538 TRANS. OP THE ACAD. OP SCIENCE. 



My explanation of the phenomena here so plainly exhibited 

 is this : The true timber line, indicated by the natural growth 

 of erect trees (and which terminates sharply at a regular line 

 of elevation), marks a point of minimum, or extreme winter 

 temperature, below which no plnenogamous vegetation can 

 survive atmospheric exposure; only two or three of the hardi- 

 est forms of tree growth reach this limit, and these, endowed 

 as it were with special powers of endurance, leave all others 

 behind to mark the barrier beyond which the rigid elements 

 absolutely forbid all organized growth that is not protected 

 by the thick covering of winter snow. Such trees as venture 

 above this limit, can only survive by submitting to the con- 

 dition of a winter burial, by which their otherwise erect 

 forms are bent down and twisted to the earth, while all am- 

 bitious branches reaching into the sunlight of this arctic win- 

 ter are inevitably nipped, and lose their vitality. The features 

 of growth thus developed in their summer resurrection pre- 

 sent a grouping of the most strange and weird forms of strug- 

 gling existence, that mark so peculiarly the alpine scenery. 

 Some future artist will here find scope for his pencil in por- 

 traying a character of Rocky Mountain scenery witnessed 

 only in its perfection by the alpine explorer — when once seen, 

 never to be forgotten. 



It appears on this view that what we have above designated 

 as the false timber line, exhibits only a particular phase of the 

 alpine flora, represented elsewhere by the growth of shrubs, 

 whose persistence to still higher elevations is due to the same 

 general cause of protection by wintry snow from the other- 

 wise killing eifect of winter temperature. That shrub growth 

 is thus limited, is evidenced by noticing that the highest ele- 

 vation of this class of alpine growth is attained by taking 

 advantage of the shelter of ravines, where snow accumulates 

 the heaviest and most constantly. The shrubs that thus attain 

 to the highest elevations belong to the genus Salix, whose 

 general mode of growth, consisting of slender flexible twigs, 

 easily bent down to the earth, peculiarly fits them for com- 

 plying with the conditions of winter burial. Among the 

 three or four species here represented, Salix reticulata takes 

 the palm as the highest climber, being found at an elevation 

 of 12,000 feet above the sea, its prostrate form enabling it to 

 secure the necessary shelter at exposed situations. 



In order to give some general idea of the character of our 

 Rocky Mountain alpine flora, I submit herewith a list, as com- 

 plete as my present means of information furnish, of the alpine 

 plants met with in the district embraced within my personal 

 observation. In this list I confine the term "alpine'' to such 

 plants as are met with on the bald exposures above the timber 

 line; by a (*) prefixed, I would indicate those species which 

 are exclusively confined to such localities, while others not 



