THE WILLOWS OF ALASKA 299 



become prostrate from the severe climatic conditions to which 

 they are exposed, particularly by their long enforced prostra- 

 tion beneath the snow and the repeated killing of upward shoots 

 by cold winds. When these adverse conditions are removed 

 such species assume their normal form. When a tree species 

 flowers young, as is often true of Salix sitchcnsis, it may easily 

 be mistaken for a bush willow ; but an observation of the aver- 

 age height of mature plants will prevent error from such a 

 source. Under very adverse surroundings even a tree willow 

 may become prostrate, a condition observed in Sali'x alaxensis 

 on the gravels at the terminal moraine of Muir Glacier. 



Only one of the willows enumerated, mn^lifolia, is confined 

 to Alaska, and on the other hand only two, retiailata and 

 glauca, are strictly circumpolar. Between these two extremes of 

 very restricted and very wide range, the species show three 

 definite geographic relationships, with Siberia, with the Pacific 

 Northwest Coast, and with the Interior of British America. 

 Excluding the strictly circumpolar species, the willows common 

 to Siberia and Alaska are 



S. alaxensis S. Ieioca?'^a 



S. arctica S. ovalifolia 



S. chamissonis S. -phlebo^hylla 



S. fuscescens S. -polai'is 



S. ^ulchra. 



One of these Siberia- Alaska species, alaxensis, is a tree, 

 another, ^?^/c//rflr, a bush, and the remainder are prostrate. All 

 the prostrate species belong to the true arctic or arctic alpine 

 flora. Some of them overlap into the Hudsonian flora, while 

 -pulchra and alaxensis are Hudsonian species which abut against 

 and even intrude into the arctic flora. The Alaskan willows of 

 Siberian relationship may be considered, therefore, prevailingly 

 arctic. 



The species showing a relationship to the flora of the interior 

 of the North American continent are : 



S. arbusculoides S. ni^hoclada 



S. bebbiana S. nuttallii 



S. myrtillifolia S. richardsoni. 



