SECTION II. THE TEMPERATE ZONES 



CHAPTER I 



GENERAL CHARACTERISTICS OF THE TEM- 

 PERATE CLIMATE AND ITS EFFECTS ON 

 VEGETATION AND FLORA 



1. General Characteristics of the Temperate Climate, i. Heaf. Great differences 

 of temperature. Maritime climate and continental climate. Isotherms of January and 

 July. Diurnal oscillations, ii. Light. Zone-like differentiation of light. Absorption and 

 diffusion of light in different latitudes, iii. Atinosphcric Precipitatioin. Periodicity 

 and amount of precipitation. Significance in relation to the soil. 2. Some General 

 Effects of the Temperate Climate on Plant-life. i. Effects of Heat. Its fundamental 

 significance. Temperatures below freezing-point. Zone with a mild winter and zone with 

 a cold winter. Distribution of species. Mesothermic plants. Lack of uniformity of the 

 oecological optimum temperature, ii. Effects of Light. Amount and intensity of light. 

 Fixed lie of leaves in relation to light. Diffuse light in temperate zones, iii. Effects 

 of Atmospheric Precipitations. Smaller significance ^^■hen compared with the tropics. 

 3. Character of the Flora of Temperate Zones. General survey of the mesothermic 

 forms. 



I. GENERAL CHARACTERISTICS OF THE 

 TEMPERATE CLIMATE. 



[i. HEAT. 



The zones between the tropical and the polar zones deserve the 

 appellation temperate solely on account of their mean temperature. From 

 the point of view of the difference between the temperatures of winter 

 and summer they should rather be termed excessive. This is specially 

 true of the north temperate zone. ' Between the tropic of Cancer and 

 the polar circle there is represented almost the entire range of temperature, 

 within which the temperature of the air on the earth's surface ever 

 fluctuates. In Central East Asia the mean temperature in January sinks 

 almost every year to — 40° C. and even lower, whilst the temperature in 

 July rises to 35° C. in the Punjab, Mesopotamia, probably also in Arabia, 

 North Africa, and in Arizona and South California. At the same time 

 the ab.solute extremes of temperature of these countries lie between — 70° 

 and + 50° and even more. As regards oscillations of temperature also, 

 the variability in temperature from one day to another reaches its highest 

 in the north temperate zone ; in it, districts with the greatest variability 

 of temperature occur ^.' 



' Hann, op. cit., Ill, p. 3. . 



