PLANT SOCIETIES. 483 



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 t}'pes of alpine 

 plants are recognized by some. ist. 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 bells 

 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 compjact 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 low- 

 lands 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, ist. 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 * 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 



* See Chapter LIV of the author's "College Texl-book of Botany." 



