Transpiration is the evaporation of water from the surfaces of plants, mostly in leaves via the stomata. Transpiration is the major driving force for the upward transport of water in a vascular plant. When water evaporates from a leaf, the hydrostatic pressure in the upper regions of the plant decreases. On the other hand the hydrostatic pressure in the lower parts of the plant is relatively higher. Water moves from an area of high to low hydrostatic pressure, thus the net effect is the upward flow of water.
Factors that affect the rate of transpiration
Plants transpire more rapidly in the light than in the dark. This is largely because light stimulates the opening of the stomata (mechanism). Light also speeds up transpiration by warming the leaf.
Plants transpire more rapidly at higher temperatures because water evaporates more rapidly as the temperature rises. At 30°C, a leaf may transpire three times as fast as it does at 20°C.
The rate of diffusion of any substance increases as the difference in concentration of the substances in the two regions increases.When the surrounding air is dry, diffusion of water out of the leaf goes on more rapidly.
When there is no breeze, the air surrounding a leaf becomes increasingly humid thus reducing the rate of transpiration. When a breeze is present, the humid air is carried away and replaced by drier air.