Q."Transpiration is a necessary evil". Explain.

Transpiration wastes huge amounts of absorbed water (the 'evil'), yet it is indispensable for water/mineral transport, cooling, and gas exchange (the 'necessity') — hence the phrase 'necessary evil'.

Transpiration is the loss of water in the form of vapour, mainly from the aerial parts of the plant (chiefly through stomata on leaves, and to a lesser extent through the cuticle and lenticels).

Why it is called an 'evil':

• Transpiration causes an enormous, seemingly wasteful loss of water — of the total water absorbed by roots, as much as 99% is lost through transpiration, and only a small fraction (~1%) is actually retained/used by the plant for growth and metabolism (e.g., photosynthesis).
• This continuous loss puts the plant at risk of water stress, wilting, or even death under conditions of limited water availability (drought), and the plant must continuously expend energy and resources to absorb replacement water.
• It appears, therefore, to be a wasteful/harmful process (an 'evil') from a strict water-economy standpoint.

Why it is 'necessary':

• Ascent of sap: Transpiration creates a negative pressure (transpiration pull) at the top of the xylem, which — combined with the cohesive and adhesive properties of water (cohesion-tension theory) — pulls water and dissolved minerals up from the roots to all parts of the plant, including the tallest trees.
• Mineral distribution: The transpiration stream carries dissolved mineral ions absorbed by the roots throughout the plant body.
• Cooling effect: Evaporation of water during transpiration has a cooling effect on the leaf surface, protecting it from excessive heating in strong sunlight (similar to sweating in animals). …