37 



which are homogeneous (Clements, 1^04: 11) throughout their ex- 

 tent have been considered members of the same association. This 

 uniformity is shown by the environment, by the behavior of the veg- 

 etation, and, above all, by the plants themselves. 



In a region of limited size, over which the climate is essentially 

 the same, the physical environment of an association is usually nearly 

 constant, although instances are not lacking of an association living 

 in the same area under widely different conditions (Cowles, ipoi: 

 79). On the other hand, it is not necessaiy that every area with the 

 same environment should be occupied by one association. It is reg"- 

 ularly the case in the sand region, and usually also elsewhere, that if 

 the areas of the associations are conditioned by the environment, a 

 considerable and obsen^able change is necessary to influence the vege- 

 tation (Clements, ipo^: 292). But in no case should the recognition 

 and delimitation of associations be based upon the environment alone, 

 which leads to a classification of habitat rather than vegetation (Grad- 

 mann, ipog) and may lead to the uniting of radically different types 

 of vegetation.* The behavior of the vegetation wdth respect to ad- 

 jacent areas is show-n by successions which take place betw^een them. 

 If two areas with essentially the same environment show^ no succes- 

 sional relations it is probable that they represent different consocies 

 of the same association. 



But the first test of a plant association must be the vegetation 

 itself. No two areas of vegetation are exactly similar, either in 

 species, the relative number of individuals of each, or their spatial ar- 

 rangement, and the smaller the areas to be compared the greater pro- 

 portionately are the differences between them. Also, w'ith continued 

 and more detailed observation the importance of these minor varia- 

 tions is magnified, and tends to lead to the recognition of an un- 

 wieldy number of minor groups unworthy of the rank of association. 

 This introduces the question of how great a variation may occur in 

 the structure of the vegetation without the identity of the associa- 

 tion being changed. Field work shows that the dominant and the sec- 

 ondary species may both vary independently. In the same association 

 the dominant species, if more than one, will have the same vegeta- 

 tive fomi, as bunch-grasses, or trees, and will be of nearly the same 

 size. Excessive development of one of them to the partial or com- 

 plete exclusion of the others makes no change in the general ap- 



* The classification of associations by Clements (1905: 302, 303) is largely 

 of this nature, and in some cases leads to the wide separation of closely related 

 associations or even to the placing of a particular area in two different groups. 

 Thus a hydrophytic sand-bar (cheradiiim) may be converted into a new xerophytic 

 "formation" (syrtidiwn) merely by the fall of the water in the river. 



