REFERENCES 143 



timber (Fig. 62). The short growing season, the low temperature of 

 the soil, and the drying effect of the winds, make conditions here so 

 difficult for growth that a tree may be several hundred years old yet 

 only a few inches in diameter of trunk and a foot or so in height. 

 Above timber line on the higher mountains a similar effect is seen 

 in the herbaceous vegetation, all of the plants being very dwarf 

 (Fig. 63). 



91. Cold-air Drainage.— Cold air is heavier than warm air. It 

 therefore has a tendency to settle down into low places and displace 

 the warmer air that is there. This takes place to a certain extent 

 each night as the air is cooling after sundown. For this reason there 

 are usually frosts later in spring and earlier in autumn in the low- 

 lands than on the uplands. Peaches, strawberries, and other crop 

 plants that are subject to late frosts in spring, are more profitable 

 on highlands than in valleys, because they more often escape the 

 effects of late frosts. 



REFERENCES 



Bergman, H. F.: The Effect of Cloudiness on the Oxygen Content of Water 

 and Its Significance in Cranberry Culture, Am. Jour. Bot., 8, 50-58, 1921. 



The Relation of Aeration to the Growth and Activity of Roots and 



Its Influence on the Ecesis of Plants in Swamps, Ann. Bot., 34, 13-33, 1920. 



Cannon, W. A.: Physiological Features of Roots With Special Reference to 

 the Relation of Roots to the Aeration of the Soils, Carnegie Inst. Washing- 

 ton Publ., 368, 169, 1925. 



Clements, Frederic E.: Aeration and Air-content, the Role of Oxygen in 

 Root Activity, Carnegie Inst. Washington Publ., 315, 183, 1921. 



Hubert, E. E.: Fungi as Contributory Causes of Windfall in the Northwest, 

 Jour. Forestry, 16, 696-714, 1918. 



Martin, E. V., and Clements, F. E. : Studies of the Effects of Artificial Wind 

 on Growth and Transpiration of Helianthus annuus, Plant Physiol., 10, 

 613-636, 1935. 



Shreve, Forrest: Cold-air Drainage, Plant World, 15, 110-115, 1912. 



