. Biology and human life. Biology. WHERE FOOD COMES FROM 115. uninjured. Such a main descend- ing root is called a taproot. The fleshy roots that have been men- tioned are all taproots; and a number of trees, as certain kinds of maples, also produce taproots. When a taproot is injured or cut off, some of the side roots turn and grow downward. In a few cases the tip of the taproot, when not too much injured, can grow a new tip and continue the main line of growth. The first function of a root may be said to be the absorp- tion of water and of dissolved substances. This work is car- ried on by t
. Biology and human life. Biology. WHERE FOOD COMES FROM 115. uninjured. Such a main descend- ing root is called a taproot. The fleshy roots that have been men- tioned are all taproots; and a number of trees, as certain kinds of maples, also produce taproots. When a taproot is injured or cut off, some of the side roots turn and grow downward. In a few cases the tip of the taproot, when not too much injured, can grow a new tip and continue the main line of growth. The first function of a root may be said to be the absorp- tion of water and of dissolved substances. This work is car- ried on by the "root hairs" (see Fig. 66). 94. Structure of roots. To understand the structure of roots, use a carrot or parsnip root that has been standing for twenty-four hours or longer with its tip in water containing red ink. Cut slices both crosswise and length- wise, and use a magnifying glass (see Fig. 67). The fiber and vessel cells can grow, but they cannot divide. In the young root there appear layers of cells which separate the water- carrying bundles from the food-carrying bundles. These cam- bium cells are capable of producing new fiber and duct cells of the two kinds—the water-carrying, or wood, and the food- Fig. 66. The tip of a young root The root hair is a single cell formed by the outward prolongation of one of the skin cells. The root hairs are the actual ab- sorbing organs. Each root hair lives but a short time and then shrivels up. As the tip of the root grows on, new root hairs are formed. The older skin cells of the root die and their contents dry out. Together with the shriveled root hairs these skin cells form a protective covering through which water does not pass very readily. As the plant becomes older and uses up more water, the absorbing area of the root is increased by the formation of many side roots and by the branching of the roots. But it is always in the region near the growing tip of the main root and of the many branch rootlets that absor
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