The loss of water as water vapour from the aerial parts of plants is called transpiration.
Difference Between Transpiration and Evaporation
|Loss of water from the surface of the water bodies in the form of vapour.||Loss of water in the form of water vapour from the aerial parts of the plants.|
|Physical process controlled by environmental conditions like relative humidity and air current.||Transpiration is a physical and physiological process.|
|It varies according to the velocity of the wind.||Rate is slightly lower than evaporation under the velocity of the wind.|
|Light does not directly affect the rate of evaporation.||Light affects the rate of transpiration. Opening and closing of stomata depend upon the intensity of light.|
Types of Transpiration
There are three types of transpiration:
- Occurs from the leaves through the stomata.
- Stomatal cells are present in the epidermis of green shoots and leaves.
- 80-90 per cent of water loss takes place through stomatal transpiration.
- Occurs from the waxy cuticle layer of leaves and stems.
- These types of cells are present at the waxy surface of the leaves.
- Occurs from the lenticels present on the surface of old stems.
- Lenticels are present in woody stem and fruits.
- 1-5 per cent of water loss takes place through lenticular transpiration.
Mechanism of Opening and Closing of Stomata
Stomata are minute openings found in the epidermis of leaves, stem and in some cases even flowers. Each stoma contains a pore surrounded by two guard cells. Both guard cells are firmly joined at both ends but separate in the mid-region of their length. Stomata are mostly present on the lower epidermis of the leaf.
The open stomata account for the diffusion of water through them. during the day the cell SAP concentration becomes high due to the accumulation of sugar in the guard cells as a result of photosynthesis.
This results in the movement of water into guard cells from the neighbouring cells. This makes the guard cells turgid and a guard cells bulbs out and pulls apart their inner walls and stomata open. if the availability of water is reduced the guard cells to lose their touch dirty and they become flaccid by exosmosis of water from guard cells.
Factors Affecting Transpiration
High humidity in the air reduces the rate of outward diffusion of water from the sub-stomatal cavity and reduces the rate of transpiration.
High temperature increases the rate of transpiration. The high temperature helps to increase evaporation of water while low-temperature reduces evaporation.
Intensity of Light
Increase in intensity of light helps to increase the rate of transpiration. Stomata open during light and thus occurs more often during the day.
Rate of transpiration increases with the velocity of the wind.
Rate of transpiration increases with the decrease in atmospheric pressure.
Carbon dioxide Concentration
If the concentration of carbon dioxide increases more than the normal, the stomata get closed. This reduces the rate of transpiration.
Water Supply to the Leaf
Shortage of water in the soil affects the water supply to the leaf and in turn the stomata close and the leaf wilts. This reduces the rate of transpiration.
How are plants adapted to reduce excessive transpiration?
- The stomata may be sunken or covered by the hair as in oleander.
- The number of stomata may be reduced as in cactus.
- The leaves may become narrow to reduce leaf surface area as in pine.
- The leaves may roll over or fold to reduce exposed surface and maintain moisture around stomata as in desert grass.
- The leaves may have thick cuticle like in mango.
it is the process by which prepared food is transferred to the different parts of plants, including storage organs. It occurs through vascular cells.
The process of exudation of water droplets from the leaf margin of plants is known as guttation.