33^ 



BOTANICAL GAZETTE 



[NOVEMBER 



scopulorum formation; 



Murrayana formation; (5) Apinus 



flexilis formation; (6) Pseudotsuga-Picea Engelmanni formation; 



formation 



While Popiih 



mountains, I have not thought best to consider it of higher rank than 

 a society. My reason for denying it formational rank is that it 

 occupies varying habitats, associating with the other formations, and 

 never dominating areas which are well defined either as to their 

 physical features or geographical position. 



All these formations are montane excepting the first, and even 

 this extends up the canons to about 1950"^. The second, on the 

 contrarv, while chieflv montane, overlaps the first on the plains - 



(l) THE POPULUS OCCIDENTALIS-SALIX FLUVIATILIS PORMATION 



{fig. 1 2) 



This formation occupies the banks of streams on the plains and, 

 as already mentioned, is found also in the lower canons. It is a 

 straggling tree formation scarcely worthy the name of forest. While 

 restricted mainly to the stream banks, groves of trees may occasionally 

 be found along the borders of the old flood plains* 



It is a ver}^ mixed formation, consisting essentially of two layers, 

 an upper woody layer and a lower herbaceous one. The former 

 may be divided into two subordinate layers, one controlled by Populus 

 occidentalis and the other by Salix fltiviatilts. These are not, how- 

 ever, usually coexistent, but alternate, the cottonwood controlling 

 here and the willows there. Associated with the two characteristic 

 species of the formation are many individuals of Populus angtistifoliaj 

 but these do not control. A few scattered shrubs occur in the for- 

 mation but are not sufficiently numerous to be considered as an inde- 

 pendent layer (see below). The herbaceous layer, moreover, might 

 be separated into two subordinate ones; but since its constituent 

 forms present every gradation in height, such a separation would be 

 entirely arbitrary and superficial. A ground layer of mosses and 

 liverworts is occasionally present. 



The formation is an open one. Occasionally the trees form a 

 sufficiently dense growth to exclude all but the more shade-loving 

 species, but for the most part they are separated by inten^als of many 



