What is Transpiration? Why it is said be a necessary evil?

The loss of water by evaporation from a plant surface is called transpiration.

Over 90% of water escapes through the open stomata, while about 5 is lost directly from the epidermal cells. The combined area of stomatal pores is on average only 1-2% of the total leaf surface.

Transpiration rates are greatest when the leaf cells are fully turgid, stomata are open and relative humidity in the atmosphere is low.

Transpiration_Anessary Evil

Upward movement of water in plants is attributed to two processes:

         i.            Root pressure    (ii)   Transpiration

1.       Root Pressure

Root pressure is capable of moving water upward in a plant, but not in the quantity and to the heights necessary for most plants. So we are left with the hypothesis that water is pulled up through the plant body due to transpiration.

2.       Transpiration

Although water is used in the maintenance of turgidity and the possible translocation of dissolved minerals, water use in plants is inefficient and can endanger their survival. So water loss by transpiration becomes necessary because of these and some other reasons (cooling effect by evaporation) it is said that transpiration is a necessary evil.