218 



PART VL-GEOGRAPHY. 



CHAPTER L FACTOKS INFLUENCING THE DISTRIBU- 

 TION OF PLANTS. 



Factors 197. The principal factors affecting 



affecting the distribution of plants will first be shortly considered under 



Distribution. ^ f H owing neads ._ 



(1) Water. (5) Air. 



(2) Soil. (6) Existence of other Plants. 



(3) Temperature. (7) Existence of Animals. 



(4) Light. (8) Fire. 



(9) Action of Man. 



(l) WATER. 198. The water available for plants 



Available depends chiefly on the rainfall. In deserts, where the amount of 

 depends moisture available for plants is exceedingly small, owing to the 

 chiefly on scanty rainfall, very few plants are able to survive and no forests 

 the Rainfall. ex i s t. I n India " really thriving forests are only found where the 

 rainfall exceeds 40", and rich luxuriant vegetation is limited 

 to those belts which have a much higher rainfall. " * Owin<* 

 to the low temperature prevailing at high elevations, mount 

 ains tend to condense aqueous vapour and thus receive more 

 rain than the adjoining lowlands. The greatest rainfall, 

 however, usually occurs at a comparatively low elevation and 

 at higher altitudes, instead of periodic heavy falls of rain 

 frequent mists and light rain occur, the rarefied air having 

 a very small capacity for aqueous vapour. This accounts for 

 the luxuriant growth of mosses and lichens often found cover- 

 ing the stems and branches of trees at high elevations. The 

 outer ridges of a range of hills, also, which first intercept the 

 currents of moisture -laden air, receive more rain than the inner 

 hills, and thus Deoban, for instance, on the outer ridge in the 

 North- West Himalayas, receives a greater rainfall than either 

 Mundali or Kathian, while, at the same time, Kalsi, at the foot 

 of the outer hills (elevation 1820'), receives a greater rainfall 



* Cn the JJislribulion of Forests in India by Dietrich Brandis, p. 6. 



