FOREST SOCIETIES. 531 



ing with a rainy season. These thickets are very highly devel- 

 oped in the so-called " thorn woods" and are often impenetrable 

 because of the very dense and intricate growth, often with a pro- 

 fuse development of thorns. In the southern Appalachians in 

 North America very dense thickets are sometimes formed by 



Fig. 4960. 



Forest with chaparral, southwestern United States. (Bureau of Forestry, 

 U. S. Dept. Agr.) 



the rhododendrons, which here reach such a high development, 

 especially R. maximum. 



1010. General structure of the forest. Structurally the forest 

 possesses three subdivisions: the floor, the canopy, and the in- 

 terior. The floor is the surface soil, which holds the rootage 

 of the trees, with its covering of leaf-mold and carpet of leaves, 

 mosses, or other low, more or less compact vegetation. The 

 canopy is formed by the spreading foliage of the tree-crowns, 

 which, in a forest of an even and regular stand, meet and form a 

 continuous mass of foliage through which some light niters down 

 into the interior. Where the stand is irregular, i.e., the trees of 



