Transpiration is the process of removing excess of water in the form of vapour from the aerial of plant such as leaves, stems, and flowers.
Transpiration occurs through the stomatal aperture.
At night, transpiration does not occur because the stomata of the most plants are closed at night. It only occurs in the daytime.
The quantity of water lost by transpiration is very large.
A herbaceous plant such as cotton or sunflower can lose 1 litre to 3 litres of water per day. It is 36 litres to 45 litres in apple, 400 litres in Palm and 600 litres in oak tree.
Why does transpiration occur?
This is because of the plant use only 1% of the total amount of water absorbed by roots. It is necessary to remove excess water. So plant removes the excess of water through transpiration and guttation.
Mechanism of transpiration
Water from root reaches to mesophyll cells by means of xylem. Water is evaporated from the walls of the mesophyll cells into the inner cellular air spaces of substomatal region.
Loss of water from the mesophyll cells results in decrease in its pressure and water potential of mesophyll cells.
The adjacent cell has higher water potential so that water is lost by the mesophyll cell, it is obtained from the adjacent cell continuously.
This results in the gradient of water potential from leaf to root which causes absorption of water.
From air spaces of a leaf the water diffuses to the atmosphere through stomata because the vapour pressure is more in air space than that in the atmosphere.
If the transpiration takes place through stomata, the guard cells control the opening and closing of stomata but if the transpiration is lenticular, the lenticel is always open.
Kinds of Transpiration
There are 4 kinds of transpiration. They are as follows
1. Stomatal transpiration
This occurs through stomata. About 90% of the total transpiration takes place by stomata.
Most of the stomata are present on leaves and some are present on young stems, flowers, and fruits.
2. Cuticular transpiration
This occurs through cuticle or epidermal cells of leaves and other exposed parts of the plants where the cuticle is not continuous.
3 -10 % of transpiration takes place through the cuticle. But in herbaceous shade-loving plants, it occurs up to 50% of the total transpiration.
3. Lenticular transpiration
In woody branches of the trees, lenticels are present on the epidermal layer.
This transpiration occurs through lenticels.
About 0.5% of the total transpiration occurs by this method.
Lenticular transpiration occurs in day and night because lenticels are open all the time.
The lenticels connect the atmosphere air with cortical tissue of stem through intercellular spaces of complementary cells.
4. Bark transpiration
It occurs through corky covering of the stem.
This occurs very little but it occurs in very large area, so the total loss of water may be more than lenticular transpiration.
Role of Stomata in Transpiration
Gaseous exchange for photosynthesis, respiration, transpiration takes place through stomata.
Number and size of stomata vary according to the plant species and availability of water.
The plant grows in dry condition have less number of stomata to prevent the loss of water.
Two guard cells surround the stomata and they control the opening and closing of stomata.
The opening and closing of stomata depend upon the heat, light, humidity and water content of the cell.
Wall of guard cell is not uniform. It is thick towards the side of the pore and thin in other sides.
When guard cells gain water it becomes turgid. As the cell becomes turgid, the outer wall of guard cell stretches outwards.
Being the inner wall inelastic, it becomes concave and also stretches outwards in some extent. It causes widening of the stomatal pore.
Opening of stomata allows the water vapour present in the substomatal air spaces to diffuse out in the water.
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