GLEASON: THE PLANT ASSOCIATION 471 
water near the shore of a pond. In such cases, regularity in the 
environment produces a similar regularity in the arrangement of » 
the associations. When this is conspicuous enough to attract the 
attention of the observer it is termed zonation. 
Zonation of associations is the exception rather than the rule 
and has no more significance than the usual irregular arrangement. 
Between the two types there are naturally intermediate stages. 
Also, irregularity in other conditions affecting the differentiation 
of associations may operate to disturb their zonation. This has 
been frequently described or figured: (see Harshberger, 8, p. 382, 
383) by investigators, generally without a statement of the cause. 
16. The area occupied by an association may be large or small, 
depending on the size of the area available without effective en- 
vironmental variation. Zoned ‘associations about a pond fre- 
quently have dimensions conveniently expressed by meters (see 
Gates, 5, p. 329), while some forest associations are better measured 
by the kilometer. The smaller associations are usually composed 
of fewer species, and are generally of shorter duration than the 
larger ones. They are the product of the same kind of causes, 
however, and are no less important in the ecological study of 
vegetation. 
Since associations often tend to extend their area at the expense 
of others, as discussed below: (part VII), the observed size of an 
association is correlated with its developmental history, and must 
always be considered from that standpoint. 
V. THE STRUCTURE OF THE ASSOCIATION 
17. The general tendency of the population of an association 
_ to migration tends to produce uniform distribution of each species 
within it, and consequently uniformity of the association as a 
whole. This uniformity is one of the most characteristic visible 
features of the association, and has by some been taken as the 
basis for a definition of it (6, p. 36). It is easily demonstrable in 
the small associations and in large ones becomes very impressive. 
The chief value of the quadrat method of expressing the structure 
of vegetation depends upon this feature, since a small area can be 
chosen for intensive study which exhibits faithfully the average 
structure of the whole association. 
