276 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 
cf 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 disseminules 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 
