The evaporation of water from the aerial part of plant is called transpiration. Transpiration is not a purely ph) sical process like evaporation. It is a vital phenomenon. It plays important role in the cell. –11e amount of water lost In transpiration is very large. It exceeds the final dry weight of the plant In se‘eral hundred times • A corn plant during its life of a fen necks transpires a large amount of water. I his water can till a barrel. Similarl . medium sited tree may lose more than [I ton of water every day. it has also been estimated that every ounce of dry matter in the aerial parts of a crop plant loses 20 —50 pounds of water by transpiration.
Demonstration of transpiration
A potted plant is placed under a bell jar. the soil in the pot is watered and the pot is covered with rubber sheet or the soil. The wails of the pot are coated %kith paraffin. After a short time. drops of water %% ill appear on the inner surface of the bell jar. Iliis water has come from the aerial parts oI the plant.
TYPES OF TRANSPIRATION
There are two type of transpiration: lot jar transpiration and lenticular transpiration.
- Foliar transpiration
the transpiration which takes place through the lea’ es is called foliar transpiration. The foliar transpiration may be stomata’ transpiration or cuticular transpiration.
(a) Stomata! transpiration: The transpiration which takes place through he stomata is called stomata’ transpiration. The lower part of leaf has the loosely packed spongy mesophyll cells. Flicy have a number of large intercellular spaces. These air spaces communicate ith the outside atmosphere hy means of stomata in the leaf epidermis.
The xylem of the leaf vein suppls water to the cells of the mesophyll by osmotic diffusion. Mcsopbyll cells become turgid and saturated with water. Water evaporates from their moist walls into the internal atmosphere of the intercellular spaces of the mesophv II. It becomes saturated with water vapour. Finally water vapours move out of stomata.
(h) Cuticular transpiration: The transpiration which takes place though the cuticle of the leaf is called cuticular transpiration. Cuticle is present on outer walls of the epidermal cells. The cuticle is not permeable for water. Therefore. the amount of water lost through it is small. It is only 3 to 10% of the total transpiration. Cuticle
transpiration depends upon the thickness of the cuticle. The cuticle is poorlv or not at all developed in herbaceous plants growing in humid and shads places. •Therelore. their cuticular transpiration becomes equals to the stomata! transpiration. Transpiration from stems, fruits and Bower parts is most!) cuticular.
- Lenticular transpiration
tenticels are pore like structures in the harks of the woodv stems. Some transpiration also takes place through the lent eels. It is called lenticular transpiration.