680 RYDBERG: PHY APHICAL NOTES 
upon the growing of trees. It is shown that the temperature in 
temperate regions in the winter often is much lower than in many 
places in the arctic. It is doubtful if the temperature in the alpine 
region of Colorado ever becomes as low as on the plains of Mon- 
tana, where some years ago it was recorded as 65° F. below zero. 
The only place within the alpine region of the Rockies where a 
record has been kept during the winter is on the top of Pikes Peak, 
and here only for a few vears. No such low temperature has been 
recorded there. It is during the growing season that a low 
temperature limit for forest growths can be spoken of, for during 
the winter, when the life functions of the plants lie dormant, a 
few degrees more or less makes in reality no difference. Képpen 
claims that no trees can grow at a place where the mean temper- 
ature during the warmest months of the year does not reach 10° C. 
Schroeter gives a table taken from the records of the meteorological 
central station at Ziirich in which are given the mean temperatures 
in July at fifteen stations at the timber line in Switzerland. The 
mean temperature ranges from 7.75° C. at Zermatt to 15.4° at 
Monte Generoso. This shows that the timber line may reach a 
little higher than the isotherm 10° C. and in other cases not reach it. 
Schroeter is particular enough to mention that at Monte Generoso 
the low timber line is not an artificial one made by man. See 
below. The arctic timber line seems to be more coincident with the 
isotherm 10° C. for July than the timber line in the mountains. 
SHORT GROWING SEASON 
Another cause of the timber line is the shortness of the growing 
season. This, it may be, is just as important a factor as the pre- 
ceding. When speaking of the arctic timber line, it is easy to see 
that this factor acts parallel to the preceding, for near the sea- 
level places of the same isotherm in the summer have about the 
same length of summer, but not so in the mountain regions. In 
the heads of valleys, where big snowdrifts are formed during the 
winter and melt late in the summer, and along glaciers and per- 
manent snow the frost is kept longer in the ground and the growing 
season is naturally shortened. Therefore, in many places in the 
Rockies, the timber line is a thousand feet or more lower in the 
valley heads than on the slopes on the sides. The shortness of 
