ALTERNATION AND ZONATION 297 



reaction of the central part has something to do with its disappear- 

 ance. The reaction of a forest or thicket, or even of a layer of 

 herbs, plays an important part in producing zones. The factor 

 chiefly concerned here is light. The intensity is greatest at the 

 edge of the formation, and also just below the primary layer. 

 It decreases toward the center of the forest and toward the ground. 

 In response to these changes, zones appear in both directions. 

 The lateral zones are more or less incomplete, and are only due 

 in part to differences of light. The vertical zones or layers are 

 characteristic of forests and thickets, and are controlled wholly 

 by differences in light intensity. 



324. Zones due to physical factors. The physical causes of 

 zonation are by far the most important. They arise from differ- 

 ences in water, temperature, and light. The great zones of vegeta- 

 tion are due to water and temperature differences. In a par- 

 ticular region or habitat, variation of water content and humidity 

 are the most important, while light acts only in the case of forests 

 and thickets. Physical factors produce zonation in a habitat 

 or a series of habitats, when there is either a gradual and cumu- 

 lative or an abrupt change in their intensity. Gradual slight 

 changes are found in single habitats; abrupt marked changes 

 in a series of them. 



The change in a decisive factor takes place in all directions 

 from the area of greatest intensity, making the habitat more or 

 less symmetrical \\ith respect to the factor concerned. If the 

 area of greatest intensity is linear, the shading out takes place 

 in two directions. The resulting symmetry is bilateral, a con- 

 dition found along streams. On the other hand, a central in- 

 tense area shades out in all directions, giving rise to radial sym- 

 metry, such as is found in ponds, lakes, etc. The close con- 

 nection between the two kinds of symmetry becomes evident 

 where a stream broadens into a lake, where a mountain ridge 

 breaks up into isolated peaks, or a peninsula or landspit is cut 

 into islands. The line that connects the points of accumulated 

 or abrupt change in the symmetrical area is a stress line or ecotone. 

 Such lines are usually well marked between formations, espe- 

 cially where the medium changes, as between a pond and a prairie. 

 They are less evident within formations. In the one case the 

 ecotone separates two distinct series of zones, and in the other 

 merely two different zones of the same formation. 



