ALTERNATION AND ZONATION 



301 



330. Vertical zonation is peculiar in that there is no ecotone 

 present, on either side of which zones arrange themselves with 

 reference to the factor concerned. This is due to the fact that 

 the controlling factor is light, which falls upon the habitat in such 

 fashion that it decreases in but one direction, i.e., downward. 

 \'ertical zones appear in bodies of water on account of the absorp- 

 tion of light by the latter. The most characteristic zones occur 

 in forests, where the primary layer of trees acts as a screen. The 

 density of this screen determines the number of zones or layers 



Fig. 12-1. Bilateral zonation of cord grass, Spartina cynosuroides , 



along a ditch. 



present. In extreme cases the foliage is so dense that the light 

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

 however, there are one or more layers present. 



A vertically zoned forest shows a complete series of reac- 

 tions. The primary layer determines the amount of light, heat, 

 and water for the subordinate layers in general. Each of the 

 latter further modifies the amount for those below it, the ground 

 layer being suljject in some degree to the reaction of every layer 

 above it. The lower layers also influence the upper by reacting 



