478 GLEASON: THE PLANT ASSOCIATION 
the other. The accompanying associations therefore lose ground 
on one side as they gain it on the other, and when the rate of gain 
and loss are about equal the association as a whole may seem to 
change its position. Such progressive changes are usually cor- 
related with a regular or symmetrical distribution of environment, 
so that zonation becomes an epitome of succession. Then the 
transition zone indicates the location of succession and endures 
in the same position merely for the transition period. 
Succession by reaction begins wherever and whenever effective 
reaction is completed. This is normally in the oldest part of the 
association, the location of which may be either central or lateral, 
depending upon the circumstances of its origin. In either case 
the result is the same. Proximity, governing the opportunity of 
immigration, limits the pioneer species in central succession to 
the most mobile forms. Thus, in the central succession of the 
black oak association by the mixed forest in Illinois, the most 
conspicuous pioneer species is the permobile avevectent Psedera 
quinquefolia (6, p..136). : 
The reaction of an association may also extend beyond its 
actual margin and produce an effect upon the margin of the con- 
tiguous association. If this weakens the environmental control 
in the latter, an advance of the former association takes place. 
This is the case in the advance of upland forest on the prairies of 
the Middle West. Since the sod-forming grasses grow less vigor- 
ously in the partial shade of the forest margin, the sod there is less 
dense and seedlings of forest species can establish themselves: 
The average rate of advance under these conditions was estimated 
by the late Professor Burrill at fourteen feet per year. 
27. Some associations make no unfavorable reaction on their 
environment, live under conditions not subject to effective change 
by the usual topographic agencies of the region, and are not near 
a different type of vegetation better adapted to the same environ- 
ment. In the.absence of all causes of succession, such associations 
occupy the area permanently and are called climax. Theoretic- 
ally, all associations of a region tend to culminate in the establish- 
ment of a climax. Many associations, however, occupy their 
ground so tenaciously that there is little or no observable evidence 
that they are ever replaced by the association ordinarily considered 
