PLANT FORMATIONS. 469 



if they exist in the portions of the temperate region where rain- 

 fall is abundant. In the spring and summer they have broad 

 and comparatively thin leaves, transpiration goes on rapidly, 

 but there is an abundance of moisture in the soil, so that root 

 absorption quickly replaces the loss and the plant does not 

 suffer. In the autumn the trees shed their leaves, and in this 

 condition with the bare twigs they are able to stand the drying 

 effect of the cold and winds of the winter because transpiration 

 is now at a minimum, while root absorption is also at a minimum 

 because of the cold condition of the soil. Perennial herbs like 

 trillium, dentaria, the goldenrods, etc., turn to xerophytic habit 

 by the death of their aerial shoots, while the thick underground 

 shoot which is also protected by its subterranean habit carries 

 the plant through the winter. 



910. While these different vegetation types are generally 

 dominant in certain climatic regions or under certain soil con- 

 ditions, they are not the exclusive vegetation types of the regions. 

 For example, in desert or semidesert regions the dominant 

 vegetation type is made up of xerophytes. But there is a 

 mesophytic flora even in deserts, which appears during the 

 rainy season where temperature conditions are favorable for 

 growth. This is sometimes spoken of as the rainy-season flora. 

 The plants are annuals and by formation of seed can tide over 

 the dry season. So in the region where mesophytes grow there 

 are xerophytes, examples being the evergreens like the pines, 

 spruces, rhododendrons; or succulent plants like the stonecrop, 

 the purslane, etc. Then among hydrophytes the semiaquatics 

 are really xerophytes. The roots are in water, and absorption 

 is slow because there are no root hairs, or but few, and the aerial 

 parts of the plant are xerophytic. 



III. Plant Formations. 



911. The term plant formation is applied to associations of 

 plants of the same kind, though there is a great difference in the 

 use of the word by different writers which leads to some con- 



