Moist-land plants, or mesophytet, stand as intermediate between the two preceding groups. The water-supply, though not excessive, is usually sufficient, and the humidity of the air is great enough to preclude the danger of excessive water-loss. In consequence, mesophytes have well-developed, more or less branched, root systems, which are usually intermediate in posi tion, but many of them are deep-seated. The development of surface is moderate, as well as that of the protective cork. Stems are for the most part tall and vigorous and much-branched. The leaves are large and mostly entire, or at least rarely finely dissected. Hairs are com mon, but seldom compacted into a dense cover ing. The epidermis is not greatly thickened and while the breathing-pores are often more abundant on the lower side, they are present in large numbers on both surfaces. The leaves of mesophytes are characterized regularly by com pact rows of oblong cells placed at right angles to the surface, which are called palisade-tissue, and by loose irregular cells with large inter cellular spaces, the sponge tissue. The former is usually in the upper, the latter in the lower half of the leaf. The palisade is differentiated in response to the action of strong light, while sponge-tissue results from the need of the rapid diffusion of the carbon dioxide and oxygen ab sorbed from the air.
Sun-plants and shade-plants are especially different in their leaves; this is to be expected, as the leaf is the organ most dependent upon light. The root-system will be more superficial in shade-plants, as the moisture is nearer the surface of the soil in shaded than in sunny places. The stem will be more slender, taller and often more branched, since it is necessary to place the leaves in the position to receive the most of the diffuse light. The leaves are broad, thin and entire, increasing the exposed surface, while in sun-plants they are thick and more or less divided. In typical shade-leaves, the pali sade-tissue is reduced to a single row, or is al together absent. The cells are not crowded closely and their longest axis often coincides with the epidermis of the leaf. Sun-leaves have one or more rows of typical palisade on the upper side and are frequently palisaded on the lower side also. The epidermis develops more wax and hairs in the sun ; the breathing-pores are more numerous on the lower than upon the upper surface, while there is little difference in the shade.
Origin and Distinction of Vege tation-forms doubtless originated in response to physical conditions, but this relation is hardly evident to-day. We can only see in trees, shrubs, herbs, etc., an expression of the success which different plants have obtained in the struggle for existence. It is also evident that the vege
tation form of a plant has much to do with its persistence and hence with its importance in vegetation. The main groups of vegetation forms are woody plants, herbs and thallus (flowerless) plants. The former are the largeit, the most dominant, and the most persistent of all forms; the latter are tiny, subordinate and fleeting. The various woody forms are trees, shrubs, bushes and climbers; the first are the most important, the last the least so. Trees con stitute the most permanent type of vegetation, the forest, to which shrubs, bushes and climbers also contribute. The relation between these forms is easily seen in the development of a forest, in which bushes precede and are fol lowed by shrubs; these give way to the trees, the climbers coming in after the latter.
Herbs are specially distinguished from woody plants by their lack of woody stems and by the fact that their persistent parts are under ground. They can never be very large, for their stems lack support, though they may per sist for years. They are either perennial, blooming each year, or annual-biennial, bloom ing at the end of the first or second year, and then dying. Various forms of herbs are dis tinguished with reference to the position of the stem and leaves. In some, like the dande lion; the leaves are grouped in rosettes in re sponse to light and heat. In others, such as the everlasting, the plants are set close together for somewhat similar reasons and perhaps for mutual protection also. Climbers, sweet peas, vetches, etc., develop partly for increased sup port, partly to secure more light. Grasses form sod because of their abundant rootstalks; it is this faculty which enables grasses to control the vegetation of meadows and prairies. Bunch grasses are an exception, but they generally i grow where the formation of a sod is impos sible on account of unfavorable physical condi tions. The position and abundance of herbs in a formation will depend also upon the char acter of the underground parts. Species with underground runners or rootstalks will be more abundant and more widely distributed than those with nearly stationary bulbs or tubers. The vegetation forms of thallus plants play a very subordinate role in vegetation. Mosses, liverworts and lichens are regularly present in tree and herb formations, but their small size and transient nature make them of little im portance. They are significant of the early stages of vegetation on rocks, new soils, etc., but they soon disappear before the grasses and other herbs. Fungi are entirely dependent upon their host plant or stratum and are relatively insignificant, except where they are necessary to the nutrition of the host, as in the case of cer tain trees.