72 FOREST DISTRIBUTION 



plants among the herbaceous perennials which flourish in such 

 numbers in the high mountain parks are conspicuous elements 

 in the spring floras of lower altitudes, and in some species almost 

 to sea-level. Still other factors besides those of altitude and 

 length of the growing season figure in determining the range 

 of these plants and such are found in their moisture reciuirements 

 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 harrier 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 sueiession of 

 parallel ridges rising 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 temperature, 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: 



Populus tremuloicles Salix cordata 



:^aU.r Candida liibfs ainericanum 



amygdaloidis Si/mphoricarpos orbiciilafus 



It is probable that some of 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 along both slopes of the Divide and wherever 

 else suitable conditions offered. 



Thdse of western or southwestern origin which have reached 

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

 have extended their ranges much further than others. 

 Pinus contorta Ccanothiis vehitinus 



Fseudotsiiga ta.rifolia Ledum plaudulosum 



Picea Engchnannii PlnjUodoce cmpitriformis 



