The loss of excess water in the form of water vapour through evaporation from the surface of the internal tissues of the aerial parts of plants especially leaves, is known as Transpiration.
Since water is one of the most important compounds needed for various important life processes, a loss of water by transpiration is obviously harmful. This harmful effect becomes dangerous when excessive transpiration leads to the wilting of the plants. To reduce the loss of water, several practices are commonly employed by the farmers and the gardeners.
One of them is to remove the weeds from the vicinity of the crop plants. These weeds transpire a lot of water and also utilize soil minerals, thus depleting the soil of its two important constituents, the water and the mineral salts.
The Hydrologic Cycle and respiration:
If it is not done, the crop plants would only be stunted. To minimize the transpiration during the summer months, the green houses are whitewashed to cut down the light intensity and the temperature. Interior of the green houses is sprayed with water to saturate the atmosphere with humidity and thus decrease the transpiration.
In propagation of plants by cutting or in transplantation of seedlings, some of the leaves are removed to reduce the transpiration and thus avoid wilting. Besides wilting, other harmful effects of excessive transpiration include inhibition of protein synthesis and breakdown of proteins and retardation of metabolic processes like photosynthesis. This is the reason why Transpiration is often called as necessary evil.