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THE AMERICAN NATURALIST [Vol. XLIII 



greater precipitation and that the vegetation of moun- 

 tains is largely dependent upon atmospheric water are 

 ideas familiar to every one. Sehimper has further 

 suggested that light showers at high altitudes favor the 

 development of grassland rather than forest. This idea, 

 to be tenable, requires the further assumption that the 

 soil in the grassland zone is deficient in water supply. 

 This may be true for lower latitudes, but it scarcely seems 

 admissible for northern mountains where the late melting 

 snows leave the soil supplied with all the water it can 

 hold. 



In quite a different way, too, precipitation in the form 

 of snow becomes a decisive factor for vegetation. In a 

 paper read before this society, a year ago, I showed that 

 the timber line of the Selkirks was due to the heavy 

 snow beds at those altitudes. By a coincidence, Cowles 

 showed elsewhere, at the same time, that in a number of 

 places in Xorth America snow beds and timber line were 

 causally related. 



3. Closely connected with the question of heat and pre- 

 cipitation is another factor of prime importance in some 

 mountains, namely, Length of Season, 



In respect to this factor at least two points of view are 

 necessary. For trees and plants growing on wind-swept 

 spots, length of season is a question of temperature. 

 For the vast majority of low-growing plants, on the other 

 hand, length of season is also a question of emergence 

 from snow beds to air and sunshine. 



Taking up, first, the question of forests, it has been 

 said that the total heat available at high altitudes is not 

 sufficient for the maturing of new wood, and more partic- 

 ularly, that the season is too short for the ripening of 

 good seed. Both of these ideas must stand or fall simply 

 upon evidence, and so far as I am aware, none of a re- 

 liable character has been brought forward. It may be 

 remarked that seedling trees usually seem abundant 

 enough at timber line. It would be of interest to gather 

 seeds of balsams, etc., growing at timber line and learn 



