34° 



THE POPULAR SCIENCE MONTHLY 



ern states and Canada printed since the invention of the half-tone proc- 

 ess, about thirty years ago. 



In the United States the jack pine prefers coarse sand and the other 

 trees above mentioned are found mainly in peat bogs ; but farther north 

 they may grow in almost any kind of soil, wet or dry. (In Alaska even 

 some of the glaciers are said to be partly covered with spruce forests.) 

 The regions where they grow are characterized by cool and moderately 

 humid climates, with an average temperature of 45° F. or less, and an 

 average growing season (i. e., period free from killing frosts) of not 



BUKNED SPEDCB SWAJIF, WITH LIVING TliEES IX BACKGROUND, CHEBOYGAN CO., 



Michigan. August, 1912. 



more than 150 days. The ground freezes several feet deep in winter, 

 and temperatures of — 30° F. or lower are likely to be experienced by 

 each tree many times during its life. 



The average annual precipitation is 20 inches or more, and in most 

 places in the boreal conifer region there is more of it in summer than 

 in winter, which tends to keep the soil moist throughout the year. 



A climatic factor which involves both temperature and precipitation 

 is the amount of snowfall; and it appears from statistics of the snowfall 

 of the United States recently published that the type of forest under 

 consideration can be correlated pretty closely with an average annual 

 snowfall of 50 inches and upward. Although it would not be exactly 

 correct from a biological standpoint to say that the narrow conical 

 form of these trees is an adaptation to heavy snows, like the steep 

 roofs of Norway, for example, it would be difficult to imagine any other 



