88 PLANT SOCIOLOGY 



Streams of cold air, 15 to 25 m. deep, flow down the radial mountain 

 canyons. By virtue of these streams the minimum temperature 

 conditions of canyons and other topographic depressions, below 

 elevations of 2,000 m., are equivalent to those of ridges and slopes 

 which are 600 m. higher up. 



Protection from Cold. — The ability to endure low temperatures 

 depends upon the specific constitution of the protoplasm and also 

 upon the physiological condition of the plant. Hence the well- 

 known fact that one and the same plant has very different degrees 

 of resistance to frost at different times. Since Lidforss' classic 

 researches on wintergreen plants (1907) an important function has 

 been ascribed to sugar as a protection to the colloids of protoplasm. 



13 



f.2 



;./ 



10 



0.9 

 0.8 

 0.7 

 0.6 

 0.5 

 O.U 

 0.3 

 0.2 

 0.1 



ZSJan. 5.Feb. Tz.Feb. ZI.Feb. 3.MaK S.Apn JV.May 



Fig. 47. — Ratio of starch to sugar in winter and spring. Solid line, Arhuhis menziesii; 

 broken line, Stellaria media. {After Rigg.) 



The presence of a non-electrolyte like glucose reduces the danger of 

 protein precipitation. Leaves of plants in the cold season, though 

 under the snow, show little or no starch. The carbohydrates are all 

 in the form of sugar, and in hardy leaves very considerable amounts 

 of sugar may have accumulated. This has the property of preventing 

 the coagulation of the albuminous parts of the protoplasm at low 

 temperatures. Sugar is therefore to be regarded as a protective 

 substance. The further the transformation of sugar into starch 

 advances the more sensitive to cold will be the protoplasm. As 

 shown in Fig. 47, sugar predominates over starch in the leaves of 

 Arbutus menziesii and Stellaria media from January to May, but in 



