Q.Why is transpiration called 'a necessary evil'?

Transpiration is the evaporative loss of water vapour from a plant's aerial surfaces, occurring predominantly through the stomatal pores of leaves (with smaller amounts via the cuticle and lenticels).

Stomata are minute pores in the leaf epidermis, each flanked by a pair of guard cells that control the pore's opening and closing. When guard cells are turgid, the pore opens, and water vapour diffuses out of the moist internal leaf tissue into the drier outside air — this outward diffusion of water vapour through stomata is transpiration. It is distinct from photosynthesis (the light-driven synthesis of sugars, which uses stomata for gas exchange of CO₂/O₂ but is not itself a water-loss process), respiration (the o …

Excessive transpiration causes excessive water loss leading to wilting/loss of turgidity, closure of stomata, reduced photosynthesis and growth, and under prolonged water stress can cause permanent wilting and death o...

Too much transpiration dehydrates the plant, wilts it, and can ultimately be fatal.

Transpiration is normally beneficial (it drives the ascent of sap, cools the leaf, and maintains a mineral-carrying transpiration stream), but when the rate of water loss through stomata exceeds the rate of water absorption by roots, the plant develops a water deficit. Cells lose turgor pressure, leaves droop and show temporary or, if prolonged, permanent wilting. To limit further loss, guard cells close the stomata, which also cuts off the CO2 supply needed for photos …