2/6 THE FORMATION 



Radial and bilateral growth play an important part in formational zones 

 in so far as they are related to migration. The growth of the runner or 

 rhizome itself is a very effective means of dissemination, while the seeding 

 of the plants thus carried away from the central mass is most effective at the 

 edge of the newly occupied area. This holds with equal force for plants 

 with a mycelium or a thallus. The circular area becomes larger year by year. 

 Sooner or later, the younger, more vigorous, and more completely occupied 

 circumference passes into a more or less complete zone. This will result 

 from the reaction of the central individuals upon the habitat, so that they 

 are readily displaced by invaders, or from their increasing senility and 

 dying out, or from the invasion of forms which seed more abundantly and 

 successfully. This result will only be the more marked if the radiating 

 migrants reach a belt of ground especially favorable to their ecesis. In this 

 connection it must be carefully noted that vegetation pressure, before which 

 weaker plants are generally supposed to flee, or by which they are thought 

 to be forced out into less desirable situations, is little more than a fanciful 

 term for radial growth and migration. It has been shown under invasion 

 that disseminulcs move into vegetation masses, as well as away from them, 

 the outward movement alone being conspicuous, because it is only at the 

 margin and beyond that they find the necessary water and light for growth. 



334. Reactions. Certain reactions of plants upon habitats produce zon- 

 ation. The zones of fungi are doubtless caused by the exhaustion of the 

 organic matter present, while in lichens and mosses the decrease in nutritive 

 content has something to do with the disappearance of the central mass. In 

 the mats of flowering plants, the connection is much less certain. The re- 

 action of a forest or thicket, or even of a tall herbaceous layer, is an ex- 

 tremely important factor in the production of zonation. The factor chiefly 

 concerned here is light. Its intensity is greatest at the edge of the formation 

 and just below the primary layer; the light becomes increasingly diffuse 

 toward the center of the forest, and toward the ground. In response to this, 

 both lateral and vertical zones appear. The former are more or less incom- 

 plete, and are only in part due to differences in illumination. The vertical 

 zones or layers are characteristic of forest and thickets, and are caused 

 directly by differences in light intensity. 



335. Physical factors. The physical causes of zonation are by far the 

 most important. They arise from differences in temperature, water, and 

 light. In the large, temperature differences are the most important, pro- 

 ducing the great zones of vegetation. In a particular region or habitat, 

 variations of water-content and humidity are controlling, while light, as 



