38 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 
fitted to support an Upper Sonoran flora could have become inhab- 
ited either by the northward extension of already adapted plants, or 
by the gradual modification of species of a colder zone, or by both. 
The evidence indicates the first method to have been the most potent. 
South of the Columbia Basin are two very distinct floral regions— 
namely, California and the Great Basin—divided by the Sierra 
Nevada. It is perfectly clear that Upper Sonoran plants of the 
Columbia Basin have been derived in part from each of these sources, 
assuming that plants which range from California or the Great Basin 
into the Columbia Basin originated in the former regions and not 
vice versa. This assumption is based on considerations heretofore 
discussed. 
The prevailing winds of the Columbia Basin are from the south- 
west. So pronounced are these winds that they have had considerable 
to do with molding the hills in the entire region. Very naturally 
plants would migrate quite rapidly with these very constant and at 
times severe southwest winds. The natural route of the Californian 
plants would be through the low gap in northeastern California 
made by the Klamath River and lakes. Some few plants may have 
reached the Columbia Basin by way of the Willamette Valley and 
the Columbia River, but this, if true, is certainly exceptional, not 
only on account of the long distance and moist region through which 
these illy adapted plants would have to migrate, but from the actual 
fact that few Sonoran plants reach the Willamette Valley, the Rogue 
River Mountains in southwest Oregon forming a sharp and effective 
barrier to them, but not to Transition plants. 
The following lists of Upper Sonoran plants indicate the relative 
importance of the Californian and Great Basin elements in Wash- 
ington: 
SPECIES OF CALIFORNIA ORIGIN, 
Alnus rhombifolia. Oenothera strigulosa. 
Aphyllon comosum. Pectocarya setosa. 
Blepharipappus glandulosus. Piscaria setigera. 
Cryptanthe subglochidiata. Psilocarphus brevissimus. 
Hemizonia citriodora. Ranunculus hebecarpus. 
Lepidium dictyotum. Rigiopappus leptocladus. 
Lupinus microcarpus. Thysanocarpus curvipes. 
Mtcroseris linearifolia. Tonella collinsioides. 
SPECIES OF GREAT BASIN ORIGIN. 
Trees. 
Celtis douglasii. Salix amygdaloides. 
