44 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM. 



A'tliird series of species, few in number, presents a puzzling prob- 

 lem. It consists of Arid Transition plants common enough east of 

 the Cascade Mountains, which are known to occur west of these moun- 

 tains only on "Whidby Island, or, in a few cases, on neighboring 

 islands. They deserve particular mention. 



Sierersia ciliata is abundant east of the Cascades, ranging as a 

 common plant to Minnesota and Nebraska, and as a rarity even to 

 New England. West of the Cascades it is known only from the 

 prairies of Whidby Island. 



Aphyllon comosum^ a parasit^ on various asteraceous plants, is not 

 rare in eastern Washington, and ranges east of the Cascades to Cali- 

 fornia. It has also been found on Whidby and San Juan islands on 

 Grindelia. 



Polemonium micranthnm and Lupinus microcarpus, both on 

 Whidby Island, together with PlatysperTnum scapigerum, Aster con- 

 spieuus, and Scntellaria angiixtifolin,, known from Vancouver Island, 

 are cases practically parallel to that of Aphyllon eoTtiosiiTn. 



Iris missmiriensis is abundant east of the Cascades, ranging to 

 Dakota and Nebraska. Its station near Coupeville is the only one 

 known in the Vancouver strip. 



Juniperus scopulorum^ which crosses the Cascades to reappear in 

 Island and San Juan counties, is a somewhat similar case. 



There needs to be mentioned also the only cactus that occurs in the 

 Vancouver strip, Opuntia polyacantha horealis, confined to the island 

 in the northern part of Puget Sound. 



Only one physical factor presents itself which may explain these 

 strange cases, namely, the fact that these islands lie in the lee of the 

 Olympic Mountains, and therefore have a lesser rainfall, as may be 

 seen by comparison with the rainfall map. The conditions, therefore, 

 more nearly approximate those of the Arid Transition area than any 

 other portion of Washington west of the Cascade Mountains. But, 

 admitting this to be true, it is difficult to see how these species could 

 have crossed the barrier of the Cascade Mountains. The only other 

 alternative would seem to he that these species once occupied much of 

 the Vancouver strip, and have persisted northward only in this some- 

 what drier region of Whidby and adjacent islands. 



The case of a few Vancouver Island plants, like Lilaea subulata, 

 Festuca reflexa^ and Microseris higelovii, not otherwise known north 

 of Oregon, and especially the cases of Baeria gracilis and Allocarya 

 (horisiana, which leap from California to Vancouver, seem, however, 

 to lend weight to the latter hypothesis. 



