The loss of excees water in the form of vapor from the aerial parts of the plant is known as transpiration. Transpiration is essential for the movement of water and minerals from the root to the healthy parts of the plant. But excess transpiration may result in drying up of the leaves or wilting and loss of soil water. Hence it is termed as a necessary evil
The absorption of excess amount of water in the form of vapour in plants is known as transpiration.In order to drain off the excess water transpiration is necessary. It plays an important role. But extra transpiration in plant may dry the leaves of the plants. So transpiration is said to be necessary evil. Hope this helps you