38 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



fitted to support an Upper Sonoran flora could have become inhab- 

 ited either by the northAvard 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 rTiomhifnlia. Oenothera strigulosa. 



Aphyllon comosum. Pcctocarya setosa. 



Blepharipappus fflandtilosus. Piscaria setipera. 



Cryptanthe subglochidiata. Psilocarphiis hrevissimvs. 



Hemisonia citriodnra. Ranunculua heheearpiiH. 



Lepidium dictyotum. ' Rigiopappus Icptoeladtis. 



Lupinus microcarpus. ThyHanncarpuft curvipes. 



Microseris UnearifoUa. Tonella collinsioides. 



SPECIES OF GREAT BASIN ORIGIN. 



Trees. 

 Celtis douglasii. i^alix amygdaloides. 



