Water Plants

plants that grow in water. They are divided into hydrophytes, or plants with only their lower parts submerged in water, and hydatophytes, which are completely or mostly submerged in water.

Water plants evolved special traits to deal with life in water. For example, the surface area of the plant is significantly increased relative to its weight; this facilitates the absorption of the necessary quantities of oxygen and other gases, which are not as plentiful in a water environment as in air. This increased surface area is achieved by the development of large, thin leaves (some pond weeds), the separation of the leaf plate into thin threadlike sections (milfoil, hornworts, water crowfoot), and the appearance of holes in the leaves or the development of air-bearing cavities and large intercellular spaces.

Water plants have various types of leaves (heterophylly); submerged, floating, and aerial leaves on the same plant are very different in both their internal and external structures. Thus, submerged leaves do not have stoma; in leaves that float on the water surface, there are stoma only on the upper side, while aerial leaves have stoma on both sides. The great density of the water medium causes weak development of the mechanical elements in the leaves and stems; the few mechanical elements that exist in the stems of water plants are located near the center, which gives them great flexibility. Since the light intensity in the water is sharply reduced, the epidermal cells of many water plants have chloroplasts. Veins and conducting bundles are weakly developed or entirely lacking. The root system is also weakly developed, and there are no root hairs. Almost all water plants are perennials that reproduce vegetatively. Some water plants (naiads, hornworts) are pollinated under water; in others the flowers rise out of the water, and pollination occurs there. Some water plants have adapted to the periodic drying up of the bodies of water (water plaintain, arrowhead, watercress).

There are more than 260 species of flowering water plants, predominantly monocotyledons, in the USSR. The seeds and fruits are dispersed by birds or water currents. Some water plants are useful; edible parts include the seeds of the water chestnut, the rhizome of the rush, and the grain from reed sweet grass. The seeds and fruits of many water plants serve as food for some birds, and the dead remains of water plants are often eaten by invertebrates, which in turn are eaten by fish. Water plants play a role in the self-cleaning of basins, although sometimes, as in the case of water thyme and some pondweed species, they themselves can be harmful if they grow too numerous, especially in reservoirs. In order to prevent their rapid growth and undesirable spread, water plants are mowed with special water mowers; the cut water plants are sometimes used as cattle fodder.

Many water plants are grown in aquariums. Water plants also include many algae (for example, green and blue-green) that can grow too thick, choking off the oxygen supply for fish, and clog canals and the cooling ponds of thermo-electric power stations. Sometimes herbivorous fish such as the white amur or silver carp are bred to clean canals and other bodies of water. Herbicides are also used to destroy water plants. Special agricultural technology has been evolved for water plants used as fish food.