as small raised pores. Because they never close, water loss through them continues day and night. The plant cannot regulate this loss. In young herbaceous plants, lenticels are few or absent.

5.5 FACTORS WHICH AFFECT TRANSPIRATION

Six external factors affect transpiration in the Class 10 ICSE syllabus. The first is sunlight intensity. Stomata open by day for carbon dioxide intake. They close at night. Cloudy days partially close them. This reduces transpiration.

The second factor is temperature. Higher temperature increases evaporation. Warm air holds more water than cold air. The third is wind velocity. Faster wind removes water vapour quickly. The air outside the leaf does not get saturated. Transpiration rises.

The fourth is humidity. Humid air slows outward diffusion. Transpiration falls. The fifth is carbon dioxide. Above the normal 0.03%, it causes stomatal closure. Transpiration decreases. The sixth is atmospheric pressure. Lower pressure increases transpiration. It enhances diffusion of water vapour.

To remember these six, use the first letters. Sunlight, Temperature, Wind, Humidity, Carbon dioxide, Atmospheric pressure. That spells S T W H C A. One mnemonic is: Sally Took Water Home, Clouds Arrived.

One internal factor matters. It is the water content of the leaves. Low water content causes wilting. Stomata close. Transpiration falls. This is a natural conserving mechanism.

Plants also carry permanent adaptations to reduce transpiration. Sunken stomata appear in Nerium. Stomata may be fewer. Leaves may be narrow. They may be wavy, rolled, or folded. Some plants drop leaves or turn them into spines, as in cacti. A thick cuticle covers Banyan and most evergreens.

5.6 SIGNIFICANCE OF TRANSPIRATION

Transpiration has three main advantages for the plant. The first is cooling. Evaporation reduces the temperature of the leaf surface. This matters on hot sunny days. Intense heat destroys enzymes.

The second is suction force. Transpiration produces a pull from the top of the plant. Evaporation concentrates cell sap. This increases osmotic pressure. Water is drawn from lower cells in sequence. This drives the ascent of sap. It also favours absorption of water by the roots.

The third is distribution. Leaves sit at the tips of branches and twigs. Transpiration draws water towards them. Water and mineral salts spread throughout the plant. Higher transpiration means higher absorption of water and solutes.

Is transpiration excretion? No. Excretion is a deliberate active process. It removes unwanted metabolic waste. Transpiration is not that. It is a physical loss of water driven by diffusion and evaporation.

Transpiration affects climate. A sunflower loses about half a litre of water a day. A maize plant loses about 2 litres. A large apple tree loses about 30 litres. Forests release huge amounts of water vapour. This increases atmospheric moisture. It brings rain. You can verify the Class 10 Biology syllabus on the official CISCE website.

Evaporation and transpiration differ. Evaporation is physical, fast, and from water bodies. Transpiration is vital and partly physical, slow, and from aerial plant parts.

5.7 DIRECT LOSS OF WATER BY PLANTS — GUTTATION AND BLEEDING

Some plants lose water directly as liquid. This ICSE chapter covers two such ways: guttation and bleeding.

Guttation means droplets along leaf margins in the morning. It happens in warm humid conditions. Humidity blocks transpiration. Roots keep absorbing water. Hydrostatic pressure builds inside the plant. Water is forced out through hydathodes. These are pore-bearing structures at the vein tips. Guttation is common in banana, nasturtium, and strawberry.

Bleeding happens only due to injury. Plant sap escapes from cut or ruptured surfaces. Root pressure assists this escape.

Three common mistakes need correction. First, transpiration is not excretion. It is a physical loss, not waste removal. Second, guttation drops are not dew. They contain mineral salts and come from inside the leaf. Dew condenses from outside air. Third, a potometer measures water uptake, not water lost. Some water is used by cells.

What to practise from Transpiration Class 10 ICSE

Work through the Progress Check blocks first. Then attempt the five question sets at the end of Section 5.7.

• Progress Check after Section 5.3: 3 questions
• Progress Check after Section 5.4.3: 4 questions
• Progress Check after Section 5.5: 2 questions
• Progress Check after Section 5.7: 36 questions
• Set A, Multiple Choice Type: 9 questions
• Set B, Very Short Answer Type: naming and fill-in-the-blanks
• Set C, Short Answer Type: give reasons, true or false, differentiate
• Set D, Long Answer Type: 11 questions
• Set E, Structured/Application/Skill Type: 6 diagram-based experiments

Set E asks you to label diagrams. One shows a stomatal apparatus from a dicot leaf. Another shows a Ganong’s potometer with its air bubble and reservoir. A third shows Garreau’s potometer with calcium chloride vials and mercury manometers. A fourth shows a vaseline-coated leaf experiment. Practise redrawing these from memory.

Transpiration Class 10 ICSE frequently asked questions

What is transpiration in class 10 ICSE Biology?

Transpiration is the loss of water as water vapour from the leaves and aerial parts of a plant. It is an evaporative process driven by diffusion. Only about 2% of absorbed water is used by the plant. The rest is lost through transpiration.

Why does more transpiration occur from the lower surface of a dicot leaf?

The lower surface of a dicot leaf has more stomata than the upper surface. More stomata means more openings for water vapour to escape. The cobalt chloride paper experiment demonstrates this. The paper on the lower surface turns pink faster.

What is the difference between transpiration and guttation?

Transpiration loses water as vapour through stomata, lenticels, and cuticle. Guttation loses water as droplets through hydathodes. Transpiration water is pure. Guttation water contains mineral salts. Guttation occurs at night or early morning in humid conditions. Transpiration occurs in daylight in dry conditions.

How does a potometer measure the rate of transpiration?

A potometer measures water uptake by a cut shoot. As the shoot transpires, suction pulls water from the beaker. An air bubble in the capillary tube moves along. The distance it moves in a set time gives the water uptake. This is nearly equal to the water lost.

Why do leaves of some plants wilt at midday during transpiration even when the soil is well watered?

At midday, the rate of transpiration exceeds the rate of water absorption. Cells lose turgidity and the leaves droop. In the evening, transpiration falls. Stomata constrict. The cells regain turgidity and the leaves stand erect again.

Reference: ICSE Class 10 Biology textbook material, chapter 5: Transpiration — CISCE-aligned.

More from this book

• Chapter 2: Structure of Chromosomes Cell Cycle and Cell Division
• Chapter 3: Genetics Some Basic Fundamentals
• Chapter 4: Absorption By Roots the Processes Involved
• Chapter 6: Photosynthesis
• Chapter 7: The Circulatory System
• Chapter 8: The Excretory System
• Chapter 9: The Nervous System
• Chapter 10: Endocrine Glands
• Chapter 11: The Reproductive System
• Chapter 12: Population
• Chapter 13: Aids to Health
• Chapter 14: Health Organisations
• Chapter 15: Pollution