1. Transpiration is the process of loss of water in the form of water vapour from various parts of the plant, mainly the aerial parts like leaves. 2. This happens through the guard cells of the plants. 3. The guard cells open up to take up carbon dioxide gas from atmosphere for photosynthesis. 4. During this process, water molecules also find a way to escape from the plant through the open guard cells, a process which can't be avoided, so its necessary. 5. However, loss of water leads to wilting of the plant, and in extreme conditions it leads to death of the plant. Thus it is a necessary evil.