382 BOTANICAL GAZETTE [November 



7. Environmental conditions are on the whole severe; the vegetation is in 

 large part open. Open associations are less definite in plant composition than 

 closed associations, in which plant competition has exercised more of a selective 

 influence (cf. footnote below). Different associations are thus as likely to 

 intergrade as to alternate sharply, the more so since environments frequently 

 pass gradually one into another. 



8. Present tendencies toward stabilization of mesa vegetation are more or 

 less obscured by artificial factors, chief of which is grazing. 



The principal effect of these complicating agencies is the presence 

 of mixed growths of all grades. Mixed associations are frequent, 

 in which plants normally dominating different associations occur 

 together. The various secondary species also are not so regularly 

 associated with particular dominants as would be expected from 

 the study of more nearly typical representations of these same 

 associations in other parts of the prairie region. At present an 

 arrangement of the minor growths and variations of associations 

 must be tentative. The associations are characterized partly from 

 their occurrence and composition in other parts of the prairie 

 province, and while certain growths in the mesa area are typical 

 in composition, the best that can be done at present with certain 

 growths within the area is to regard them as mixtures of two certain 

 associations, or as a representative of one association varying in the 

 direction of another, or with an added plant element from some 

 particular source. On the other hand, certain appearances of some 

 of the associations 1 are quite definite in composition, quite regular 

 and uniform in occurrence in different stations of the mountain- 



1 Definitions of terms applied to plant growths of different rank, as herein used, 

 may conduce to accuracy. The unit of vegetation is taken to be the plant associa- 

 hon } meaning an essentially uniform assemblage of plants living together in an area 

 essentially uniform in environmental conditions. No interrelationship of different 

 plants is necessarily implied, nor is uniformity taken to exclude internal local varia- 

 bility of either environment or vegetation. Those who have worked principally with 

 closed vegetation in the humid eastern states are likely to formulate certain attributes 

 of plant associations which will not apply to open growth in the less favorable environ- 

 ments of semi-arid regions, or to primitive stages in development of vegetation, or 

 to mixed growths in habitats internally variable. In areas of vegetation permanently 

 or temporarily open, plant competition is usually not an important limiting factor; 

 there is no competition for light as in forest, and very little for above-ground space 

 as in closed grassland; root competition is perhaps not so general as accommodation 

 of root systems, so that different plant species derive moisture from different soil 

 depths. Physical conditions may thus exert more direct control upon plant indi- 



