HYDROTROPISM (from Gk. iniwp, hydor, water + rpor*, trope, a turning, from rptreiv, trepein, to turn). The sensitiveness of certain plant organs to the presence of unequal moisture, which causes them to curve toward the source of moisture. Among the fungi hydrotropism is shown particularly by those filaments (hyphen) which bear the sporangia. While most of the hypha ramify through the moist substance on which the fungus grows, the sporangial hyphn rise at right angles to it. If they start from a salient angle on the substratum (e.g. the edge of a slice of bread), they may take the position shown at Fig. 1. growing equidistant from the two moist surfaces. If they arise from a re entrant angle, they place themselves likewise so as to be as far away from each moist surface as possible. The roots of higher plants are also sensitive to moisture, the stimulus of which may exceed that of gravity, as shown in Fig. 2. Here roots of corn planted in the heaped-up sand have come over the edge of the glass funnel, and have started to grow downward in the moist air. As
they get farther from the surface of the wet paper covering the outside of the funnel, they find drier and drier air. A stream of water par ticles, however, is reaching them from one side. At a certain point the stimulus of the diffusing moisture overcomes the geotropism (q.v.) and the root, growing more rapidly on one side, is directed toward the moist surface. When it comes into the neighborhood of the moist paper again the sides are less unequally stimulated, the stimulus of gravity reasserts itself, and the tip again grows downward. The alternating preva lence of hydrotropism and geotropism is shown in the wavy course of the longer roots. It will be observed that the more rapid growth which directs the tip toward the moist surface is not due merely to the absorption of water, for it occurs on the drier side of the root. Hydrot ropism is a form of ebemotropism (q.v.).