44 CONTRIBUTIONS FROM THE NATIONAL HERBARIUM, 
A third 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. 
Steversia 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 parasite on various asteraceous plants, is not 
‘are 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 micranthum and Lupinus microcarpus, both on 
Whidby Island, together with Platyspermum scapigerum, Aster con- 
spicuus, and Seutellaria angustifolia, known from Vancouver Island, 
are cases practically parallel to that of Aphyllon comosum. 
Iris missouriensis 1s 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 oecurs in the 
Vancouver strip, Opuntia polyacantha borealis, 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 he 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 be 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, ike Zilaea subulata, 
‘Festuca reflewa, and Microseris bigelovit, not otherwise known north 
of Oregon, and especially the cases of Baeria gracilis and Allocarya 
chorisiana, which leap from California to Vancouver, seem, however, 
to lend weight to the latter hypothesis. 
