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Transpiration is the evaporation of water into the atmosphere from the leaves of the plants. The water is lost in the form of water vapour. about 90% of the total transpiration occurs from the leaves. The leaf surfaces on their upper and lower epidermis, contains pores called stomata through which transpiration takes place.

Significance of Transpiration

If the transpirational loss of water is higher than the amount of water absorbed by the roots from the soil, plants undergo water stress. This results in decline of growth in plants.

The question is, then why is transpiration important and what is its significance? Transpiration helps in the movement of xylem sap, which simultaneously increases the absorption of mineral nutrients by the roots from the soil. As evaporation causes cooling it assists in the cooling of leaves. Not all the solar radiations absorbed by leaves are used in photosynthesis, but some will cause heating of the leaves. Transpiration, however, reduces the heating of the leaves.

Factors affecting transpiration

Transpiration is affected by the environmental factors, that is, humidity, temperature and wind speed. These factors influence the rate of water diffusion from the leaves to the atmosphere.

If the humidity is high, water is lost slowly into the air as a result of transpiration and vice versa. The rate of evaporation doubles with every rise in temperature by 10oC. As transpiration causes cooling of leaves, the leaf temperature does not rise as much as that of the air. High temperature, influencing the closure of stomata, reduces transpiration. Wind speed also has an influence on the rate of transpiration. A high wind speed or a dry breeze will greatly increase transpiration.
Some other plant factors also affect the rate of transpiration. These factors include: efficiency of water uptake by roots, the leaf area and the leaf structure. Larger leaf area will transpire more water into the atmosphere. Leaves of plants which grow in a dry habitat develop structures like thick cuticle and sunken stomata (deep-seated) to cut down the loss of water through transpiration.

Water escapes from the aerial parts of the plant in the form of water vapour. This takes place chiefly through the stomata of the leaves. Transpiration is an evaporation process. Evaporation is also a cooling process. It helps prevent leaves from being damaged by intense heat from the sun.

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