=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. [Illustration: 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.)] [Illustration: Fig. 494. Zonal distribution of plants, South Shore, Cayuga Lake.]