KINDS OF ZONATION 157 



other. Incompleteness is more frequently found in bilateral 

 zonation, though this is a question of distance or extent, 

 rather than one of symmetry. 



Vertical zonation is peculiar in that there is no primary 

 ecotone present, on either side of which zones arrange them- 

 selves with reference to the factor concerned. This arises 

 from the fact that the controlling factor is light, which im- 

 pinges 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 light 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, especially 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 dete^-mines the number of zones 

 found beneath it. In extreme cases the foliage is so deuse 

 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: (1) 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 

 formation shows a complex series of reactions. The pri- 

 mary 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 definifceness and permanence of this layer. The 



