ZONATION 279 



the zones produced are obscure. Asymmetry of a habitat or a region prac- 

 tically does not exist. Central areas of excess and deficiency may be very 

 large and in consequence fail to seem symmetrical, or the space between 

 them so great that the symmetry is not conspicuous, but they are everywhere 

 present, acting as foci for the intervening areas. 



The response of vegetation to habitat is so intimate that physiographic 

 symmetry everywhere produces vegetational symmetry, which finds its ready 

 expression in plant zones. The reaction of vegetation upon habitat causes 

 biological symmetry, typical of growth zones and light zones. From these 

 facts it is clear that zonation will be regularly characteristic of the vegetative 

 covering. The zonal arrangement of formations is usually very evident; 

 the zones of a formation are often obscured, or, where the latter occupies 

 a uniform central area of excess or deficiency, they are rudimentary or lack- 

 mg, 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 transi- 

 tions between the various stages, the plant population is so scattered, so 

 transient, or so dense as to respond 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 physiographic feature as a hill may have its sym- 

 metry interrupted by ridges or ravines, which deflect the zones downward 

 or upward, or cause them to disappear altogether, while the shallows or 

 depths of a pond or lake may have the same efrect. An entire absejice of 

 zones, i. e., azonation, is exceptional in vegetation. Almost all cases that 

 seem to exhibit it may be .shown by careful examination to arise in one of the 

 several ways indicated above. 



KINDS OF ZONATION 



337. 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 temperature, 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 



