Plants have evolved an elegant and highly effective means of transporting water from the soil to tremendous heights without the use of metabolic energy. The key is the surface tension of water. Water has a much higher surface tension than most other liquids because of the higher internal cohesion related to hydrogen bonding between the water molecules. If a column 10 micrometers in diameter and 3,000 meters tall were filled with water, it would hold the column of water and not drain as a result of the surface tension of water. Therefore, the transport of water is not limited by the height of fruit trees. The movement of water within a tree requires a continuous column of water; however, breaks in the column of water moving through the xylem do occur. Water stress occurs when the environmental demand for water exceeds the plant's ability to transport water to the leaves. As the demand for water begins to exceed the transport capacity, greater tensions develop within the xylem, and at high tensions, columns of water in tracheids and vessels break, leaving cavities. A bubble of air is generally contained within the individual element. Some cavitations in the xylem permanently block water movement through that xylem element. In other xylem vessels and tracheids, the cavita-tion may be filled when the tree is rewatered through irrigation, or it may fill overnight due to root pressure. Root pressure develops in plants because water is drawn through the endodermis by the concentration of salts and organic molecules dissolved in the stele. Water moves into the stele by osmosis, in which the endodermal cells are the selectively permeable membrane. The water that moves into the stele is forced up the xylem, and it is important for refilling the xylem during the night when transpiration ceases.
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