7'J FORKST DlsTKIIJrTION 



plants among the herbaceous perennials which flourish in such 

 numbers in the high mountain parks arc conspicuous elements 

 in the sprini: floras of lower altitudes, and in some species almost 



a-lcvcl. Still other factors besides those of altitude and 

 Icnirth of the Crowing season figure in determining 1he range 

 of these plants and such are found in their moisture requirements 

 and the trying conditions which at all times beset the crest of the 

 main Divide and all exposed places in its immediate vicinity. 



The continental crest in itself is hardly a barrier of suf- 

 ficient height to prevent transmigrations. Flanked, however as 

 it is by massive ridges and high parallel ranges, its effect is re- 

 inforced by repetition. The topography offers a succession of 

 parallel ridges rising 1 on both sides to the main Divide. These 

 cut by valleys would have even less effect individually as bar- 

 riers, but they serve to increase the mass elevation and contrib- 

 ute to the general effect upon the climate over a region 250 to 

 300 miles from east to west, and north and south for some dis- 

 tance beyond the limits of the region here under discussion. The 

 proximity of surrounding peaks has of course a marked influ- 

 ence on t< mperature, humidity and precipitation and conse- 

 quently upon the flora of localities, themselves not of great ele- 

 vation. 



The following species of eastern origin have reached the west- 

 ern slope: 



us tremuloides Salt'.'- cm-data 



Candida Kiln* <nnu-ir<uuun 



amygdaloides Symphoricarpos <-l>iculnl us 



It is probable that SOUK- ,,f the species have traversed the 

 Rocky Mountain barrier in many places. In the case of trans- 

 COntinental species in the north their southward extension has 

 doubtless followed aloii'j both slopes of the Divide and wherever 



else suitable conditions offered. 



Thise of western or southwestern origin which have reached 

 the eastern slope or the plains are listed below. Some of these 

 have extended their ran-j.-s much further than others. 

 tltorta ('iii<>thi<s r< lii fill iis 



I lottuga f'l.ri folia Ledum (jlandulosum 



/ Kngelmaniin J'lnjllodoce empetriformis 



