SOURCES OF THE VEGETATION 67 



Salix Candida Symplwricar'pos orbiculahis 



" amygdaloides Fraxinus lanceolata 



Eihes americanum 



Bessey (3) concluded from a study of the trees of Ne- 

 braska that most of the species had entered that state from the 

 southeast, influenced largely by the direction of the prevailing 

 winds in the spring and early summer. The cottonwoods, two 

 willows (8. amygdaloides and fluviaiilis) and the boxelder were 

 included in his list. Popvlus Sargentii, which by some authors 

 is considered as at best only a variety of F. deltoides, is contin- 

 uously distributed along all the bottom-lands of the Missouri and 

 the Yellowstone and their tributaries, and evidently has followed 

 up these streams from the lower regions to the southeast. The 

 favorable conditions which woody species usually find on river 

 bottoms and the continuity of such conditions along the many 

 drainage channels across the plains should have provided a 

 larger representation of such species in the Montana flora. The 

 low relative humidity of the plains and the desiccating winds 

 that sweep across them serve apparently as effective preventives 

 of any departure of woody plants from the lower and more 

 favorable levels. The tops of the cottonwoods along the upper 

 tributaries (16) are distorted and repressed as they reach above 

 the protection of the bordering benches. The matter of at- 

 mospheric humidity is as important for certain trees as the mat- 

 ter of soil moisture, and specific requirements must be met in 

 one condition as well as another. Whether this is the determining 

 factor or not in this case is impossible to say without further 

 stud.y, but for various reasons it seems probable. Fruiius Ameri- 

 cana is reported only from the eastern part of the state, and on 

 uncertain authority as far west as the Bearpaws, south of Havre. 

 The ash (Fraxinus lanceolata) probably occurs sparingly in the 

 eastern end of Montana. 



As the western or Pacific slope of the divide is the more 

 favorable for forest growth and is tenanted by a greater variety 

 of species, it naturally follo^'s that the western element is the 

 most conspicuous in the forest flora of the Rocky Mountains. 

 On this point several considerations should be noted. Aside 

 from the more favorable climatic conditions enjoyed by this 

 region, its more ample precipitation, its higher relative humidity 



