In lower plants (and also protozoans)
exchange of gases takes place through the general body surface as they are not
highly modified or specialized. Also, the body surface allows the diffusion of
gases. In higher plants the exchange of gases takes place through stomata.
Gaseous exchange
through Stomata
Stomata are openings generally present on the lower surface
of the leaves through which the gases and water vapour diffuse in and out
easily. The oxygen diffuses in through the stomata and then enters the leaf
cells. Similarly, the carbon dioxide produced by the leaf cells diffuses out
through the stomata.
Gaseous exchange through the Lenticels
In
woody stems, the entire surface is covered by bark which is impervious to gases
or water. However, there are certain openings or pores in the layer of bark.
These are called the lenticels. They are visible slightly more raised than the
general surface of the stem. At the base of the lenticels are loosely arranged
cells which allow the diffused gases to pass through them.
Gaseous
exchange in the Roots
Gases diffuse in and out of the general
surface of the roots. The gases are found in the soil surrounding the roots.
Plants which grow in salty water show specialised roots called the
pneumatophores. These are roots growing out of the surface of water with
numerous pores on their surface
Importance of Gaseous Exchange.
1. It enables
plants to obtain carbon dioxide, which is one of the raw materials necessary
for photosynthesis
2. Plants obtain oxygen which is necessary for the production of energy.
3. It enable the plants to eliminate excess carbon dioxide at night of which if left will harm the plant.