38 



CONTRTP,I"TTONi=; FROM THE NATIONAL HERBARIUM. 



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

 ited eitlier by the northward extension of alread^y adapted pLmts, 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 hax'e been derived in j^art from each of these sources, 

 assuming- that plants which range from California or the (Ireat Basin 

 into the Columbia l^asin originated in th<^ formei" regions and not 

 vice versa. This assumption is based on considerations heretofore 

 discussed. 



The prevailing winds of the Columbia Basin are from the south- 

 w^est. So pronounced are these winds that they have had considerable 

 to do wnth 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 l)e through the low gap in northeastern California 

 made by the Klamath Kiver and lakes. Some few plants may have 

 reached the Columbia Basin by way of the Willamette Valley and 

 the Columbia Iviver, but this, if true, is certainly exceptional, not 

 only on account of the long distance and moist region through wdiich 

 these illy adapted plants would have to migrate, but from the actual 

 fact that few Sonoran plants reach the Willamette Valley, the liogue 

 River Mountains in southwest Oregon forming a sharp and efi'ective 

 l)arrier to them, but not to Transition j^lants. 



The following lists of Upper Sonoran plants indicate the relative 

 importance of the Californian and Great Basin elements in AVash- 

 iuirton: 



SPECIES OF CALIFORNIA ORIGIN. 



Ahiii.'i rhnnthi folia. 

 Ai>h}ilU)ii coiiiosuiii. 

 Blcph(irli>(tppHf< t/landinoaii-^. 

 Cnjptinithr i^ulH/IocIiididtd. 

 Hem izon la citrlodorii. 

 Lepidium dictyotum. 

 Lupin ».s- micrncarpus. 

 Mieroseris Uncarifolia. 



OoKttJirra ■<<tri(7iiioi^a. 

 Prctocari/d Mctosa. 

 Piscaria scii(jcra. 

 PsiJocurph ».< brcvisMm us. 

 Rail iDWtil lis hchccarpus. 

 Rif/iopappufi lepfocludus. 

 Til ]/s(i nocarp ».s cii r ripcs. 

 Toiiclla colli nsioidcs. 



SPECIES OF GREAT BASIN ORIGIN. 



Trees. 

 Celtis dou(/Utsii. Sali.r aiiim/dahiidrs. 



