GLEASON: THE PLANT ASSOCIATION 469 
13. One of the most important features of the environment is 
the control or modification of the original physical factors by the 
plant population itself. This action of plant life has been discussed 
repeatedly and requires no further argument here. As simple 
examples only, there may be mentioned the reduction of the light 
_ for the smaller plants by the crowns of the larger ones, and the 
modification of soil by the addition of organic substance. This 
control is in general directly proportional to the density of the 
plant population. In areas whose physical environment is gener- 
ally genial and favorable to the development of numerous indi- 
viduals, the resulting dense vegetation controls to.a large degree 
the original physical factors, and this environmental control is of 
greater importance than those in the selection of the plant popu- 
lation. 
It is at once obvious that, as a result of environmental control, 
the associated species of even the most limited area do not always 
enjoy the same environment. Thug the relations of a mature 
forest tree to light, wind, and soil are very different from those 
of the shallow-rooted, shade-loving, secondary species beneath it. 
The physical factors of the environment generally vary gradu- 
ally in space. Exceptions to this condition may easily be found 
in the soil factor, where sharp variations frequently occur, such 
as the contact of sand with clay at the foot of an advancing dune, 
or less obviously in the water factor, as on the rocky shore of a 
lake. Such gradual and progressive variation of environment 
would normally lead to equally gradual and progressive changes 
in the vegetation and to the establishment of broad transition 
zones between adjacent associations, in which the species of both 
mingle. In most regions of extreme physical environment, occu- 
pied by sparse vegetation exerting little environmental control, 
this condition holds, and the transition from one association to 
another is just as gradual as the change of environment.* . 
In regions of genial environment and dense vegetation, on the 
other hand, the nature of the environmental control of two ad- 
* This condition has in fact led some students to complain that it was difficult 
or impossible to distinguish associations in the vegetation of os western states. 
This would ye a sad state of affairs if all vegetation were compose of definite organic 
entities, but is quite to be expected when vegetation depends bees pig ster guarmycapee 
selection of favored individuals. 
