Excretion is the process by which metabolic wastes and other non-useful materials are eliminated from an organism. In vertebrates this is primarily carried out by the lungs, kidneys and skin.This is in contrast with secretion, where the substance may have specific tasks after leaving the cell. Excretion is an essential process in all forms of life. For example, in mammals urine is expelled through the urethra, which is part of the excretory system. In unicellular organisms, waste products are discharged directly through the surface of the cell.Green plants produce carbon dioxide and water as respiratory products. In green plants, the carbon dioxide released during respiration gets utilized during photosynthesis. Oxygen is a by product generated during photosynthesis, and exits through stomata, root cell walls, and other routes.
Plants can get rid of excess water by transpiration and guttation. It has been shown that the leaf acts as an 'excretophore' and, in addition to being a primary organ of photosynthesis, is also used as a method of excreting toxic wastes via diffusion. Other waste materials that are exuded by some plants — resin, saps, latex, etc. are forced from the interior of the plant by hydro static pressures inside the plant and by absorptive forces of plant cells. These latter processes do not need added energy, they act passively. However, during the pre-abscission phase, the metabolic levels of a leaf are high.Plants also excrete some waste substances into the soil around them.
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