rule, these merge into a single heterogeneous zone of thickets. =339. Bilateral zonation= differs from radial only in as much as it deals with linear elevations and depressions instead of circular ones. With this difference, the zones of ranges and ridges correspond exactly to those of peaks and hills, while the same relation is evident between the zones of streams, and of lakes and ponds. The ecotones are identical except as to form; they are linear in the one and circular in the other. Incompleteness is more frequently found in bilateral zonation, though this is a question of distance or extent, rather than one of symmetry. =340. Vertical zonation= is peculiar in that there is no primary ecotone present, on either side of which zones arrange themselves with reference to 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, 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 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 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 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 permanence 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 influence 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.