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- 
ing, 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 effect. An entire abseace 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 
