ZONES AND REGIONS 



INTRODUCTION 



The summer and winter isotherms subdivide the earth's vegetation 

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

 orders and families of plants are dependent on constantly high tempera- 

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

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

 for the development of other plants other temperatures, alternately high 

 or low, are required (Mesotherms). The plants of the second category also 

 exhibit differences in their susceptibility to temperatures below zero, 

 and these differences call forth corresponding ones in the characters of 

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

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

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

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

 (Microtherms). 



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

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

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

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

 at its base an equatorial climate, but at its summit one that may be 

 termed polar as regards the temperature of the air. 



The resemblance between the change in climate in passing from the 

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

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

 faction of the air bestows on other climatic factors very characteristic 

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

 It is accordingly inadvisable to apply the term zones to the climatic 

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

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

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

 earlier geographical botanists, and by Grisebach among others. 



Zones of temperature are by no means identical floristically and 

 occologically all round the earth ; indeed they show themselves to be 



