The living organisms differ in their methods of excretion. Generally there are no specialized organs or systems in plants for excretion. The unwanted metabolites may be secreted into intra or inter-cellular spaces. The aerating system of plants makes possible the ready diffusion of respiratory CO2 into the surrounding water or atmosphere. In some plants the waste metabolites may be deposited in leaves, seeds and fruits which when shed can easily carry with them these accumulated un-necessary substances. It can be noted that such elimination is purely secondary to the true purposes of leaf fall or seed and fruit dispersal.
13.2.1 Removal of carbon dioxide and oxygen
CO2 is produced as a result of respiration. As the process of respiration takes place continuously in all living cells, CO2 is produced continuously and discharged through the stomata, or lenticels. On contrary, oxygen (O2) is released only during the day-time, because it is a by-product of the process of photosynthesis. The CO2 thus produced during daytime is consumed by the mesophyll cells, in photosynthesis. Only a small quantity of oxygen produced during photosynthesis is used for respiration. The excess oxygen is released from the leaves through stomata.
“Green plants give off oxygen as waste in the day time and CO2 at night.”
At night the photosynthesis stops, the atmospheric O2 is used for respiration in plants and CO2 is evolved as a by-product.
13.2.2 Excretion of extra water
Water is one of the major basic needs of the plants the water in plants is used for photosynthesis, for transpiration and carry out many other functions like transportation of different materials etc. the plants obtain water largely from soil. Whereas a small percentage of it is provided as a result of respiration. The excess water is removed in the form of water vapours by the process of transpiration. Sometimes, the removal of this excess water takes place by guttaion. For example in cabbage, potato and strawberry etc., the droplets of excess water (similar to the dew drops) often appear at the tips and margins of leaves which enlarge and drop-off later on. In this way such plants and few grasses get rid of the extra water (Fig. 13.1)
“Transpiratin: it is the process by which the water gets evaporated from the plant cells, through stomata etc.”
13.2.3 Latex, Resins and gums
In some plants the products like latex, resins and gum are secreted in special canals or ducts, present in the plant body. Such products are not ordinarily excreted from the plant, but they trickle out (leak slowly) of the plant when some part of it is damaged. (Due to this unique behavior, some scientists are also reluctant to regard them as waste products of plants.) The common examples of latex producing plants are rubber plant and the pines. Resins and gums are secreted by Acacia Arabica (keekar) etc and mucilage is secreted by Hibiscus (ladyfinger).
