66 



FOREST DISTRIBUTIOX 



Southwest and the :Mexiean Plateau may be regarded as ex- 

 amples. The representatives of these groups in the Montana 

 flora can be regarded only as having a southern origin. With 

 such points in mind a grouping of species is attempted accord- 

 ing to the sources from which they seem to have migrated into 

 the northern Rocky ilountains. 



The following species mostly have a northern transcontinent- 

 al range and appear to have moved southward along the Rocky 

 Mountains into ilontana and in some eases far beyond: 



Dasiphora frniicosa 

 Elaeagnus argentea 

 Lepargijraea canadensis 

 Rhus glabra 

 Bhamuits ahii folia 

 Corniis stolonifrra 

 " canadensis 

 Andromeda polifolia 

 ^ Arctostaphylos Uva-ursi 

 Cliiogenes hisjyidida 

 ^Vaccinium idiginosum 

 " caespitosum 



" ovalifolium 



SambucHs raccmosa 

 Sij)nplioricarpos pauciflorus 

 Lonicera involucrata 

 There is, of course, no barrier to the southward movement 

 of species along this liighA\a.\- through the region here under con- 

 sideration. Five of the above species (marked *) occur around 

 the world in the northern hemisphere. ^lost of the others are 

 middle or northern Atlantic Coast or eastern species which 

 apparently have extended their ranges far to the northwest, 

 passing north of the Great Plains to the RocIvA' ^lountains and 

 thence southward; some are extensively distributed from the 

 coast of New England and Labrador to Alaska. 



The following have entered ^lontana and the Rocky Moun- 

 tains from the east : 

 Populus angnstifolia Pninus americana 



" Sargentii Acer Xcgundo 



Salix pedicellaris Stambucus canadensis 



* Jumper us communis 



* " Sahina 

 Populus fremuloides 



" balsamifera 

 Salix cordaia 



" chlovophijlla 



" fluviatilis 



" Bebbiana 



" Barclaiji 



pscudomyrsiniies 

 Betula papyrifera 

 *Alnus incana 

 Eibcs lacustre 



" liudsonianum 

 Rubus strigosus 

 Rosa acicularis 



