39 



near the center of the association, and tend to increase progressively 

 toward its boundaries. The best idea of the structure of an asso- 

 ciation is accordingly gained at its center. The presence of a derived 

 element is well illustrated in the black oak association, in which every 

 spot of unshaded exposed sand is occupied by interstitial annuals 

 of the bunch-grass association, while near the margin of the forest 

 numerous prairie perennials and grasses also occur. Many species, 

 naturally, are almost equally typical of two or more associations. 



The boundary of an association is frequently sharp and well de- 

 fined, especially if the dominant species of the adjoining areas are 

 of different vegetation fonns, as between prairie and forest, or if 

 the associations are correlated with considerable and relatively con- 

 stant differences in the environment, as between the windward slope 

 and basin in the blowout formation. In other cases the boundary is 

 broad and more or less indefinite. This is particularly true if the 

 dominant species are of the same vegetation form or if the environ- 

 mental difference is fluctuating, as, for example, between the bunch- 

 grass and the Panicuni pseiidopnbescois associations. The vegeta- 

 tion of these transition zones is a mixture of usually indefinite and 

 frequently highly variable character. The species in them should be 

 referred as far as possible to their respective associations, and not al- 

 lowed to modify the ideas of structure gained from an examination 

 of more typical localities. 



Besides these transitions in space, there are also transitions in 

 time. An early stage in the development of an association may re- 

 semble but little its mature condition. Certain members of the asso- 

 ciation with excessive seed production, with more mobile seeds, or 

 better adapted to the somewhat aberrant environment, appear first 

 and for a time dominate the area. Thus, in the stabilization of a 

 blowout, the redevelopment of the bunch-grass association begins 

 with a growth of Lespedeza capitata and Oenothera rhomhipetala in 

 large quantities. This condition lasts but a short time before they 

 are replaced by the usual bunch-grasses. Such an area is at first sug- 

 gestive of a distinct association, but examination shows that it has no 

 species, aside from relics of the preceding vegetation, not found also 

 in the bunch-grass, and that the environmental conditions are very 

 similar to those of the spaces between the bunches of grass, where 

 these interstitials (p. 54) grow. A knowledge of the habitat prefer- 

 ences and habits of the component species and of the general dynam- 

 ics of the area is necessary to decide upon the proper classification 

 of these transitional stages. 



In estimating the unifonnity of the vegetation, direct observation 



