Students' handbook to accompany Plants and their uses . ent is a response to the stimulus ofgravity, acting upon the very sensitive young root and caus-ing unequal growth in the upper and lower sides. It is not amere bending, like that of an unsupported piece of wet string,for the moving end-portion of the root will be found to pushdownward with a force of more than ten times its own weight. 19. Turgidity. Hoot, hairs and other cells of plants usuallytake up water until the cell walls are distended with waterand protoplasm. The outward pressure which distends andstretches the walls is called /
Students' handbook to accompany Plants and their uses . ent is a response to the stimulus ofgravity, acting upon the very sensitive young root and caus-ing unequal growth in the upper and lower sides. It is not amere bending, like that of an unsupported piece of wet string,for the moving end-portion of the root will be found to pushdownward with a force of more than ten times its own weight. 19. Turgidity. Hoot, hairs and other cells of plants usuallytake up water until the cell walls are distended with waterand protoplasm. The outward pressure which distends andstretches the walls is called ////•//<//•, and the resulting condition ROOTS AND THEIR RELATION TO PLANTS 23 is called turgidity. Turgor doubtless helps to force water up-ward through the stem. The distention of cells due to turgoralso accounts for the rigid or erect position of most leaves,growing shoots, and succulent stems. Each distended cell, likean inflated balloon, assumes a semi-rigid position, and a mass ofdistended cells pressing against one another makes the whole. FIG. 17. A young cucumber plant (grown in a flowerpot) At left, in normal condition; at right, much wilted from having been leftunwatered for several days structure rigid. But just as, when the air escapes from a balloon,its wall collapses of its own weight, so the cells of the leaves andshoots collapse when, on account of loss of water, they losetheir turgidity. When soil water is not available to the plant,the outgo from the leaves is often greater than the income fromthe roots, and in such cases wilting follows (fig. 17). If waterdoes not again become available, the plant will die, but with arenewed supply turgidity and the resulting rigidity soon return. 24 INTRODUCTION TO BOTANY 20. Amount of water pressure in woody plants. When verylittle water is briii.;- lost by evaporation from the leaves, thesap pressure in trees and large shrubs is often great. As aresult of this pressure, in early spring sap escapes freely fromcuts or
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