Chap. I] CHARACTERS OF THE TFAIPl'.RATI-: 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 arc killed or seriously 

 injured b\" air-temperatures of 0° to ;^° C. arc far more numerous than 

 arc 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 

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

 tlic tempera le zones into tzoo belts, one with a mild winter — tlie warm 

 temperate belt, and tite otJier with a eold winter — the eold temperate bell : 

 the warm temperate belt is character ized by broad-leaved trees t hat are 

 evergreen or g reen du r ing 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 

 appro.Kimately corresponds to the isotherm of 6 C. for the coldest month. 



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

 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. 

 isotlierm 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 distributi oi i of the constituents of the flora, tempera ture plays 

 a m uch m orgimpoilaivL_4iarl_irt-thp tpmp. erate zones than in the jropics. 

 whe re in this respect it g jves_ wa>- to the ^action of at mosph ejic prccipita- 

 tio ns. The distributional areas of many European, North Asiatic, and 

 North Ainerican species of plants have been defined, and corrcctl\-, as 

 functions of the temperature. 



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



SCHIMPER E e 



