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 crythrorrhisus 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 ccotone. 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 



