Why Transpiration is known as Necessary Evil?

Transpiration is the evaporation of water from the surface of leaf cells in actively growing plants .

many authors described this  phenomenon of transpiration from time to time in the following words:

• Transpiration is the loss of water in the form of water vapour from stem and leaves. (Miller 1930)
• The loss of water vapour from aerial parts of living plants is known as transpiration (Meyer, 1953)
• Transpiration is giving off of water vapour from the surface of a plant. (Meyer, 1956)
• Transpiration is necessary evil - Curtis

Why Transpiration is known as Necessary Evil?

Transpiration causes loss of huge amount of water absorbed by plants and leads to wilting and injury in plants. It also check photosynthesis, reduces growth and if too severe may cause death from desiccation. 

Inspite of various detrimental disadvantages the plant cannot avoid transpiration due to their peculiar structure of leaves which is basically meant for gaseous exchange during respiration and photosynthesis. Therefore, transpiration is also regarded as “necessary evil” by Curtis or “unavoidable evil” by Steward.
Read more: Difference between Transpiration vs Guttation