Question

Transpiration has been described as a 'necessary evil' because it is inevitable, but potentially harmful. Which of the following statements describe this?

(i) lt causes absorption of mineral salts.

(ii) lt regulates plant temperature.

(iii) lt causes wilting and injury in plants.

(iv) lt causes ascent of sap.

(A) (i) and (iv) only

(B) (i), (ii) and (iii) only

(C) (i), (ii) and (iv) only

(D) (i), (ii), (iii) and (iv)

Answer 1

Correct option (D) (i), (ii), (iii) and (iv)

Explanation:

Transpiration is the loss of water in the form of vapour from the living tissues of aerial parts of the plant. lt mainly occurs by the process of diffusion through stomata.

It is regarded as 'necessary evil' due to some of the following advantages and disadvantages.

Advantages :

(i) Transpirational pull created by transpiration of water from leaf surface causes ascent of sap (upward movement of water and mineral salts).

(ii) The sap absorbed from the soil contains low concentration of mineral salts. The loss of water through transpiration increases the concentration of mineral salts in the plant.

(iii) Due to the heat of the Sun, temperature of the plant increases which may be harmful for the plants. Transpiration lowers down the temperature of the plant by 10° - 1 5°C.

Disadvantages :

Due to transpiration being higher than the rate of water absorption at noon time wilting or loss of turgidity is quite common. Excess dehydration reduces the rate of photosynthesis and other metabolic activities, which may further lead to injury in plants.