152 ZONATION 



no genetic connection between a tiny plant zone, and a zone 

 of vegetation. 



Radial, and bilateral growth play an important part in 

 format'onal 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 com- 

 pletely 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 connec- 

 tion, it must be carefully noted that vegetation pressure, 

 before which weaker plants are generally supposed to flee, 

 or bv which they are thought to be forced out into less de- 

 sirable situations, is little more than a fanciful term for 

 radial growth and migration. It has been shown under in- 

 vasion 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, light and room for 

 growth. 



Certain reactions of plants upon habitats produce zon- 

 ation. The zones of fungi are doubtless caused by the ex- 

 haustion 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 reaction of a forest, or thicket, or even of a 

 tall herbaceous layer is an extremely important factor in the 

 production of zonation. The factor chiefly concerned here 



