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Transpiration-Cohesion Theory
Water entering the xylem in roots must travel through the stem to the leaves. Sometimes this movement involves a distance greater than one hundred meters. How can this process occur?
Water enters a root and passes into the xylem of the stele. The water in the xylem exerts a pushing force called root pressure. Perhaps you have seen the effects of root pressure. Sometimes, when root pressure is high, water is forced out of the ends of leaf veins. The water forms droplets around the edges of the leaf, a process known as guttation. Root pressure can also be shown by an experiment. A piece of tubing can be attached to the cut stem of a plant whose roots are immersed in water. Water will rise in the tube. Root pressure may play a role in transport of water in small plants. However, it cannot explain water transport in most plants. Most biologists favor another explanation, called the transpiration –Cohesion theory for water transport through a stem. According to this theory, water must form continuous columns from roots to leaves. Two properties of water, cohesion and adhesion, make this water movement possible. Cohesion is the clinging together of the same kind of molecules, and adhesion is the attraction of unlike molecules. Cohesion causes water molecules to be “sticky.” Also, they adhere to the walls of the xylem tissues. In these very narrow tubes, the combination of cohesion and adhesion gives a column of water the properties of a metal wire – continuous with tightly packed molecules. This assumption is important to the theory. What causes the pull? Water constantly is leaving the leaves of plants via stomata. This water loss is called transpiration. As water molecules leave, other water molecules from the xylem vessels of the leaf replace them. Because of cohesion, this movement creates a tension, or pull, on the rest of the water column. This pull can be thought of as a “stretching” effect. The “stretching” force extends all the way to the roots. The more quickly water is lost, the more quickly it is pulled up through the plant. As the water in the root xylem is pulled up, more water moves into the xylem from the surrounding root cells. The water comes from spaces in the soul. Transpiration pull is not the same as drawing a liquid through a straw. Even a vacuum pump could not lift water to the height it travels in tall trees. The pull occurs only because the column of water is continuous. Without cohesion and adhesion of water in the narrow xylem tubes, transport could not occur. The combination of these properties produces an unbroken column of water that can be pulled.
áËÅ觢éÍÁÙÅ: BIOLOGY living system. Page 454.
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