Classification of Habitats are usually grouped with respect to the two direct factors, water and light. They are first classed as wet, moist and dry and the moist habitats are further divided into sun and shade. Wet habitats comprise all bodies of water, oceans, lakes, ponds, springs, streams, swamps, marshes, river-banks, seashores, tanks, etc. Dry habitats are principally deserts, sandhills, prairies, gravel-slides, strands, dunes, bad lands, cliffs, rocks, heaths, humus-marshes, moors, alpine and polar barrens. Sunny moist lands are meadows, pastures, grain-fields and waste places. Shady moist habitats are forests, groves, woodlands and thickets.
Effects of These, as regards the individual, are either evident or demonstrable, as in the case of the habitat form seen in bog plants, shade-plants, etc., or they are obscure and remote and can in consequence no longer be traced. The latter is true of vegetation forms trees, shrubs, bulb-plants, etc. Three well defined groups of habitat forms are recognized, based upon the water-content of habitats. These are water-plants (hydrophytes), moist-land or middle plants (mesophytes), and desert plants (xerophytes). Upon the basis of light differences, mesophytes are further divided into sun-plants (heliophytes), shade-plants (scio phytes) and darkness-plants (scotophytes). Water plants owe their peculiar stamp to the fact that the water-supply is always greatly in excess of the water-loss. The roots are super ficial in position, owing to the abundance of water at or on the surface of the soil. Root surfaces are slightly developed and root-hairs often lacking, because the amount of water ren ders absorption easy. The surplus of water is a disadvantage, however, as it reduces the amount of air in the soil and hence cuts off the supply of oxygen necessary for the activity of the roots. This lack of aeration is compensated by the development of large air-passages leading down from the leaves through the stem and roots. Stems and leaves are almost invariably smooth and without any sort of protective cov ering. Breathing-pores are usually abundant and the necessity that the plain should lose a large amount of water has led to the develop ment of water-pores and papilla. This struc ture is typical of amphibious plants, that is, those that grow in the mud or in shallow water. Floating plants are usually much the same, with the exception that the breathing pores become useless and disappear on the under surface of the leaf, which is in contact with the water.
Certain plants, such as the duckweed, have be come greatly reduced in consequence of the floating habitat, and consist merely of a tiny, leaflike disc, with a few rootlets. Submerged plants grow entirely beneath the water and are not subject to water-loss. As a result their leaves and stems are greatly reduced. The leaves are thin and divided into narrow seg ments; in structure they are almost uniform. The characteristic air-passages of the other water forms are lacking, as all the air must be dissolved in water.
Desert-plants are in most respects the exact opposites of water-plants. Not only is the water-supply scanty, but all the factors which increase water-loss are present in a large de gree. Such plants must use all their power of adaptation to absorb and store all the water they can and to lose just as little by evaporation as possible. The roots of desert-plants are for the most part deep-seated and branch for the most part only in the deeper, moist soil. They are covered elsewhere with a corky layer to pre vent the loss of water where the root passes through the dry upper layers. In the moist soil, root-hairs are produced in large numbers. In many cases the upper portion of the root consists of tissue especially adapted to the stor age of water. The stems of xerophytes are short and stout. The leaves are thick and much reduced in size; and in extreme forms they are entirely lacking. Both leaves and stems are covered with a thick coating of hairs, or wax, or the epidermis is greatly thickened, all for the purpose of protection against water-loss. The breathing-pores are generally confined to the underside of the leaf and are often sunken far below the surface for still greater protection. In the cactus the leaves are reduced to mere scales and the stem often contracted into a cylinder or ball, thus decreasing the exposed surface to the minimum. Succulent plants, such as the live-for-ever and ice-plant have, on the other hand, been modified so that the leaves serve for the storage of water. Lichens and mosses which grow on rocks are capable of withstanding extreme dryness, a faculty seem ingly inherent in their protoplasm, as they are without ordinary protective contrivances.