344 RYDBERG: PHYTOGEOGRAPHICAL NOTES 
Some of the plants of the first category may have a range 
limited 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 others might 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 (t). 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 tremuloides,* 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. 
