Transpiration — ICSE Class 10 Biology Important Questions

Answer: (c) Stomata.

Over 90% of transpiration is stomatal, occurring through the tiny stomatal pores of the leaves.

Answer: (b) Guard cells.

The two bean-shaped guard cells surrounding each stoma open or close the pore by changes in their turgidity.

Answer: (b) High humidity.

High humidity reduces the difference in water-vapour concentration between the leaf and the air, so transpiration slows down. The others increase it.

Answer: (c) Bleeding. Sap flows out of a cut or wounded stem, pushed up by root pressure. In guttation, by contrast, water droplets come out of the hydathodes at the edges of an uninjured leaf.

Answer: (a) Both are true and R explains A. Spines give a very small surface for transpiration, which saves water in a dry habitat. Other adaptations that reduce transpiration are a thick waxy cuticle, sunken stomata, fewer stomata, and rolled or folded leaves.

Transpiration: The loss of water in the form of water vapour from the aerial (above-ground) parts of a plant, mainly through the stomata of the leaves.

Types: (i) Stomatal transpiration (through stomata), (ii) Cuticular transpiration (through the cuticle of the epidermis), and (iii) Lenticular transpiration (through lenticels of woody stems).

Transpiration: Water is lost in the form of vapour, mainly through stomata, usually during the day, and the water is pure.

Guttation: Water is lost as liquid droplets, through special pores called hydathodes at leaf margins, usually at night/early morning when root pressure is high, and the droplets may contain dissolved salts.

A stoma is a pore in the leaf epidermis bounded by two bean-shaped guard cells containing chloroplasts; the inner walls of the guard cells (towards the pore) are thicker than the outer walls.

When the guard cells are turgid, their thin outer walls stretch more than the thick inner walls, so they curve apart and the pore opens.

(i) Light: Light causes stomata to open, so transpiration increases in bright light and decreases in the dark.

(ii) Temperature: A rise in temperature increases evaporation of water, so the rate of transpiration increases.

(iii) Humidity: High atmospheric humidity lowers the rate of transpiration (less difference in vapour concentration), while dry air increases it.

(Wind speed and available soil water also affect the rate — higher wind increases it.)

The opening and closing of stomata depend on the turgidity of the guard cells, explained by the potassium ion exchange theory.

Opening (in light): Potassium ions ($K^+$K^+) move from the neighbouring epidermal cells into the guard cells. The solute concentration of the guard cells rises, so water enters them from the neighbouring cells by osmosis and they become turgid. The thin outer wall stretches more than the thick inner wall, so the guard cells curve apart and the pore opens.

Closing (in darkness): $K^+$K^+ ions move back out of the guard cells, their solute concentration falls, water leaves by osmosis and they become flaccid. The guard cells straighten and the pore closes.

Thus stomata are generally open in the day and closed at night.

(a) Significance of transpiration:
(i) Ascent of sap: Transpiration creates a transpiration pull that draws water and dissolved minerals upward from the roots to the leaves through the xylem.
(ii) Absorption of water: By continuously removing water from the leaves, it helps the roots to absorb more water from the soil.
(iii) Cooling effect: Evaporation of water during transpiration cools the leaf surface and protects the plant from overheating in strong sunlight.
(iv) Distribution of minerals: The upward stream of water helps distribute mineral salts throughout the plant, and helps keep cells turgid and the concentration of cell sap suitable.

(b) Harmful effects of excessive transpiration:
(i) It can cause wilting and even drying up of the plant when water loss exceeds absorption.
(ii) It reduces the plant's efficiency, as the plant must spend energy and absorb large amounts of water (most of which is lost), and may limit growth during water shortage.

Aim: To measure the rate of transpiration of a leafy shoot.

Apparatus: Ganong's potometer — a graduated horizontal capillary tube connected to a wide tube holding a leafy shoot, with a reservoir; the whole apparatus is filled with water and the capillary tube dips into a beaker of water.

Method:
(i) Cut a healthy leafy shoot under water and fix it air-tight into the potometer, with all joints sealed with grease/vaseline.
(ii) Fill the apparatus completely with water so there are no air bubbles inside.
(iii) Introduce a single air bubble into the open end of the capillary tube by lifting it out of the water for a moment.
(iv) Note the position of the air bubble on the scale and record the distance it moves in a fixed time (say 5 minutes).

Observation: As the shoot transpires, water is drawn in and the air bubble moves towards the plant.

Result: The distance moved by the bubble per unit time gives the rate of water uptake, which is taken as a measure of the rate of transpiration. The reservoir is used to push the bubble back to the start to repeat readings under different conditions (light, wind, humidity).

Precautions: (i) The shoot must be cut and fixed under water and all joints made air-tight to avoid air entering. (ii) Only one air bubble should be introduced and the apparatus kept free of other bubbles.

Limitation: The potometer actually measures the rate of water uptake (absorption), which is only approximately equal to the rate of transpiration, since some absorbed water is used by the plant and not all is transpired.

(i) The cobalt chloride paper turns from blue to pink, because it absorbs the water vapour given off by the leaf.

(ii) The paper on the lower surface turns pink faster, because a dorsiventral leaf has more stomata on its lower surface, so more water vapour is lost there.

(iii) The glass slides keep moisture in the surrounding air from reaching the papers, so only the water vapour from the leaf is detected.

(iv) The pot and the soil surface are covered with a polythene sheet, so that water cannot evaporate from the soil. Any loss in weight is then due to transpiration from the shoot alone.