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1




TRANSPIRATION

Basic Concepts :

*) Transpiration : Transpiration is the loss of water in the form of water

vapours from the leaves and other aerial parts of the plant.

*) Measurement of transpiration :

Potometer Method: Potometer is a device which is used to measure

the rate of transpiration

Ganong’s potometer :

Precautions in the use of potometer :

i)The potometer should be water tight.

ii)The twig should be cut obliquely ( to allow larger surface for the water

intake) and under water to avoid suction of an air bubble into the twig

which will stop the absorption of water into the xylem.

Limitations in the use of potometer :

i) Introducing the sir bubble is not very easy.

ii) The twig may die.

iii)Any changes in the outside air temperature may affect the position of

the air bubble in the capillary tube.

2




TYPES OF TRANSPIRATION

StomatalTranspiration

CuticularTranspiration

LenticularTranspiration

i) Stomatal Transpiration : Occurs through stomata Which are minute

openings in the epidermal layer of leaves.

ii) Cuticular Transpiration : Occurs through cuticle Which is a waxy layer

secreted by the epidermis on the two surfaces of the leaf. The The

greater the thickness of the cuticle, the lesser is the transpiration.

iii) Lenticular Transpiration : Occurs through Lenticels which are special

openings that develop on the barks of older stems in the place of

stomata. Lenticels never close. They remain open all the time.

The amount of transpiration from lenticels is certainly more than the

cuticular transpiration, but very much less than the stomatal transpiration.

i.e,

Stomatal > Lenticular > Cuticular

3




*) Factors that affect Transpiration :

A. EXTERNAL FACTORS :

1. Intensity of sunlight : Rate of transpiration increases with the increase

in the intensity of sunlight.

2. Temperature : Rate of Transpiration increase with temperature.

3. Velocity of wind : Transpiration increases with the velocity of wind.

4. Humidity : Transpiration is reduced if the air outside is humid.

5. Carbon Dioxide : Increase in CO2 results in less Transpiration.

6. Atomoshperic Pressure : Increase in Atmospheric pressure results in less Transpiration.

B. INTERNAL FACTORS :

1. Water content of the leaves : If there is shortage of water content in

the plant the leaves will wilt and transpiration is reduced due to

closure of stomata.

4




*) Adaptations in plants to reduce excessive Transpiration.:

1) Sunken Stomata/ covered with hairs.

2)Fewer stomata

3)Narrow leaves to reduce surface area.

4)Reduced exposed surface , rolled , folded leaves.

5)Loss of leaves

6)Thick cuticle formation.

*) Significance of Transpiration :

1)Cooling effect

2)Creating suction force to draw water.

3)Distribution of water and minerals.

Transpiration Increases the moisture in the atmosphere and brings rain.

*) Guttation – Guttation is the loss of water as droplets along the margin

of leaves trough hydathodes in some plants Like banana.

*) Bleeding – Bleeding is the direct flowing out of plant sap from any cut

surfaces in case of injury.

5




Give Reasons :

1. Leaves of the plants wilt during the hot day.

Ans> When the surrounding temperature is high , the rate of transpiration

increases; But the rate of absorption of water & minerals from the root

by the plants remain the same as a result, The rate of transpiration

exceeds the rate of Absorption which creates a scarcity of water within

the body of plants. Hence Plants generally wilt during hot day.

2. Stomata are more numerous on the lower surface of dorsiventral leaf.

Ans> All surfaces of the leaf have some amount of stomata for regulating

gas exchange for photosynthesis. However, The lower epidermis has

more stomata because it is more often in the shade and so it is cooler,

which means evaporation won’t take as much.

3. Leaves sometimes drip water at night.

Ans> The stomata remains closed during night , so no stomatal

Transpiration occurs during Night. Though Lenticular Transpiration

occurs During night but it’s not enough to release the extra water

intaked by the plant, So this extra water escapes from small pores

known as hydathodes. Water is released in the form of droplets. Thus,

leaves Sometimes drip water at night.

4. During The day Transpiration and Photosynthesis are interlinked.

Ans> In the daytime, Stomata are open to facilate the inward diffusion of

carbon dioxide for photosynthesis and water vapours are also lost to

the outside during transpiration. So, Stomatal Transpiration can take

place and Carbon dioxide can also enter the green leaf to enable

photosynthesis to take place. Thus, Transpiration And Photosynthesis

are interlinked.

6




5. While setting up the potometer, The leafy shoot should be cut under

water.

Ans> While setting up the potometer, the leafy shoot must be cut under

water because if air gets into the xylem vessels of the plants, it can

form air locks which will prevent the plant taking up water.

6. Forests tend to bring more rainfall.

Ans> Transpiration helps in bringing more rain because more the number

of trees in a particular locality implies more humidity which on

condensation the upper layer of atmosphere precipitates down as

rain.

Name the following.:

1. Openings on the stem through which transpiration occurs. – Lenticels

2. The process by which the intact plant loses water in the form of

droplets- Guttation

3. An instrument used to find the rate of transpiration – Potometer

4. A plant in which the stomata are sunken. – Nerium

5. The apparatus to record the rate of transpiration in a cut shoot. –

Ganong’s Potometer.

6. Any two parts of a leaf which allows transpiration. – Stomata, cuticle

7. The structure in a leaf that allows guttation. – Hydathodes

8. Loss of water as droplets from the margin of certain leaves. –

Exudation/ Guttation.

7




Review questions.

1. What do you mean by transpiration?

2. Define Stomatal & cuticular transpiration.

3. Write a short note on lenticular transpiration.

4. Write the factors and conditions affecting transpiration.

5. What are the adaptations In the plants to reduce transpiration?

6. What is potometer? Write its limitations.

7. Write any four Significance of transpiration.

8. Write any two disadvantages of transpiration.

9. What is the transpiration stream?

10. Distinguish between transpiration & guttattion.

11. Given below is the diagram of an experimental set-up to study the

process of transpiration in plants. Study the same and then answer

the questions that follow:

a) What is the colour of dry cobalt chloride paper?

b) Is the experimental leaf a monocot or a dicot? Give a reason to

support your answer.

c) Why are the glass slides placed over the dry cobalt chloride paper?

d) After about Half an hour what change, if any, would you expect to find

in the cobalt chloride paper placed on the dorsal and ventral sides of

the leaf? Give a reason to support your answer.

8




Answers:

1. Transpiration is the loss of water in the form of water vapours from

the leaves and other aerial parts of the plant.

2. Stomatal transpiration : The epidermis of the leaf has numerous pores

called stomata. These pores remain open during the day so that CO2

of the air can diffuse in an enable the plant to prepare food by

photosynthesis. Simultaneously the excess water in the form of Water

vapour is screted out through Stomata. This process is known as

Stomatal Transpiration.

Cuticular Transpiration : The epidermis of the leaves secrete a thin

layer of waxy, known as cuticle. The cuticle is not very permitable to

water However, Its molecules absorb water from the epidermal cells

by imbibition. The imbibed water is slowly Lost to the atmosphere.

This is known as cuticular Transpiration.

3. Stems of the woody plants have openings on their surface called

lenticels. Transpiration continues throughout day and night through

this lenticels since they are always open and the Water vapour easily

escapes through the loose mass of cell of each lenticel. This is known

as lenticular transpiration.

4. Intensity of sunlight : Rate of transpiration increases with the

increase in the intensity of sunlight.

Temperature : Rate of Transpiration increase with temperature.

Velocity of wind : Transpiration increases with the velocity of wind.

Humidity : Transpiration is reduced if the air outside is humid.

Carbon Dioxide : Increase in CO2 results in less Transpiration.

Atomoshperic Pressure : Increase in Atmospheric pressure results in less

Transpiration.

9




5. i) Sunken Stomata/ covered with hairs.

ii)Fewer stomata

iii)Narrow leaves to reduce surface area.

iv)Reduced exposed surface , rolled , folded leaves.

v)Loss of leaves

vi)Thick cuticle formation.

6. Potometer Method: Potometer is a device which is used to measure the

rate of transpiration

Limitations in the use of potometer :

i) Introducing the sir bubble is not very easy.

ii) The twig may die.

iii)Any changes in the outside air temperature may affect the position of

the air bubble in the capillary tube.

7. i)Removal of excess water from the plants

ii) Cools the plants

iii)Cools the enviroment.

iv)Prevents the plant from drying up.

8. Two disadvantages of transpiration are:

i) Plants often wilt and die when excessive transpiration takes

place,that is , when the rate of transpiration through leaves exceeds

absorption of water through the roots.

ii) Since most of the water absorbed is lost by transpiration, plants have

to work more to compensate the loss.

9. Tanspiration stream is the path of movement of mineral sap through the

vessels and tracheids of xylem tissue, Due to transpiration.

10




10.

Transpiration Guttation

i)Water is lost in the form of

vapour.

ii)It cools the plant body.

iii)It occurs in the presence of

sunlight.

iv)It occurs during dry

conditions.

i)Water is lost in the form of

water droplets.

ii)It has no such effect on the

plant.

iii)It generally occurs at night.

iv)It occurs in humid conditions.

11. a) The colour of dry cobalt chloride paper is Blue.b) The experimental leaf is a dicot leaf because the experimental leaf

have sub-veins.c) The glass slides are placed over the dry cobalt chloride paper

because to block the Atmospheric water vapour to come in contact with the cobalt chloride paper. If the Cobalt chloride paper comes in contact with Atmospheric water vapour then the reading would be inaccurate.

d) After half an hour we will observe that the cobalt chloride paper which is placed on the dorsal side of the leaf will be coloured more deep pink than that of the Ventral side because there are more stomata in the dorsal side than there are in ventral side so more transpiration occurs in dorsal side than ventral side thus the cobalt chlorde paper turns pink more faster than of ventral side.

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