ZONES AND REGIONS 



INTRODUCTION 



The summer and winter isotherms subdivide the earth's vegetation 

 ito more or less parallel zones of different systematic character. Certain 

 ders and families of plants are dependent on constanth" high tempera- 

 n-es, such as prevail in the tropics alone, and perish at the freezing- 

 jint of water or even at some degrees above it (Macrotherms) ; whereas 

 ir the development of other plants other temperatures, alternately high 

 • low. are required (Mcsotherms). The plants of the second category also 

 chibit differences in their susceptibility to temperatures below zero, 

 id these differences call forth corresponding ones in the characters of 

 ;e flora as the latitude increases. Finally, polar zones are inhabited 

 r plants that can support without injury, not only very low winter 

 mperatures, but also frosty weather during the vegetative season, and 

 e able to complete their cycle of development in a short time 

 .licrotherms). 



A reduction in the temperature, similar to that from the Equator to 

 le Poles, occurs in a vertical direction up mountains that exceed the 

 nit of perpetual snow. But here the isothermal zones are much 

 UTOwer, so that, for instance, Kilimanjaro, 6,000 meters high, possesses 



its base an equatorial climate, but at its summit one that ma\" be 

 rmed polar as regards the temperature of the air. 



The resemblance between the change in climate in passing from the 

 quator to the Poles, and from the base of mountains to their summit, 



limited to the temperature of the air ; the vertically increasing rare- 

 ction of the air bestows on other climatic factors ver}' characteristic 

 aturcs, which are not at all comparable with those in the lowlands. 



is according!}- inadvisable to apply the term zones to the climatic 

 :lts of mountains, though this has frequently been done in recent times ; 

 ice by this means a partial analogy receives an exaggerated and con- 



sed expression ; we should rather retain the term of regions used by 

 '.rlier geographical botanists, and by Grisebach among others. 



Zones of temperature are by no means identical floristically and 

 :coIogically all round the earth; indeed they show themselves to be 



SCHIMI'ER p 



