The volume of water that becomes incorporated in higher plants during growth is infinitesimal in comparison to the volume that flows through the plant in the transpiration stream. Nevertheless, water is a critical resource. Hydration is a necessary condition for metabolic reactions to proceed, and because no organism is completely watertight its water content needs continual replenishment. Most terrestrial animals drink free water and also generate some from the metabolism of food and body materials; there are extreme cases in which animals of arid zones may obtain all their water from their food.
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