ZONATION 277 
shown above, is important in the reactions of forest and thicket. Physical 
factors produce zonation in a habitat or a series of habitats, when there is 
either a gradual and cumulative, or an abrupt change in their intensity. 
Gradual, slight changes are typical of single habitats; abrupt, marked 
changes of a series of habitats. This modification of a decisive factor tends 
to operate in all directions from the place of greatest intensity, producing 
a characteristic symmetry of the habitat with reference to the factor. con- 
cerned. If the area of greatest amount is linear, the shading-out will take 
Fig. 72. Zones of Cyperus erythrorrhisus produced by the recession 
of the shore-line. 
place in two directions, and the symmetry will be bilateral, a condition well 
illustrated by rivers. On the other hand, a central intense area will shade 
out in all directions, giving rise to radial symmetry, as in ponds, lakes, etc. 
The essential connection between these is evident where a stream broadens 
into a lake, or the latter is the source of a stream, where a mountain ridge 
breaks up into isolated peaks, or where a peninsula or landspit is cut into 
islands. The line that connects the points of accumulated or abrupt change 
in the symmetry is a stress line or ecotone. Ecotones are well-marked be- 
tween formations, particularly where the medium changes; they are less 
distinct within formations. It is obvious that an ecotone separates two 
