ABSTRACT
Water is the only inorganic liquid that occurs naturally on earth. It is also the only chemical compound that occurs naturally in all three physical states: solid, liquid and vapor. Water existed on the planet long before any form of life evolved but since life developed in water, it is the universal internal medium of all organisms. Water surfaces are poor reflectors of solar radiation and therefore serve as a good sink for solar energy. The volume of water that becomes incorporated into the body of a plant is infinitesimal compared with what passes through the plant in transpiration. Hydration is a necessary condition for metabolic reactions to proceed within the organism; water is the medium in which the reactions occur. Water may move through the soil towards a root or the root may grow through the soil towards the water. In aquatic environments, the availability of water as such is not a problem.
