PIPER—FLORA OF THE STATE OF WASHINGTON. 51 
cially sticky laurel (Ceanothus velutinus) and thimbleberry (Rubus 
parviflorus), form dense thickets. 
Such Arid Transition forests, in which the red fir becomes the most 
abundant tree, resemble very closely the Humid Transition forests 
of western Washington. Indeed, they are intermediate in character 
petween the two, merging at lower altitudes into the ordinary vellow 
pine forests, and at higher altitudes into the Canadian or white pine 
zone. Inasmuch as the Canadian and higher zones in the Bitterroot 
Mountains have a flora very similar to that of the Cascade Mountains, 
1t is not surprising that the higher and moister portion of the Arid 
Transition shows a like similarity. 
ORIGIN OF THE ARID TRANSITION FLORA. 
~The Arid Transition flora, like the Upper Sonoran, is clearly made 
up for the most part of three sets of species. These may be denomi- 
nated the Californian element, consisting of plants which have 
migrated from California; the Rocky Mountain element, species 
whose distribution is easterly and southerly from the region here 
considered; and the Columbian element, those species which are con- 
fined to the Columbia Basin and have in all probability originated 
there. 
THE CALIFORNIA ELEMENT. 
This consists of species which range from California through the 
low passes in the Klamath region into the Columbia Basin. <A large 
number of the species range also into the Willamette Valley on the 
west side of the Cascade Mountains, most of these also reaching the 
gravelly prairies of western Washington. These species have been 
discussed in connection with the Humid Transition area. The fol- 
lowing list is composed of species confined to the Arid Transition area 
east of the Cascade Mountains: 
Festuca pacified, 
Irasera nitida, 
Agoseris grandiflora. 
Agoseris retrorsa, 
Alopecurus californicus, 
Amsinekia intermedia. 
Arabis subvillosa, 
Arenaria pusilla, 
Calandrinia menziesii. 
Calochortus macrocarpus. 
Ceanothus prostratus. 
Clarkia rhomboideda. 
Cuscuta californica. 
Deschampsia calycina, 
Flatine californica. 
Hriogonum sphaerocephaltum, 
Eryngium articulatum. 
Festuca confusa. 
Gilia harknessii. 
Lagophylla ramosissima. 
Lepidium nitidum. 
Linum digynum. 
Lomatium piperi. 
Melica fugaxr. 
Merathrepta californica. 
Microseris nutans, 
Mimulus pulsiferae, 
Orthocarpus hispidus., 
Orthocarpus tenuifolius. 
Pectocarya penicillata. 
Pectocarya pusilla. 
Plagiobothrys tenellus. 
