HYDROPHYTES, plants which grow in water or mud. They may be wholly submerged, completely without roots, and derive their suste nance wholly from the water; or may live amphibiously, rooted in soil and lifting some or all of their leaves into the air, and so differ only in a greater or less degree from land plants. Adaptations of water plants are especially to meet the difficulty of obtaining oxygen and of effecting pollination under water. In plants which grow wholly in or under water, roots, when present, are comparatively small and free from hairs, stems are slender and abound in air-space, and leaves are, as a rule, either long and narrow, or else greatly subdivided, so as to expose the greatest possible amount of surface. The cuticle of the leaf, also, is very thin, and, lacks several of the structures, such as palisade cells and stomata, always present in aerial leaves. Water therefore enters easily into the tissues of the plant and carries with it a large amount, not only of oxygen, but of dissolved nutriment, so that in any oceanic plants, and plants of ponds and rivers, no roots whatever are developed, and these live practically inde pendent of any connection with the land. The
fertilization of submerged cryptogams is ef fected by the passing of generative elements through the water, but only a few submerged phanerogams make such use of the agency of the water. The pollen of the eel-grass (Zostera) has 'been modified for under-water efficiency. It does not form round grains, but elongated thread-like filaments which have the same specific weight as the water, and hence neither float nor sink, but move about at the level of eel-grass growth until they come in contact with the stigma of some neighboring flower. In the duckweeds and some other submerged plants, the male flowers break loose, rise to the surface and float away like little boats carry ing pollen to the female blossoms, which at that time have risen to the surface but sink again as soon as fertilized. The hydrophytes show many examples of exceedingly wide distribu tion, as might be expected of oceanic plants, but is not so easily explained of those of fresh waters, many genera and species of which, nevertheless, are cosmopolitan.