HY'DROPHYTES (from CIk. Mop, hyd5r, water (istrr6e, phytoa, plant). Plants which grow naturally either in water or in very wet. soil. This term is contrasted with mesophytes and xerophytes (qq.v.). Common illustrations of hydroph•tie plant societies are swamps of all kinds, pond societies, and plants. Various classes of hydnqihytes are taken up under sepa rate heads, where the main features of the va rious hydrophytie societies will be discussed. It will he desirable. however, to give a short ac count of the eharacteristie lildrophytie struc tures. The roots of hydrophytie plants are in most eases very greatly reduced, and in sonic eases altogether lost. as in some of the duck Hoot-hairs are also commonly lacking in water plants. The stems and leaves are not as a whole conspicuously reduced in water plants, but they show peculiarities of structure that are quite interesting. The leaves of hydro phytes are frequently finely divided, as in the water-milfoil and mermaid-weed. In many eases where the leaves are not finely divided, they are very thin; for example, in tape grass. An ex amination of the structure of the leaves shows the entire absence of stomata in the submerged parts, complete or almost complete absence of palisade cells. and a very thin epidermis which contains chlorophyll. The stems sillier a noteworthy re duetion in the size and development of the water conducting vessel: and mechanical tissues. and a great increase in air-spaees. The slruetures just described are found in their highest develop ment in submerged water plants. Ilydrophytes whose leases are ai:rial show no conspicuous dif ferences from ordinary land plant"; in most re spects. One olas: of hydrophytes. which may perhaps he called amphibious plants. shows sonic remarkable variations, especially in structure.
Leaves which are developed under the water show the characteristic structures outlined above, including the line leaf division, whereas leaves of the Slane plaint developed in the air show typical leaves without these divisions and with palisades, stomata, and a thick cuticle. The Fi lint] Ins or stimuli which pause these wide varia tions are not certainly known, but they are dis cussed to some extent in the article LEAF'. Com mon American plants, which show variations to a high degree, are the aquatic buttercups, the mermaid-weed, some of the tresses, and water hemlocks.
the hydrophytie struetuies that have been described in the preceding paiagraph may have arisen. certain advantages can be clearly seen, at least in some cases. The thin walls of the epidermis. which are in striking eontrast to the thick entinized walls of ntany leaves, permit the cast' entrance of water and substances dissolved in the water. On this account litany suianerged plants are practically independent of soil relations; they take in most of their male• vial directly from the water. It is easily possible to grow cultures of many of these plants without having root connection while certain forms, such as the water-milfoil and the water-weed (Elodea), roots in ordinary aquarium cultures, other forms, such as horn. wort (Ceratophyllum). never velop roots and yet grow quite as vigorously as in their natural rooted condition. A few forms, such as the bladderwort (Ctrieularia) and some of the duckweeds, have no roots in nature. Of course, in such eases, the entire absorption of nul rimont must take place through the leaf epidermis. The fact that the leaf epidermis contains chlorophyll is also a matter of advantage, since water very soon destroys the efficiency of rays of light. At
a comparatively shallow depth there is a cessa tion of the development of green A re duction in the water-condueting tissues. while not necessarily an advantage. is. nevertheless, not harmful, inasmuch as the absorption is so largely through the leaf. instead of the root as in land plants. The reduetion in root develop ment is not so easy to understand, since it would seem that holdfast organs would ordinaril• be of advantage; then again. any absorption whielt the roots might make would so much the more increase the capacity of the plant. A high de velopment of air-cavities is a distinct advantage, not only to help limit the plant. hut probably to a much higher degree to net as a sort of air storage. It can readily be seen that [lie condi tions for obtaining air underneath the water are not of the best, and that any additional means for obtaining or for storing air would increase the plant's eflieieney. The reduction in the lie. N clopment of mechanical tissues, of palisade cells, and of stomata is not necessarily au advantage to water plants. bud since these tissues arc not actually needed. the plant loses nothing by its failure to develop these structures. The peculiar leaf forms that have been noted above are. per• haps, not necessarily of arty exceptional ad van• lage. Tt must not be supposed that everything in a plant can be explained in accordance with the need of the plant. It is much more likely that the explanations should be referred to deli idle chemical and physical causes. However. in the ease of finely divided leaves. it ean be seen that a much larger proportion of cells conies in contact with the material than is true with the more compact air leaves. Thus the absorption capacity of the leaf is increased. Finely divided leaves are also doubtless more able to escape the dangers coining from currents of water than leaves which are more compact.
The hydrophytie plant societies are essentially all edaphie, that is, they are conditioned by local causes. in this respect there is a wide contrast as compared with the xerophytic plant societies. Perhaps some of the ocean formations may be conditioned by climatic causes to some extent, but the ordinary hydrophytic societies of swamps and ponds are due to essentially local conditions. Perhaps no plants have such a wide distribution as certain of the hydrophytes. This is particu larly true of ocean plants, where it can easily be accounted for by the almost universal distribu tion of the oceans themselves. it is true, to a striking degree, as well of the pond and swamp plants. Such plants as the pond wecd=. eattails, and bulrushes. are found almost throughout the world the habitats are favorable. Perhaps the chief reason for the wide distribution of hydrophytie species is the great ease of dispersal by means of the water itself, but a reason. almost if not quite as is furnished by the wide degree of uniformity of hydrophytie con ditions. Since water is colder in summer and warmer in winter than adjoining portions of the land, it is obvious that water plants can. for reasons of temperature. have a much wider dis tribution than land plants.
The hydrophytie plant societies may be rough ly subdivided into those associated with salt water and those with fresh water. The former are treated under the heads PLANKTON; BEN TIIOS ; MANGROVE SWAMP: and HALOPHYTES : the latter under the head of SWAMPS, where it will be convenient, not only to treat the swamps proper. but also, to some extent, the development of the swamps from ponds and lakes. See Pis TRIBUTION OF PLANTS.