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When water is lost by evaporation (transpiration) heat energy is taken up from the plant body to vapourize the water, here the water is rapidly taken up by the roots and again released through the leaves by transpiration. In the air above, the moisture forms clouds and soon falls as rain.

Forests have large number of plants especially trees. Each plant loses water in the form of water vapour everyday into the atmosphere through transpiration. A large apple tree loses as much as 30 litres of water per day. So huge amount of water is escaped into the atmosphere by forests. This increases the moisture in the atmosphere and brings more frequent rains.

The factors that accelerate the rate of transpiration are:

(i) High intensity of sunlight

(ii) High temperature

(iii) Higher wind velocity

(iv) Decrease in atmospheric pressure

(Any three)

Guttation: It is the process in which water droplets oozes out from the hydathodes present at the tip and margins of leaves.

Bleeding: It is the process in which plant sap oozes out through injured or cut ends of the plant.

Stomata: Stomata pass out water in the form of vapours.

Hydathodes: Hydathodes send out water in the form of droplets.

(b) Soil is wet and the atmosphere is humid

Cobalt chloride paper: It is used for the process of transpiration to check the presence of water.

Goat’s Bladder: It is used for the process of osmosis as semipermeable membrane.

(b) The turgid and flaccid state of the guard cells

Cobalt chloride paper

Guttation excess of water is forced out directly from the tips of veins in the leaf.

No, they are not dew drops.

This is water given out by the plant body through guttation. Since the banana plant is growing in humid environment, transpiration is hampered. But the roots continue to absorb water from the soil. This builds up a huge hydrostatic pressure within the plant and forces out the excess water from the hydathodes, which are pores present at the tips of veins in the leaf. This is observed especially during the mornings.

(i) The leaf D would become most limp. This is because water would be lost through transpiration from upper as well as the lower surface of leaf D since it is uncoated. 

(ii) The least limping would be shown by leaf C since its upper and lower surfaces have been coated with vaseline. So no water is lost from the leaf through transpiration since the stomatal openings get blocked by vaseline.

(a) Lenticels

(b) Guttation

(c) Potometer

(d) Nerium

(e) Ganong's photometer

(f) Stomata and cuticle

(g) Hydathodes

(h) Guttation

(i) The CaCl₂ vials are kept inside the cup to absorb water.

(ii) After few hours, the weight of the CaCl₂ vials will increase because they will absorb the water which is transpired by the leaf of the plant.

(iii) Manometers are used to indicate the unequal transpiration from two surfaces of a dorsiventral leaf by showing difference in rise in their mercury levels.

(iv) Transpiration is the loss of water in the form of water vapour from the aerial parts of the living plants.

Opening found on the under surface of dorsiventral leaf is Stomata.

(a) 

(i) False 

(ii) True 

(iii) True 

(iv) False

(b) (i) Most transpiration occurs at mid-day. 

(iv) Potometer is an instrument used for measuring the rate of transpiration in green plants.

The loss of water due to transpiration tends to lower the concentration of water in the cell sap. Thus, the root hair with its semi-permeable membrane and hypertonic cell sap establish an osmotic system with the water available in the soil surrounding the root hair. This is how the two processes are interrelated.

When cobalt chloride paper is placed on lower and upper surface of the leaf, the paper of Lower side becomes more pink.

At the time of opening of stomata, the guard cells are in the turgid state.

Lower side of the leaf has more stomata.

(a) Loss of water vapours from leaves 

The plants absorb much more water than their requirement because:

(a) They require more mineral salts for their growth and minerals are present in very small quantity in water

(b) Most of the water is lost through transpiration. Therefore, to prevent wilting they require excess of water.

Because of their small size, guard cells are rapidly influenced by turgor changes. Because of their kidney shaped outline, they remain joined at their ends. This concave-convex curvature of the two guard cells can be varied easily and this further facilitates the stomatal pore to open and close easily.

Two equal pieces of cobalt chloride paper are dried and placed one on the upper surface and the other on the lower surface of dorsiventral leaves. They are firmly held in place by pieces of cellotapes or glass slides. After a few minutes the cobalt chloride paper shows change in colour from blue to pink. The time taken for the cobalt chloride paper on the upper surface of the leaf to turn pink is much longer than for the under surface. Thus, it proves that the rate of transpiration is greater from the under surface of dorsiventral leaves.

Older portions of the root do not have root hairs. They are covered with a protective layer of dead calls having tiny openings called lenticels. Through these lenticels, gaseous exchange occurs between the soil and inner living cells. If the roots of land plants remain waterlogged, then lenticels will not be able to do gaseous exchange and as a result of this plants will die.

The lower surface of leaf is sheltered from direct sunlight. If more stomata are on the upper surface of a leaf, then excessive transpiration would occur, resulting in quick wilting of the plant. Hence most plants have more numerous stomata on the lower surface of a leaf to control the rate of transpiration.

The stomata close and there is a general reduction in the loss of water by the leaves due to transpiration.

During day time transpiration rate is very high. As a result to excessive transpiration and the seedling will wilt and ultimately die. However in the evening, transpiration rate is very slow and the seedling will be able to retain the water absorbed from the soil and as a result of this they will remain transplanted.

Wilting refers to the loss of cellular turgidity in plants which results in the drooping of leaves or plant as a whole because of lack of water. During noon the rate of transpiration exceeds the rate of absorption of water by roots. Due to the excessive transpiration, the cells of leaves lose their turgidity and wilt.

During mid-day the rate of transpiration is maximum. During this time, in some herbaceous plants, the rate of transpiration exceeds the amount of water absorbed. Thus, the leaves loss its turgid state and show wilting.

Lenticels are minute, permanent openings which may develop in the young stems of woody plants. Some water is lost continuously through the lenticular openings in the form of water vapour.

Stomata are generally found on the epidermis of the leaf.

Stomata present, on the leaf and open only during the day while lenticel present on the stem of woody plants and open at all the times.

Hydathode: A water secreting gland found on the edges and tips of leaves of many plants.

The escape of plant sap from the ruptured or cut surfaces of the plant due to root pressure is Bleeding.

Lenticel: A pore in the periderm of a woody stem. It acts as an organ of gaseous exchange.

A pulling force called suction, caused by the evaporation of water in a leaf draws a long, continuous column of water through the xylem from the root to the leaf. This is called the transpiration stream.

The structures through Hydathodes guttation occurs.

Bleeding is the flow of the plant sap at the sites of injury or cuts.

Transpiration is defined as the loss of water vapour by the plant. It may take place from any part of the plant body.

Guttation is defined as the loss of water in the form of water droplets from the leaves of intact plants. It is also called exudation.

A plant having sunken stomata is Xerophyte.

Cuticular transpiration takes place through the cuticle (waxy layer) covering the leaves.

Stomatal transpiration takes place through the stomata on the leaves.

Woody trees are lenticular transpiration.

Lenticular transpiration takes place through the small openings in the corky tissue covering the stem.

(i) Transpiration: Loss of water as water vapour from aerial parts of the plant.

(ii) After 2-3 days the level of water falls in test-tube A while it remains constant in test t ube B.

(iii) The fall in the level of water in test-tube A due to the absorption of water by the roots.

(iv) The oil has been put in each test-tube to prevent the loss of water by evaporation.

(v) The purpose of setting up test-tube B is to show that in the absence of plant, there is no change in the level of water.

(a) False 

Reason: Potometer is used to measure the rate of transpiration in a plant. Demonstration of transpiration occurring from the lower surface of a leaf is done by analyzing the changes in colour of pieces of dry cobalt chloride paper attached (and held in place) to the two surfaces of a leaf.

(b) True 

Reason: Transpiration carried out by the large number of trees in a forest. This increases the moisture in the atmosphere and brings rain.

(c) False 

Reason: Hydathodes are special pores present on the ends of leaf veins through which guttation occurs and water droplets are given out. Their openings cannot be regulated. Stomata on the other hand are minute openings in the epidermal layer of leaves through which exchange of gases as well as transpiration occurs. Water is given out as water vapour. Stomatal opening is regulated by guard cells.

(d) False 

Reason: Transpiration is reduced during high atmospheric humidity. High humidity in the air reduces the rate of outward diffusion of the internal water vapour across stomata, thereby reducing the rate of transpiration.

(e) True 

Reason: Desert plants need to reduce transpiration as much as possible so as to survive in the hot and dry environment. Hence some of them have sunken stomata as an adaptation to curtail transpiration.

True 

Reason: During the day, the stomata are open to facilitate the inward diffusion of carbon dioxide for photosynthesis. During mid-day, the outside temperature is higher, due to which there is more evaporation of water from the leaves. Therefore more transpiration occurs during mid-day.