Chap. I] CHARACTERS OF THE TEMPERATE CLIMATE 417 



a slight frost at night is more fateful to vegetation than are long and 

 severe periods of winter cold occurring in districts accustomed to annually 

 recurrent low temperatures. Species of plants that are killed or seriously 

 injured by air-temperatures of o° to 3 C. are far more numerous than 

 are those which withstand — 3 C, but not lower temperatures occurring in 

 nature. 



One may reasonably assume that near the tropics it is the winter 

 temperatures which are extremely important in relation to plant-life in 

 the temperate zones, but that at a greater distance from the tropics it 

 is the summer temperatures which are important ; so that, for instance, the 

 difference in plant-life between Southern and Central Europe must depend 

 chiefly on the winter temperature, that between Central and Northern 

 Europe on the summer temperature. This consideration leads us to divide 

 the temperate zones into tzvo belts, one with a mild winter — the warm 

 temperate belt, and the other with a cold winter — the cold temperate belt : 

 the warm temperate belt is characterized by broad-leaved trees that are 

 evergreen or green during the rainy period, and by only a partial winter- 

 rest of its woody plants ; the cold temperate belt is characterized by trees 

 that are bare in winter and green in summer, and by a general winter-rest 

 on the part of its woody plants. The border line between the two belts 

 approximately corresponds to the isotherm of 6° C. for the coldest month. 



A more exact coincidence of the border line of the belts of vegetation with the 

 isotherms would possibly be attainable if attention were also paid to the isotherms 

 of the hottest month, as we should exclude from the belts having a mild winter, 

 in the northern hemisphere, districts north of 20 C. isotherm in July, in the southern 

 hemisphere, where heat due to insolation is stronger, districts south of 14 C. 

 isotherm in January, and should incorporate them with the belts having a cold 

 winter. Too much importance, however, should not be attached to such attempts, 

 as an exact coincidence between zones of heat and of vegetation is impossible, 

 at any rate under the present mode of defining climatic factors. 



The general effects of temperature on vegetation in the temperate zones 

 are particularly exhibited in the phenomena of periodicity. Indirectly, by 

 its influence on the absorption and emission of water, heat also affects the 

 distribution of woodland and grassland. Special chapters are devoted to 

 both groups of phenomena. 



In the distribution of the constituents of the flora, temperature plays 

 a much more important part in the temperate zones than in the tropics, 

 where in this respect it gives way to the action of atmospheric precipita- 

 tions. The distributional areas of many European, North Asiatic, and 

 North American species of plants have been defined, and correctly, as 

 functions of the temperature. 



The flora of the temperate zones as a whole is described as mesothermic, 



SCHIMPER JT Q 



