Thursday, October 13, 2011
Most animals stop growing when they reach maturity and each species has a fixed shape. But plants go on growing throughout their lives and although the leaves and flowers of a particular species generally have a fixed shape, the overall shape and size of the plant can vary a good deal according to its situation.
Leaving aside the flowers or other reproductive organs, a typical plant has three main regions – root, stem and leaf. The root anchors the plant and absorbs water and dissolved minerals from the soil by means of osmosis. Growth takes place by elongation of the cells just behind the tip and this forces the root down. Delicate root hairs just behind the growing regions absorb the water and pass it to tubes in the centre of the root.
The stem which may be soft or woody carries water and dissolved minerals from the roots to the leaves through a ring of tubes. Another set of tubes carries manufactured food down from the leaves and distribute it around the plant. The stem also arranges the leaves in the most efficient way for catching sunlight and in flowering plants, displays the flowers in the best position for achieving pollination. The leaves are the plant’s food factories. They are usually flat and have a number of veins. Continues with the tubes in the stems, the veins carry water to the leaf tissues for photosynthesis and also carry away the manufactured food. The undersides of the leaves have thousands of microscopic breathing pores that allow carbon dioxide in and oxygen out. They also release a lot of water vapor by evapo-transpiration.