ZONATION 281 



the factor concerned. This arises from the fact that the controUing factor 

 is Hght, which impinges upon the habitat in such manner as to shade out in 

 but one direction, i. e., downward. Vertical zones appear in bodies of 

 water, on account of the absorption of Hght by the water. In a general way, 

 it is possible to distinguish bottom, plancton, and surface zones, consisting 

 almost wholly of algae. There is little question that minor zones exist, es- 

 pecially in lakes and seas, but these await further investigation. The most 

 characteristic vertical zones occur in forests, where the primary layer of trees 

 acts as a screen. The density of this screen determines the number of zones 

 found beneath it. In extreme cases the foliage is so dense that the light 

 beneath is insufficient even for mosses and lichens. As a rule, however, 

 there will be one or more zones present. In an ordinary deciduous forest, 

 the layers below the facies are five or six in number : (_i ) a secondary layer 

 of small trees and shrubs, (2) a tertiary layer of bushes, (3) an upper 

 herbaceous layer of tall herbs, (4) a middle herbaceous layer, (5) a lower 

 herbaceous layer, (6) a ground layer of mosses, lichens, other fungi, and 

 algae. The upper layers are often discontinuous, the lower ones are more 

 and more continuous. As a forest becomes denser, its layers disappear from 

 the upper downward, the ground layer always being the last to disappear 

 because of its ability to grow in very diffuse light. A vertically zoned for- 

 mation shows a complex series of reactions. The primary layer determines 

 the amount of heat, light, water, wind, etc., for the subordinate layers in 

 general. Each of these layers then further determines the amount for those 

 below it, the ground layer being subject in some degree to the control of 

 every layer above it. This accounts probably for the definiteness and per- 

 manence of this layer. The degree to which the lower layers influence the 

 upper by reacting upon the habitat is not known. It is evident that this in- 

 fluence must be considerable by virtue of their control 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 dis- 

 appear in this transition zone, but in opposite directions. The overlapping 

 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. 



341. Vegetation zones. 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 con- 

 tinental as well as insular vegetation. The continent of North America 



