PIPER FLORA OF THE STATE OF WASHINGTON. 53 



Parrya menziesU. Platyspermum scapigerum. 



Pentstemon attenuatus. Polygonum polygaloides. 



Pentstemon pinetorum. Ranunculus triternatus. 



Pentstemon richardsonii. Silene spaldingii. 



Pentstemon triphyllus. Sphaeralcea longisepala. 



Phaca aiTecta. Trifollum douglasii. 



Phaca conjuncta. Trillium petiolatum. ^ 



Phaca spaldingii. Viola trinervata. 

 Physaria geyeri. 



COMPARISON OF THE HUMID AND ARID TRANSITION FLORAS. 



As before stated, the Cascade Mountains form, a sharp and efficient 

 barrier between the areas of the Transition zone. The Humid Transi- 

 tion does not cease abruptly with the crest of the Cascade Mountains, 

 but many species descend for some distance on the eastern slope, at 

 lower altitudes becoming mixed indiscriminately with Arid Transi- 

 tion plants. 



In the moisture portions of the western slopes of the Bitterroots 

 and in the Blue Mountains the climatic conditions approach those of 

 western Washington. It is therefore not surprising that in the 

 moister portions of the yellow pine subarea in western Idaho a great 

 many of the Humid Transition plants of western Washington reap- 

 pear. Indeed at least one-half of the species are thus common, and 

 it is therefore largely an arbitrary matter to class the one as Humid 

 Transition, the other as Arid. 



The complex nature of the problem is realized, however, when we 

 consider the remaining species. They may be thus grouped : First, 

 those that occur in the Vancouver strip, but not in North Idaho ; sec- 

 ond, those that occur in Idaho, but not in the Vancouver strip, and 

 third, those that are abundant in Idaho, but very rare in the latter 

 area. The following tables bring out clearly the relative importance 

 of these elements: 



PLANTS EQUALLY COMMON IN THE HUMID TRANSITION AREA OF THE VANCOUVER 

 STRIP AND THE MOISTER PORTION OF THE YELLOW PINE SUBABEA IN NORTH IDAHO. 



Trees. 



Abies grandis. Rhamnus purshiana. 



Acer douglasii. . Salix scouleriana. 



Pvnus contorta. Tnxtts brevifolia. 



Pinus monticola. Thuya plicata. 



Populus trichocarpa. Tsuga mertensiana. 

 Pseudotsuga mucronata. 



