sap, fluid in plants consisting of water and dissolved substances. Cell sap refers to this fluid present in the large vacuole, or cell cavity, that occupies most of the central portion of mature plant cells. The term sap is generally applied to all the fluid that travels through the vascular tissues (xylem and phloem) of higher plants. Water containing dissolved minerals enters the plant through the root hairs by osmosis and is transported upward through the xylem to the parts containing chlorophyll, usually the leaves. There, large amounts of water leave the plant by transpiration, although some is used in photosynthesis to produce food materials. The phloem carries the resulting highly concentrated colloidal solution down to the other plant parts for storage. Sap ascends at a rate of from 1 to 4 ft (30–122 cm) per hr; in the coast redwood it rises easily to a height of almost 400 ft (120 m). The exact mechanisms behind this enormous lifting force are not certain, although several principles are thought to be involved. Chief among them is the pull of transpiration; as water evaporates from the leaf cells, they draw in liquid osmotically from the xylem tubes to replace it. Because of the great cohesiveness of water molecules, the resulting tension affects the entire continuous column of water down to the root tips, which in turn absorb more water from the soil. Root pressure is another factor, although it can force the sap up only a limited distance and operates chiefly in the nongrowing season, which explains the sap flow when a leafless tree is tapped in winter. Atmospheric pressure and capillary attraction are minor factors. The sap of some plants (e.g., sugarcane, sugar maple) contains much sugar and is an article of commerce. The name sap is sometimes applied to latex (e.g., rubber), resin, and other specialized plant fluids.
The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.