CHAPTER XXXVIII 



PLANT GEOGRAPHY * 



459. Regions of vegetation. The earth's surface (that of the 

 land) has been described by one of the greatest of geographical 

 botanists 1 as divided into twenty -four regions of vegetation. 

 His grouping takes account of all the principal continental areas 

 which have a characteristic set of plants of their own, as well 

 as of the most important islands. But a simpler arrangement 

 is to consider the plant life of the earth as distributed among 

 the following regions : 



1. The tropical region. 3. The arctic regions. 



2. The temperate regions. 4. Mountain heights. 



5. Bodies of water. 



Any good geography gives some account of at least the 

 land vegetation of the earth. It is only necessary in the pres- 

 ent chapter to point out a few of the most important charac- 

 teristics of the plants of the areas mentioned above and to give 

 some reasons why the plant population of each has its special 

 characteristics. 



460. Tropical vegetation. Within the tropics two of the 

 great factors of plant life and growth, namely, light and heat, 

 are found in a higher degree than elsewhere- on the earth. 

 Moisture, the third requisite, is in some regions very abundant 

 (over forty feet of rainfall in a year), or sometimes, in desert 

 areas, almost lacking. We find here, accordingly, the greatest 

 extremes in amount of vegetation, from the bare sands or rocks 

 of the Sahara desert (Fig. 367) to the densely wooded basin of 



* To THE INSTRUCTOR : Unless the present chapter can he discussed in 

 considerable detail, it might better be omitted than hastily dealt with. 

 1 A. Grisebach, in Die Vegetation der Erde. 



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