Perennial rosette plant from alpine flora of the Andes, showing short stem, rosette of leaves, and large flower. (After Schimper.)
926. Arctic-alpine societies.—The most striking of the arctic plant societies are the “polar tundra,” extensive mats of vegetation largely made up of mosses, lichens, etc., only partially decayed because of the great cold of the subsoil, and perhaps also because of humus acid in the partially decayed vegetation. These tundras are brightened by numerous flowering plants which are characterized by short stems, a rosette of leaves near the ground, and by large bright-colored flowers. Heaths, saxifrages, and dwarf willow abound. Alpine plant societies are similar to the arctic, although some of the conditions are more severe than in the arctic region. This is principally due to the fact that during the summer while the plants are growing they are subject to a high temperature during the day and a very low temperature at night, whereas during the summer in arctic regions while the plants are growing there is continuous warmth for growth and continuous light for photosynthesis. Five types of alpine plants are recognized by some. 1st. Elfin tree. This type has short, gnarled, often horizontal stems, as seen in pines, birches, and other trees growing in alpine heights. 2d. The alpine shrubs. In the highest alpine belts they are dwarfed and creeping, richly branched and spreading close to the ground, while at lower belts they are more like lowland shrubs. 3d. The cushion type. The branching is very profuse and the branches are short and touch each other on all sides, forming compact masses (examples saxifrages, androsace, mosses, etc.). 4th. Rosette plants. These are perennial, short stems and very strong roots, and play an important part in the alpine meadows. 5th. Alpine grasses. These usually have much shorter leaves than grasses of the lowlands and consequently form a low sward.
927. Edaphic plant societies.—These are equivalent to edaphic plant formations, and the vegetation is of course controlled by the peculiar conditions of the soil. There are a number of different kinds of edaphic plant societies determined by the character of the physiographic areas. 1st. Sphagnum moors. These are formed in shallow basins originally with more or less water. The growth of the sphagnum moss along with other vegetation and its partial decay in the water builds up ground rapidly so that in course of time the pond may be completely filled in. This filling in proceeds from the shore toward the center, and in the early stages of course there would be a pond in the center. The partial decay of vegetation creates an excess of humus acid which retards absorption by the roots. The conditions are such, then, as require aerial structures for retarding the loss of water, and plants growing in such moors are usually xerophytes. Some of the plants are identical with those growing in the arctic tundra. 2d. Sand[53] strand of beach. The quantity of sand with very little or no admixture of humus or plant food makes it difficult for plants to obtain a sufficient amount of water even where rainfall is abundant. The same may be said of the sand dunes farther back from the shore. The plants of these areas are then usually xerophytes. Some of the plants accustomed to growing in such localities are American sea-rocket, seaside spurge, bugseed, sea-blite, sea-purslane, the sandcherry, dwarf willow, marram-grass, certain species of beard-grass, etc. 3d. Rocky shores or areas. Here lichens and mosses first grow, later to be followed by herbs, grasses, shrubs, and trees, as decayed plant remains accumulate in the rock crevices. 4th. Shores of ponds, or swamp moors. Here the vegetation often takes on a zonal arrangement if the ground gradually slopes to the shore and out into the pond. In [Fig. 493] is shown zonal distribution of plants. The different kinds of plants are drawn into these zones by the varying amount of ground water in the soil, or the varying depth of the water on the margin of the pond as one proceeds from the land towards the deeper water. On the border lines or tension lines between the different zones the plants are struggling to occupy here ground which is suitable for each adjacent individual formation. Other edaphic societies are those of marl ponds, alkaline areas, oases in deserts, warm oases in arctic lands, the forested areas along river bottoms in prairie or plains regions, etc.
Fig. 493.
Macrophytes in the upper zone of the photic region. Ascophyllum and Fucus at low tide, Hunter’s Island, New York City. (Photograph by M. A. Howe.)
Fig. 494.
Zonal distribution of plants, South Shore, Cayuga Lake.
928. Aquatic plant societies.—In general we might distinguish three kinds, 1st. Fresh-water plant societies, with floating algæ like spirogyra, œdogonium, etc., the floating duck-meats, riccias; the plants of the lily type with roots and stems attached to the bottom and leaves floating on the surface, like the water-lily and certain pondweeds, and finally the completely submerged ones like certain pondweeds, the bassweed (Chara), etc. 2d. Marine plant societies, which are made up mostly of the red and brown algæ or “seaweeds,” though some green algæ and flowering plants also occur. 3d. The salt marshes where the water is brackish and there is usually a luxuriant growth of marsh-grasses.