ZONATION 281 
the factor concerned. This arises from the fact that the controlling factor 
is light, 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 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, 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: (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 
aleae. 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 
