THE SUBCLIMAX PRAIRIE. 131 



single locality is studied. Since the fluctuations of societies are of great 

 importance for indicator studies as well as for climatic correlation, it seems 

 clear that they should always be treated as such, with the proper statement 

 as to their reduced significance. It is perhaps even more necessary to main- 

 tain the distinction between fragmentary consocies or socies, and clans. A 

 host of minor disturbances may denude a small spot or displace the dominants 

 sufficiently to start a minute succession. To the unpracticed eye, the com- 

 munity will appear as a clan, while it is really a stage in succession. Its real 

 nature is readily disclosed by comparison with other areas where disturbance 

 is obvious, or by following its development during a few years. 



Because of their subordinate importance, the factor relations of clans have 

 secured little attention. They are clearly controlled by water relations, as is 

 shown by their topographic position, and their seasonal appearance. They 

 are also more or less influenced by light as an outcome of their competition 

 with the subdominants. 



Vernal Clans: Estival Clans: Estival Clans — continued. 



Delphinium penardi. Asclepias syriaca. Evolvulus argenteus. 



Oxalis violacea. Asclepias sullivantii. Gerardia purpurea. 



Oxalis stricta. Asclepias tuberosa. Gerardia aspera. 



Scutellaria parvula. Asclepias verticillata pumila. Cacalia tuberosa. 



Astragalus canadensis. Lactuca pulchella. Lythrum alatum. 



Specularia perfoliata. Desmodium illinoense. Lechea minor. 



Pentstemon cobaea. Schrankia uncinata. Ruellia ciliosa. 



Pentstemon albidus. Desmanthus illinoensis. Triosteum perforatum. 



Onosmodium molle. Lathyrus ornatus. 



Baptisia leucantha. Acerates viridiflora. Serotinal Clans: 



Erigeron philadelphicus. Psoralea esculenta. Liatris squarrosa. 



Potentilla arguta. Hieracium longipilum. 



Physalis lanceolata. Gentiana puberula. 



Physalis virginiana. Gentiana andrewsii. 



Dalea aurea. Solidago graminifolia. 



THE SUBCLIMAX PRAIRIE. 



ANDROPOGON ASSOCIES. 



Nature. — East of the Stipa-Koeleria association lies a belt of prairie more or 

 less interrupted by woodland. In general character the two are very similar, 

 so much so that at first thought it seems impossible to draw a valid distinction 

 between them. The difficulty arises from the very gradual increase of rainfall 

 from 30 to 40 inches and the correspondingly broad transition from the one 

 to the other. In spite of this, the two communities are at least as different as 

 the other associations of this formation. The climatic difference of 10 inches 

 of rainfall is reflected in the close sod and the taller growth-form, both more 

 typically developed than in any other association of the grassland. The 

 greatest distinction arises from the fact that the dominants are nearly all 

 different, though their similarity in requirements is attested by the degree to 

 which they mingle and alternate. Andropogon is typical of the community to 

 an almost exclusive degree, but the species often mix with Stipa, Agropyrum, 

 and Koeleria to such an extent as to make the exact relationship of a particular 

 area difficult to determine. All of these differences are summed up in the 

 fact that the Andropogon prairie over most of the region is subclimax in charac- 

 ter, i. e., it will be replaced by scrub, woodland, or forest wherever cultivation, 



