The loss of water from aerial parts of the plant in the form of water vapour is termed transpiration and in the form of liquid is termed guttation.
Transpiration may occur through three main sites in the plant :
(i) Cuticle : Cuticle is the wax like covering of the epidermis of leaves and herbaceous stems. Though it is meant to check transpiration, still about 10% of the total transpiration may take place through cuticle and known as cuticular transpiration.
(ii) Lenticels : Lenticels are areas in the bark of a tree which are made up of loosely arranged cells and about 0.1 percent of water loss take place through it. It is known as lenticular transpiration.
(iii) Stomata : Stomata are minute pores on the epidermis of leaves, whose opening and closing are controlled by guard cells. About 90 percent of water loss from plants take place through stomata known as stomatal transpiration.
Mechanism of transpiration
Transpiration occurs in two stages :
(i) Evaporation of water from the mesophyll cells into the intercellular spaces.
(ii) Diffusion of this water vapour of the inter cellular spaces into the outside atmosphere, when the outside atmosphere is drier.
Factors affecting transpiration
There are many external and internal factors that affect the process :
(i) Temperature : The increase in temperature increases the rate of transpiration by increasing the rate of evaporation of water from cell surface and decreasing the humidity of the atmosphere.
(ii) Wind velocity : The increase in wind velocity increases the rate of transpiration by removing the water vapour of the atmosphere and lowering the relative humidity.
(iii) Light : Light has got no direct effect in the rate of transpiration but indirectly it affects the rate in two ways, firstly by controlling the stomatal opening and secondly by affecting the temperature. With increase in intensity of light rate of transpiration increases because stomata get opened and the temperature
(iv) Water supply : Deficiency of water supply in the soil decreases the rate of transpiration by decreasing the rate of absorption. When the deficiency of water in the soil becomes too much then the plants with and do not recover from wilting unless water is supplied in the soils. This is known as permanent wilting. When in a hot and dry summer day the plant transpires more than the roots are able to absorb, even though there is enough water in the soil, it is known as temporary wilting as the plant recovers from such wilting in the late afternoon or at night.
(v) Atmospheric pressure : Reduction of atmospheric pressure reduces the density of external atmosphere thus permitting more rapid diffusion of water. Plants growing on high will show higher rate of transpiration hence they develop xerophytic characters.
(vi) Atmospheric humidity : Humidity means the amount of water vapour present in the atmosphere. The diffusion and evaporation of water depends on the vapour pressure gradient or the difference of water potential gradient between the atmosphere and the inside of the leaf. More the difference more will be the rate of transpiration.
Internal plant factors
Certain plant adaptations reduce transpiration
– Reduced size of the leaves, thereby reducing transpiring surface. Some xerophytic plants have needle like or spine like leaves (Pinus and Opnuntia)
– thick deposition of cutin (wax like substance) on the leaf surface.
– stomata found sunken in the cavities surrounded by hairs as in Nerium and Cycas.
– root shoot ratio, when there is more root and less of shoot system or leaves, there will more of transpiration. Root is the water absorbing surface and shoot or leaves is the transpiring surface; high root shoot ratio will cause more transpiration.