1. Urat.-Vvom the days of Humboldt to the present, 

 the vast importance of temperature in this connection 

 has been recognized. In spite of this, our knowledge is 

 rather scanty and vague. It has been usual to attribute 

 great importance to the severe cold of mountain tops. 

 This view, however, is entirely open to challenge. 

 Mountains which rise up from warm plains certainly can 

 not be invaded by plants which are killed by frost, hut 

 is there warrant in any physiological knowledge we 

 now possess for supposing that extreme low temperatures 

 are of any especial significance for plants which endure 

 freezing ? There is reason for believing that some woody 

 plants are cracked by severe cold, but I believe the gen- 

 eral question must be answered in the negative. That 

 treeless mountain tops are not so because of great cold 

 is shown by two facts: First, in cases that have been 

 investigated, timber lines do not bear any direct relation 

 to isothermal lines ; and furthermore, forests do exist in 

 the coldest districts on the globe. Herbaceous plants, 

 too, taken even in the active condition, are known to sur- 

 vive extremely low temperatures, undisturbed. 



We may note in passing the obvious fact that, for most 

 low growing mountain plants, deeply buried as they are 



in for consideration at all. 



That the lower temperatures of air and soil during the 



hardly point to a single feature of anatomy or histology 

 which is called forth by or conditioned upon heat or 

 cold. In order to make further progress, recourse is 

 naturally had to instrumental study. In the use of this 

 method, many pitfalls await the worker. It is relatively 

 ' " temperature, but vastly difficult 



