Alpine and Boreal Floras of Western North America. 253 
Ribes sanguineum Pursh. Colorado and California. 
Rosa Fendleri Crep. New Mexico, western Texas, sierras of California, 
and northward beyond boundary. 
Symphoricarpos oreophilus Gray. Colorado, Utah, Arizona, California and 
regon. 
Tetradymia canescens DC. New Mexico to northern Wyoming, Arizona, 
California, British Columbia. 
Tetradymia spinosa Hook. & Arn. Southern Wyoming to Arizona, south- 
eastern California and eastern Oregon. 
Two plants have entered the Rocky mountain region from the southeast, 
viz, Rhamnus caroliniana Walt. which ranges from Montana through Colorado 
western Texas, northern Florida and northeastward to Long Island, New York, 
and Sapindus marginatus Willd. from southern New Mexico and Arizona, 
eastward through the Gulf States to the Atlantic coast. 
3. Origin of Alpine and Boreal Floras of Western America’). Before con- 
sidering the origin of the flora of the Great Basin, which now concerns us, 
it is important to study the origin of the alpine and boreal floras of the moun- 
tains of western North America, which consist of first, the Rocky Mountains, 
and second, the Sierra Nevada and Cascade ranges. As, from what 
follows, the origin of the boreal floras of these mountains can be traced to 
the same source, it is expedient at this juncture to tabulate the boreal plants 
of a common origin and group them according to the mountain ranges on 
which they are found. This will simplify matters, so that when the general 
floral development of the Sierra Nevada and Cascade ranges is described, a 
consideration of the boreal floras of these mountains can be illiminated from 
the discussion. 
The two great mountain ranges at their southern ends are widely separated 
by the deserts of the Great Basin; but in British Columbia the two systems 
draw closer together, the depressed plains between them become studded with 
other mountains, and the cold of the northern latitude allows the boreal flora 
of the mountain crests to descend more nearly to the base level of the country. 
Under these conditions, we should expect the boreal flora of any portion of 
one system to be most nearly similar to that of other portions of the same 
system; and, indeed, that each system would constitute a distinct line of south- 
ward boreal migration and a distinct center for the differentiation of new forms 
distinet from their progenitors, the migrating boreal species. The following 
list adapted from CoVvILLE°) with the addition of the northern Sierran plants 
constitutes a fair representation of the boreal Sierran flora which has been 
separated into four groups: a) those confined to the Sierra Nevada; b) those 
common to the Sierra Nevada and Cascade Mountains only; c) those common 
1) See the foregoing page 243. : BR 
2) CoviLLE, FREDERICK VERNON: Botany of the Death Valley Expedition Contributions from 
U. S, National Herbarium IV: 27—29. 1893. 
