D. Michael Glenn
Water constitutes 80 to 90 percent of leaf and fruit tissue and more than 50 percent of an entire fruit tree by weight. Over 90 percent of the water entering a tree is lost through the leaves, and 95 percent of this water passes through the stomates, or pores in the leaves, which occupy less than 1 percent of the leaf area. The process of leaf water loss is termed transpiration, and a large fruit tree can transpire more than 400 liters of water in a day if the environmental demand for water is great. Over 90 percent of living cells are water because water is needed for both chemical and physical requirements. Water is a medium for chemical reactions in the cell, is used in the chemical reactions of photosynthesis, and is an agent for the transport of chemicals in the diffusion process. Water is the medium for the transfer of carbon dioxide from the atmosphere into the mesophyll cells where photosynthesis occurs and for the transport of sugars from leaves to storage organs, such as fruit. Water is needed structurally for cell turgor and physical cell expansion. In addition, transpiration of water cools the leaves through the process of water evaporation.
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