KINDS OF ZONATION 155 



rudimentary or lacking, as in shallow ponds. Zones are 

 frequently imperfect, though rarely entirely absent, in new 

 soils, such as talus. They are rendered obscure in several 

 ways. In the initial stages of a succession, as well as in 

 the transitions between the various stages, the plant popu- 

 lation is so scattered, so transient, or so dense as to res- 

 pond not at all to a degree of symmetry which produces 

 marked zonation in later formations. The alternation of 

 conspicuous species not only causes great interruption of 

 zones, but often also completely conceals the zonation of 

 other species, such as the grasses, which though of more 

 importance in the formation, have a lower habit of growth. 

 Furthermore, the ecotones of one factor may run at right 

 angles to those of another, and the resulting series of zones 

 mutually obscure each other. Finally, such a physiogra- 

 phic feature as a hill may have its symmetry interrupted by 

 ridges or ravines, which deflect the zones downward or up- 

 ward, or cause them to disappear altogether, while the 

 shallows or depths of a pond or lake may have the same 

 effect. An entire absence of zones, i. e. azonation, is ex- 

 ceptional in vegetation. Almost all cases that seem to ex- 

 hibit it may be shown by careful examination to arise in 

 one of the several ways indicated above. 



Two kinds of zonation are distinguished with reference to 

 the direction in which the controlling factor changes. 

 When this is horizontal, as with water content and tem- 

 perature, zonation will be lateral : when it is vertical, as in 

 the case of light, the zonation is vertical. There exists an 

 intimate connection between the two in forests, where the 

 secondary layer of small trees and shrubs is continuous with 

 a belt of trees and shrubs around the central nucleus, and 

 the lower layers of bushes and herbaceous plants with 

 similar zones still further out. This connection doubtless 

 arises from the fact that conditions are unfavorable to the 

 facies, outside of the nucleus as well as beneath it. Floris- 

 tically, each layer and its corresoonding zone are distinct, 

 as the one consists of shade, the other of sun species. 

 Lateral zonation is radial when the habitat or physiographic 



