172 



The Plant World. 



hindering of reproductive processes; maturing of seed; (b) Of 

 vegetative processes; manufacture of food; growth of new wood. 



It is probably true that trees in high altitudes are often 

 unable to mature their seeds. It is not clear, however, that 

 this would limit the development of the forest. Wind-dispersed 

 seeds apparently would have no difficulty in travelling up from 

 below. The idea that insufficient growing time prevents the 

 development of woody shoots must depend wholly upon evi- 

 dence, and of this little has been brought forward. 



3. Dry Killing in Winter. Since trees during the resting 

 season are unable to replace the water they may lose, they 

 must be exposed to serious danger from winter drying. Kihl- 

 man* reached the conclusion that the arctic limit of the forest 

 is set by the drying action of the winter wind. Schimperf 

 accepted his view and extended it to apply to the timber line 

 of mountains. Diminished pressure and increased insolation 

 would tend to increase this danger for trees at high altitudes, 

 and dessication by wind is probably more emphasized in this 

 connection by present writers than any other cause. 



4. Besides the foregoing, many other factors have been 

 suggested. Character of rainfall, summer frosts, soil tempera- 

 ture, possible absence of the necessary myeorhiza, of the necessary 

 soil bacteria — all have been speculated upon more or less and 

 for all it is a question of evidence. Of the latter, very little has 

 been given. 



5. Snow. That the heavier and longer persisting beds of 

 snow at higher altitudes may have something to do with pre- 

 venting forest growth has been more or less remotely suggested 

 by several writers, and distinctly by Buehler.J: However, the 

 factor has not, if the ground taken in'the present paper be cor- 

 rect, been given anything like its due importance. The view 

 here presented is that for the Selkirks and neighboring moun- 

 tains, at least, and probably for the Alps, the late lingering beds 

 of wet snow are altogether the most important factor in in- 

 hibiting the development of a forest and bringing about the ex- 

 istence of alpine grassland. 



*Cited from Schimper's Pfianzengeographic. 

 tPflanzengeographie. 



JA. Buehler. Studien ueber die Baumgrenze im Hochgebirge. Ber. d. schweiz. but 

 Gesell. Heft VIII 1898, p. 19-38. 



