470 GLEASON: THE PLANT ASSOCIATION 
jacent associations may be, and usually is, very different. Species 
~ of one association are then excluded from the margin of the other 
by environmental control, when the nature of the physical factors 
alone would permit their immigration. The adjacent associations 
therefore meet with a narrow transition zone, even though the 
variation in physical environment from one to the other is gradual. 
In such regions of great environmental control, the boundary 
between two associations seldom marks the extreme location of 
suitable physical environment for either, but rather some inter- 
mediate position (see also paragraph 25). The removal of either 
association at one portion of the boundary results accordingly in 
an immediate extension of the other. This may be seen around 
many small lakes in Michigan, where a zone of sedge is followed 
in deeper water by a zone of water lilies. Destruction of the 
sedge zone in the shallower water is followed by the extension 
of the lily zone almost or quite to the shore line. The normal 
landward margin of the water lily association is therefore deter- 
mined not by depth of water alone, which represents the chief. 
physical environmental variant, but also by the environmental 
control of the sedge association. 
14. Instances may be cited where the sole environmental dif- 
ference between two adjacent associations lies in the environ- 
mental control. This is notably the case with some types of 
prairie and upland forest in the Middle West. The differentiation 
of two distinct associations in the same environment is then to be 
explained chiefly, if, not entirely, by developmental conditions, as 
discussed below under Succession. 
15. The line which marks the boundary of the distribution of 
a species is located at right angles to the direction of the effective 
environmental variant. Consequently the boundary of the whole 
assemblage of species constituting the plant association has the 
same position, whether the variant be purely physical or biotic. 
Since physical factors generally vary irregularly from place to 
place, the boundary and shape of an association are also usually 
irregular. 
Symmetry or regularity in the variation of an important envi- 
ronmental factor may however occur. The most conspicuous 
example is the variation in depth of water or in amount of soil 
