placed. By placing the hydrometer in liquids of known densities it may be graduated empirically; and may henceforth be used to measure the unknown density of a liquid. The distinguishing feature of this form of hydrometer is that the weight of liquid displaced is a constant. The second form, which is generally ascribed to Nicholson but is really due to Robert Boyle (1675), is a floating body provided with a fiducial mark. The density of any liquid is determined by the weight which must be placed upon the hydrometer to sink it to the fiducial mark in this particular liquid. [A fiducial line or point is a line or point of reference, as for setting a scale used for measurements.] This form of hydrometer is sometimes used for weighing bodies in water and out of water; and in this way it may be used to determine the specific gravity of solids. The characteristic feature of this second form of hydrometer is that the volume of liquid displaced is constant. A great many special modifications of these two forms are to be found. For these consult any good laboratory manual of physics or chemistry.
Hy′drophytes. The name literally means water-plants. The water conditions in which such plants grow range from complete submergence to swamps. Such plants have various adaptations for living in connection with a large amount of water. If the plant lives submerged, there is a feeble development of the tissues for mechanical support, and when it is removed from the water it collapses. Another common adaptation of water-plants, found also in swamp forms, is the development of conspicuous air-passages for aeration. Stalks of water-lilies, calladiums etc. easily show these large passageways. It is necessary for air to be brought into the plant and carried to parts which are too much shut off from air by water. In certain plants which float, bladder-like floats are provided, as in the common bladderworts. There are three conspicuous types of hydrophytes. The first type is made up of the “free floating societies,” that is, those in which the plants are entirely sustained by water and are free to move either by locomotion or water currents. To this group belong the ordinary pond societies, composed of algae, duckweeds etc., which float in stagnant or slow-moving water. The second type is made up of the “pondweed societies,” in which the plants are anchored, but their bodies are submerged or floating. Here belong the associations of seaweeds, among which there are often elaborate systems of holdfasts. Another conspicuous pondweed society is that which contains among its representatives the water lilies with their broad floating leaves and the pondweeds or pickerel weeds with entirely submerged leaves. The third type is made up of “swamp societies,” in which the plants are rooted in water or in soil rich in water, but the leaf-bearing stems rise above the surface. The conspicuous swamp societies are “reed swamps,” characterized by tall rushes, cat-tails and reed grasses, wand-like monocotyledons which usually form a fringe about shallow margins of small lakes and ponds; “swamp moors,” the ordinary swamps, bogs, marshes etc., which are covered by coarse grass; “swamp thickets,” in which there is a tangle of willows, alders etc.; “sphagnum moors,” in which the sphagnum moss prevails and is accompanied by numerous orchids, heaths etc.; “swamp forests,” in which the tamarack (larch), spruce, pine, hemlock and juniper are the prevailing trees.
Hydrotropism (hī̇-drŏt′rō̇-pĭz’m), the sensitiveness of a plant to the presence of moisture in greater amount on one side, to which it responds by so altering the rate of growth as to curve toward or away from the source of moisture. Thus the mycelium or nutritive part of fungi grows towards moisture; the reproductive parts generally away from it. In the higher plants this sensitiveness is shown most strikingly by roots. If roots of corn are growing straight downward by geotropism (which see), they will be deflected in a few hours toward a sheet of wet blotting paper hung near them. Growth is accelerated on the side where the air is drier, but is retarded on the moister side. As this is exactly the reverse of the direct influence of moisture, it is clear that the result must be an indirect one, due to irritability (which see).
Hy′drozo′a. Sometimes casual visitors at the seacoast gather certain animal forms from rocks and submerged objects, and press them on paper under the name of seaplant or seamoss. This shows how much some of the hydrozoa resemble delicately branching plants. The general stem is usually horny and hollow. At various points are small, tubular polyps resembling the fresh-water hydra in form and structure. While it is solitary, they are colonial. In this group also the polyps have become modified into several kinds of persons. The feeding persons—with tubular body disc and tentacles—are the most typical. The reproductive persons are often shaped like small jellyfish, and are set free from the colony when fully formed. They are called medusoids. There are in some colonies, in addition to the above, protective persons, swimming persons and stinging persons. These are all modified polyps, and the variety of form gives rise to what is called polymorphism. The hydrozoa make a natural division of the subkingdom Cœlenterata (which see). See Hydra.
Hye′na an ungainly, carnivorous mammal of southern Asia and the continent of Africa. The hind quarters are lower and weaker than the fore quarters, which gives them an awkward shambling gait. The body is covered with rather long, coarse hair,
