Transpiration

 We can show that water evaporates from the leaves of a plant by placing a dry piece of cobalt chloride paper on a leaf surface. The paper changes from blue to pink after some time, showing that water is given off. The rate of transpiration from the lower surface of a leaf is Much greater than that the upper surface. This is because most leaves have more stomata on the lower surface than on the upper surface. Transpiration also occurs through the cuticle of leaves and through lenticels.

    Stomata are open in the daytime when the leaves are actively photosynthesizing, so that gases can be exchanged. When stomata are open , transpiration is unavoidable. 

   Transpiration is an important process since it sets up the transpiration stream that enables

 • Water to move up the xylem vessels so that it can be distributed to all the plant cells for their cellular activity; and

 • Minerals salts dissolved in the water to be transported to the leaves for the leaves for the synthesis of complex substances.

     Transpiration also helps to cool the plant since the heat needed to convert water yo vapour comes from the cells of the leaves. However, under hot and dry conditions, transpiration can cause too Much to be lost from the plant body. If this loss exceeds the uptake of water by the root, the plant t will wilt.

 Some plants have special features to reduce loss of water by transpiration. The rate of transpiration will be less in a cold, wet, dull day than a hot dry, bright one. Similarly, on a windy day, a plant loses more water than on a calm day.