344 Rydberg: Phytogeographical notes 



Some of the plants of the first category may have a range 

 Hmited in the Middle Province to the Canadian Rockies alone; 

 others may extend throughout the Northern Rockies, but not be 

 found in the Southern; while still others may extend throughout 

 the Rocky Mountain system. Of the second category, some plants 

 may be common to the Rockies and the Hudsonian Zone of the 

 east, some others common to the Rockies and the Pacific moun- 

 tains, or to the Sierra Madre in Mexico, or to the Alaskan moun- 

 tains, or to the arctic coast. Some might have their home in the 

 Rockiesand spread toother regions, and othersmight be immigrants 

 into the Rocky Mountain system. Among the plants of the third 

 category, many are distributed throughout the Rockies, others 

 have a distribution limited to either the Northern or the Southern 

 Rockies, or to certain parts of either, while still others are purely 



local. 



The strictly subalpine element of the flora of the Rockies is in 

 fact very small, the plants consisting mostly of species that are 

 found also in the Montane Zone, especially the upper part thereof, 

 and of alpine-arctic species, running down in the swales, along 

 the streams, or along the wind-swept hog-backs. In fact some 

 plants, especially aquatics and hydrophytes, are common to three 

 or more zones. In the following lists the plants common to the 

 alpine-arctic zone are designated by a dagger (f). No attempt 

 is made to designate those common to the Montane Zone, as 

 they would probably constitute 90 per cent, of the remaining 



species. 



I 



I. Transcontinental species ranging throughout the 



Rockies 



This element is represented among the trees by the quaking 

 aspen, Populus tremidoides* but this tree is not one of the char- 

 acteristic trees of the Subalpine or Subarctic Zone either of the east 

 or of the Rockies, nor is it limited to the subalpine regions. One 

 should therefore not lay too much stress upon this tree and its 



* It is true that some botanists, such as Tidestrom, Wooton and Standley, regard 

 the Rocky Mountain aspen as a distinct species, Populus aurea. differing in smaller, 

 thicker, less toothed leaves and different colored anthers and bark, but as the dis- 

 tribution of this species has not been worked out, it is better here to ignore the same 

 and include it in P. tremuloides. ^ 



