Unlike evaporation, the transpiration is a vital physiological process and is regulated by living cells.

There are three types of transpiration.

1. Stomatal Transpiration
2. Cuticular Transpiration
3. Lenticular Transpiration

Lenticles are small pores broder by parenchyma and are present in the woody stems. Some transpiration takes place though these pores are called Lenticular Transpiration.

Typically each stomata is surrounded by two specialized kidney cell, the guard cells. These cells, unlike the epidermal cells contain chlorophyll. The inner wall of each guard cell, adjacent to stomata bore is thicker than the outer wall which is thinner and more elastic. This factor is important in regulation of rate of transpiration.

The opening and closing of stomata depends on the turgidity of the guard cell. When the guard cell is turgid, its outer thinner and elastic wall stretches more than the inner thicker wall, thus the guard cells become wider, increasing the transpiration. When the guard cell loses water, it becomes flaccid and the stomata is closed, reducing the transpiration rate generally the stoma open during the day and close at night. According to one hypothesis this happen because:

• During light photosynthesis takes place and the CO2 from the guard cells is used up thus increasing the acidity of the cell sap. As a result the enzyme phosphorylase become active and changes the starch to glucose-1-phosphate. This increases the osmotic concentration of the cell sap therefore the water flows inside the guard cell from the surrounding cells, make them turgid and open the stomata.
• During nigh consumption of CO2 decreases because the photosynthesis stops. CO2 accumulates and increases the acidity of the cell sap of the guard cells. The whole sequence is reversed; the glucose is changed to starch which dresses the concentration of the cell sap. As a result water flows out the guard cells which become flaccid and close the stomata.

Both the environmental condition and the internal water contents of the cells effect the rate of transpiration. Some of these are:

High temperature provided more heat and thus the rate of vaporization increased. The rise in temperature also decreased the humidity of which increases the rate of transpiration.

Due to humidity of the air, the difference between water vapour concentration inside the leaf and outside is increased. Thus the dry air accepts water vapour more readily and the rate of transpiration increases.

The air surrounding the stomata is saturated with water vapours due to transpiration and cannot accept more water vapour. The rapid movement of strong wind increases the rate of transpiration.

If sufficient water is not available in the soil, the rate of transpiration exceeds the rate of absorption. In such condition the guard cell become flaccid and the stomata close down and the rate of transpiration is reduced.

The size and number of stomata affects the rate of transpiration normally larger size and number increases the rate of transpiration.

Many structure peculiarities like thick cuticle, sunken stomata, waxy surface etc reduce the rate of transpiration.

Transpiration has some beneficial and some harmful effects on the plant;

• Transpiration causes the absorption of water and mineral from the soil. Less water is removed by transpiration.

• Transpiration to some exert cooling effect on the plant.

High rate of transpiration causes water deficiency and thus the excessive transpiration leads to wilting and death of plant.