ALTERNATION AND ZONATION 299 



In consequence zones are regular features of vegetation. The 

 zonal arrangement of formations is usually evident, but the zones 

 of a formation are often obscured and sometimes lacking. The 

 latter is regularly the case in a uniform area, such as a shallow 

 pond or a new soil. 



Zones are obscured in several ways. The plants are some- 

 times too scattered to make the response to physiographic sym- 

 metry evident. The alternation of conspicuous species not only 

 interrupts zones, but often it also completely hides the zonation 

 of species of lower habit. The ecotones of one factor may run 

 at right angles to those of another, and the resulting series of 

 zones may obscure each other. A physiographic feature such 

 as a hill may have its symmetry interrupted by ridges or ravines, 

 which deflect the zones downward or upward, or cause them to 

 disappear altogether. 



327. Kinds of zonation. Two kinds of zonation are dis- 

 tinguished with respect to the direction in which the controlling 

 factor changes. When this is horizontal, as with water content 

 and temperature, zonation is lateral. If the direction is vertical, 

 as in the case of light, zonation is vertical. There sometimes 

 exists a close 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. Lateral zonation is radial when the habitat or 

 physiographic feature is more or less circular in form, and it is 

 bilateral when the latter is elongated or linear. Vertical zona- 

 tion is unilateral, i.e.. the zones extend in but one direction. 



328. Radial zonation is typical of elevations and depressions, 

 especially mountain peaks, islands, lakes, ponds, etc. The zones 

 of peaks are ordinarily quite perfect. They are due largely to 

 temperature, though humidity also plays a part. The zonation 

 of islands, hills, etc., is jjroduced by water content. The zones 

 of islands are often regular and complete, while those of hills 

 are more often incomplete or obscure. Prairies and steppes do 

 not show a series of zones, but their hills and ridges are more 

 or less zoned. Ponds and lakes usually show comj^lete zones, 

 except in ponds so shallow that the ordinary marginal zone is 

 able to extend over the entire bottom. 



The line between an elevation and a depression, i.e., the edge 



