158 ZONATION 



degree to which the lower layers influence the upper by re- 

 acting upon the habitat is not known. It is evident that 

 this influence must be considerable by virtue of their con- 

 trol of the water supply in the upper soil strata, by virtue 

 of their transpiration, their decomposition, etc. 



The ecotone between two formations is never a sharp line, 

 but it is an area of varying width. The edge of this area 

 which is contiguous to one formation marks the limit for 

 species of the other. Both formations disappear in this 

 transition zone, but in opposite directions. The over- 

 lapping which produces such zones arises from the fact that 

 the physical factors tend to approach each other at the line 

 of contact between formations, and that many species are 

 more or less adjustable to conditions not too dissimilar. 



As a fundamental expression of progressive change in the 

 amount of heat and water, zonation is the most important 

 feature of vegetation. It constitutes the sole basis for the 

 division of continental as well as insular vegetation. The 

 continent of North America furnishes striking proof of the 

 truth of this. Conforming to the gradual decrease of 

 temperature and water content northward, three primary 

 belts of vegetation stretch across the continent from east to 

 west. Thtsa are forest, grassland, and polar desert. The 

 first is further divided into the secondary zones of broad- 

 leaved evergreen, deciduous, and needle-leaved forests. At 

 right angles to this temperature-water symmetry lies a 

 symmetry due to humidity alone, in accordance with wGich 

 forest belts touch the oceans, but give way in the interior to 

 grasslands, and these to deserts. It is at once evident that 

 the mutual interruption of these two series of zones has pro- 

 duced the primary features of North American vegetation, 

 i. e.. tropical forests where heat and water are excessive, 

 deserts where either is unusually deficient, grassland when 

 one is low, the other moderate, and deciduous and conifer- 

 ous forests, where the water content is at least moderate, 

 and the temperature not too low. Such a simple yet funda- 

 mental division has been modified, however, by the disturb- 

 ing effect which three continental mountain systems have 



