MESOPHYTE SOCIETIES. 243 



are far more brilliant in certain years than in others, and 

 that the coloration must be connected in some way with 

 the food relations of the plants. Recent experiments have 

 shown that the red coloration is largely dependent upon 

 low temperature, which affects certain of the food-stuffs, 

 and the red stain is one of the products. 



The autumnal colors are notably striking in American 

 forests on account of the fact that in these forests there 

 is the greatest display of species, and hence not only are 

 more colors produced, but they are usually strikingly 

 associated. 



Xot only is protection during the cold period secured 

 by deciduous forests through the falling of leaves, but the 

 development of scaly buds is an adaptation to the same 

 end. By means of these overlapping, often hairy, and 

 even varnished structures, delicate growing tips are pro- 

 tected during the cold season. The development of cork, 

 also, on the older parts, is a measure of protection. 



As in the case of thickets, deciduous forests may be 

 pure or mixed. A very common type of pure forest is the 

 beech forest, which is a characteristic dark forest. The 

 wide-spreading branches of neighboring beeches overlap 

 each other, so as to form dense shade. As a consequence, 

 in a pure beech forest there is little or no undergrowth ; 

 in fact, no lower strata of vegetation until the lowest 

 ones are reached, made up of grasses and mosses. An- 

 other type of pure forest, which belongs to the drier re- 

 gions, is the oak forest, which forms a sharp contrast to 

 the beech, in that it is a light forest, permitting access of 

 light for lower strata of plants. Hence in such a forest 

 there is usually more or less undergrowth, consisting of 

 shrubs, etc., which may develop regular thickets. The 

 typical American deciduous forest, however, is the great 

 mixed forest, made up of many varieties of trees, such as 

 beech, oak, elm, walnut, hickory, gum, maple, etc. These 

 great mixed forests, with their remarkable autumnal 



