HYDROTHORAX. 887 HYDEOZOA. of the pleura, great advantage may often be de- rived from occasional cupping and repeated blis- tering, hydragogue catharsis, tapping of an ascitic abdomen, etc., according to the cause. HYDROT'ROPISM (from Gk. vAup, hydSr, water + Tpmr//, trope, a turning, from TpcKciv, Fig. 1. NEGATIVE HYDHOTROPISM. Culture of the black mold on bread in a moist chamber. The hyph(e carrying the thread-like sporangia grown in Buch directions as to be as far as possible from the surfaces of bread and glass from which moisture is diffusing. Dia- grammatic. trepein, to turn). The sensitiveness of certain plant organs to the presence of xmequal moisture, Fig. 2. positive hydrotropism. Grains of corn, planted in wet sand in a funnel whose outer surface is covered with wet blotting-paper, have germinated. The roots, r, instead of growing straight down, as they would do in saturated air, have applied themselves to the wet paper. Hydrotropism under these conditions overpowers geotropism. which causes them to curve toward the source of moisture. Among the fungi hydrotropism is shown particularly by those filaments (hyphae) which bear the sporangia. While most of the hypha" ramify through the moist substance on which the fungus grows, the sporangial hyphse rise at riglit 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 chcmotropism (q.v.). HY'DROZO'A (Xeo-Lat. nom. pi., from Gk. vdup, Ai/dor, water + ^ipov, 2oo«, animal) . A class of Coelenterata in which the simplest form of the body, as in the polyps, is a sac attached by its posterior end, and with a digestive cavity communicating with the exterior by a mouth only. The mouth is surrounded by hollow ten- tacles. The body is composed of two cellular layers, the ectoderm and endoderm, separated by a gelatinous, non-cellular 'mesogloea.' Tlie body- is usually differentiated into two sorts of zo"- oids, i.e. 'polyps' or nutritive zoiiids, which are usually sexless, and "medusae,' or reproductive zoiiids. The latter produce medusa buds, which on being set free are called medusie, and are bisexual. In these medusa- the body is greatly more complicated than in the polyps, being bell or umbrella shaped, with a well-developed ner- vous system, composed of a thread-ring around the eye of the disk and with ganglia near the 'eyes' or sense organs. The digestive cavity is differentiated into a central and a peripheral portion, the latter forming radial and circular canals. The eggs and sperm are discharged ex- ternally, and fertilization of the egg takes place in the sea. Xettling organs (see XEM.TOcysT) are usually present; they arise from some of the interstitial cells, and are most abundant in the tentacles. Many Hydrozoa (Hydra and Hydrac- tinia excepted) exhibit the phenomenon of alter- nation of generations (q.v.). The asexual polyps give rise to buds which develop into medusx, and the latter lay the eggs from which are hatched the polyps. The medusie are more or less phos- phorescent. They abound in all seas, while Hydra is a minute fresh-water form. Very primitive
