Leaves and green shoots are the principal organs of transpiration. Transpiration may occur through the cuticle, lenticels or stomata. Accordingly it is called cuticular, lenticular or stomatal transpiration.
i. Cuticular Transpiration
Cuticle is a layer of wax-like covering on the epidermis of leaves and herbaceous stems. It is meant to check transpiration. However, some water may be lost through it. Thus the loss of water in the form of water vapour through the cuticle is known as cuticular transpiration. Cuticular transpiration accounts nearly 20% of the total water loss by a plant.
ii. Lenticular transpiration Lenticel
Loss of water in the form of water vapour taking place through the lenticels present in woody stem and fruits is called as lenticular transpiration. It amounts 1-5 percent of the total water loss by the plant.
iii. Stomatal transpiration
Stomata are minute pores confined to epidermis of green shoot and leaves. Opening and closing of stomata are controlled by guard cells. Maximum loss (80-90 percent of the total water loss) of water from the plant tissues takes place through the stomatal openings. The loss of water in the form of water vapour through the stomata of the plants constitutes stomatal transpiration.