94 PLANT SOCIOLOGY 



the interior of the continents (Table 5). In the interior the decrease 

 of temperature toward the poles is more rapid than on the coasts. 

 Large bodies of water have an equalizing effect, since the surface water 

 is somewhat cooler than the air at low latitudes and considerably 

 warmer at higher latitudes. 



Table 5. — Difference between the Mean Temperatures of the Coldest 



AND Warmest Months of the Year 



(After Hann) 



Degrees 

 Centigrade 



Valentia, Ireland 7.8 



Miinster, Westphalia 16 . 



Warsaw, Poland 23 . 



Orenburg, Russia 36 . 9 



Irkutsk, Siberia (490 m.) 39.2 



Nerchinsk, eastern Siberia (600 m.) 51.8 



From the above it clearly follows that the temperature zones 

 must be divided in two directions. The simple classification of 

 de Candolle is : 



Megathermic, the zone of warm climate. 



Xerophilous, dry regions. 



Mesothermic, warm temperate climates (mean annual temperature 15° to 



20°C.). 



Microthermic, cold temperate climates (mean annual temperature 0° to 14°C.). 

 Hekistothermic, cold climates. 



This arrangement does not do justice to the actual conditions. 

 Even with subsequent changes, the scheme gives only a very general 

 and superficial basis for the arrangement of zones of vegetation 

 according to temperature. Following De Martonne, but also taking 

 into consideration oceanic and continental influences, it seems neces- 

 sary to recognize the following latitudinal zones: equatorial (without 

 dry season), tropical (with a dry season), subtropical, warm temperate, 

 cold temperate, subarctic, and arctic. These are subdivided into 

 oceanic, suboceanic, median, subcontinental, and continental regions 

 (Gebiete). From the standpoint of temperature, the daily and annual 

 range of temperature determines the oceanic or the continental 

 character of the climate. 



On this basis, the climate of north central Siberia is subarctic- 

 continental, the eastern Mediterranean region is subtropic-sub conti- 

 nental, and Great Britain is mostly cold temperate-oceanic. In 

 characterizing these climates, humidity and cloudiness must be 

 included (c/. p. 106). McDougall (1925) has attempted to express 

 temperature and humidity of cHmate graphically (Fig. 50), and 



