Hydrophytes which live in fresh water show many features which adapt them to the peculiar conditions of their environment, and are given the name of aquatics. Some aquatics, such as the mare ’s-tail and water milfoil, live totally immersed in water, and are rooted in the mud at the bottom of the stream or. The Canadian water weed, though submerged, has no and floats freely. The white and the water crowfoot are aquatics which, in addition to submerged , also bear leaves which float at the surface of the water.
The conditions of life of a plant living in water are very different from those of a land plant. Since all or part of an aquatic is submerged in water, the plant has not the difficulty of a land plant in obtaining water. The cuticle of an aquatic is thin and easily permeable by water, so that the plant can absorb water and mineral salts, as well as carbon dioxide and oxygen, over the whole of its surface. Water plants, therefore, are not dependent on roots for their water supply, and these may be absent altogether, as in the bladderwort, or developed mainly for anchoring purposes, as in theand water crowfoot.
Since aquatics can obtain their water so easily, the conduction of water up theis not so essential as in land plants, hence there is but little conducting tissue and this is confined to a small space at the centre of the . The xylem is poorly developed and is only partly lignified, while there is little phloem since most parts of the plant are green and can easily manufacture or obtain the elaborated food materials.
In land plants the wood plays the important mechanical function 44° of making the plant rigid and enabling it to maintain an upright. In the water such rigidity is not required, for the aquatic is supported and held up by the water, and consequently the wood is not required as a mechanical tissue. The chief strain to which a rooted water plant is subject is the pull exerted by running water of a stream. The little wood that is present in the of aquatics is found at the centre of the stem, where it can best resist the pulling strain. The same kind of arrangement is seen in a rope-covered wire hawser which has the strongest material at the centre. The leaves of submerged plants are also exposed to the pulling effect of running water, and since large leaves are liable to be torn many aquatics have finely divided leaves which offer little resistance to the flow of the water. Water milfoil has leaves arranged upon the stem in whorls of four, and each is divided into many slender filaments. The water crowfoot has finely cut submerged leaves, and larger rounded leaves which float on the surface. The floating leaves of aquatics show features of both land and water plants. The upper surface of the floating of a water lily, for example, is adapted for life in air, and has a well-developed epidermis in which stomata occur. The epidermis has a waxy cuticle which prevents the surface of the leaf from being wetted, so that the stomata are not blocked if the leaf surface is temporarily swamped with water. Beneath the upper epidermis is palisade tissue containing chlorophyll, and below this is loose parenchymatous tissue containing large air spaces. The lower epidermis is thin-walled, and has neither waxy cuticle nor stomata.
The proportion of carbon dioxide is greater in water than in air, so that aquatics have no difficulty in obtaining sufficient quantities of this gas, but owing to the absorption of light by the water they receive a diminished light supply. As a result, the stems of water plants rooting in the mud at the bottom of aor stream often show the very long internodes characteristic of etiolated plants. There is less oxygen in the mud at the bottom of the water or in still water than in air, and in order to obtain sufficient oxygen for respiration aquatics have a very well-developed system of air spaces, which occur in all parts of the plant that are submerged. In the water-lily petiole the air spaces are so large that they can be seen by the naked eye. The oxygen that is absorbed from the water, and also that which is set free during , is stored in the air spaces and can diffuse down these to the underground stems rooted in the mud, where oxygen is scarce.
Theof most aquatics float at the surface, as is the case in water crowfoot and water lily, and are pollinated by wind or insects.
Elodca furnishes a good example of rapid multiplication by vegetative, for though female flowers are found, the male flowers do not develop in this country, so that no are set. The plant branches frequently, and whenever a branch shoot becomes detached it continues growth as a separate plant. Elodea was introduced into this country from America in 1S42, and multiplied so fast that for some time many rivers and canals were completely choked by it.