THE PHYSICAL BASIS. 39 



other so closely, however, that they occupy the same sublicmate, in which they 

 mix or separate in accordance with local variations. An interesting regional 

 separation occurs with the two species of Stipa, as well as in the case of 

 Agropyrum. Stipa spartea marks the eastern portions of the true prairies 

 and S. comata the western; Agropyrum glaucum is typically associated with 

 Stipa comata, while A. spicatum is best developed in the Northwest, especially 

 in the Palouse. The essential point is that each consociation or mixture of 

 two or more marks a subdivision of the association habitat, and is the indicator 

 of it. Similar though minor habitat divisions are indicated by such character- 

 istic societies as those of Glycyrhiza lepidota, Amorpha canescens, Psoralea 

 argophylla, P. tenuiflora, Petalostemon candidus, and P. purpureus, the water 

 relations of which are essentially in the order given here. In the eastern 

 prairies, where water is abundant, several of these may occur together more or 

 less constantly, but farther west each tends to form a distinct society, and to 

 indicate a corresponding water-content. The differences are slighter than in 

 the case of consociations, and hence society habitats do not necessarily fall 

 in the habitat of a particular consociation. This is probably to be explained 

 partly also by the action of climatic cycles. For example, the wet phase would 

 favor the local extension of Psoralea argophylla and Petalostemon candidus for 

 a few years, while during the dry phase the less mesophytic Psoralea tenuiflora 

 and Petalostemon purpureus would have the advantage. 



Since the habitat, like the formation, shows development in the course of 

 succession, it exhibits developmental divisions and subdivisions. Each of 

 these necessarily has its own indicator community, namely, the associes, con- 

 socies, and socies. The habitats which correspond to these have a time as 

 well as a space relation. If the best-known succession, the hydrosere, be 

 taken as an example, these two relations are shown in the familiar zones of 

 lakes and ponds. Each plant zone or associes from the center of submerged 

 • plants to the surrounding climax of forest or prairie indicates a major develop- 

 mental habitat, e.g., the habitat of the floating aquatics, of the reed-swamp 

 the sedge-swamp, etc. 1 Each of these associal habitats is subdivided into the 

 habitats of consocies indicated in the reed-swamp, for example, by Scirpus, 

 Typha, and Phragmites, respectively. Within the latter may be minor habitats 

 characterized by such socies as Sagittaria, Alisma, Heleocharis, etc. As a 

 result every region is a complex of climax and developmental habitats of vary- 

 ing rank and extent, each controlling a plant community which serves as the 

 indicator of it. 



Variation of climate and habitat. — While many reasons make it desirable if 

 not necessary to regard each habitat as a unit, it should be clearly recognized 

 that it varies from place to place and from year to year. The seasonal varia- 

 tions are more or less of the same character and they are marked by their own 

 indicators in the form of the seasonal societies. A grassland climate is char- 

 acteristically different from a forest climate by virtue of its product, the 

 grassland climax. This has its explanation in the average difference between 



1 Pearsall (1917 : 78) has recently recognized three associes of submerged plants, namely, (1) 

 linear-leaved associes of Naias, etc.; (2) Potamogeton associes; (3) Nitella associes. This is in 

 full accord with our growing knowledge of vegetational development, which must result in the 

 general acceptance of more rather than fewer units (Clements, 1916 : 132). However, the latter 

 must be based upon quantitative studies and checked by extensive scrutiny of other vegetations 

 if the results are not to be mere personal judgments, leading to the condition in which taxonomy 

 finds itself to-day. 



