231 



CHAPTER II. PRINCIPAL TYPES OF VEGETATION, 



210. The vegetation of the earth Types of 

 may be broadly divided into three great types : Vegetation. 



(1) Woodland in which woody plants predominate. 



(2) Grassland 'in which grasses predominate, usually in 



company with other herbaceous plants. 



(3) Desert where the climatic conditions render luxuriant 



vegetation of any kind impossible and only a few 

 plants are able to survive. 



Grassland containing isolated trees is usually called savannah. 

 Of each of these great types a multitude of varieties exist 

 which however do not concern us at present. The distribution Factors 



of the above types is influenced chiefly by three factors, viz.,' 11 . 



(1) Moisture which depends principally on the amount and Distribution. 



distribution of the rainfall, (2) Soil and (3) the Action of Man. 



These will be shortly considered below : 



211. Typical deserts are charac- (i) Moisture- 



teristic of areas with a very small rainfall. Grasses, being Different 

 usually shallow-rooted plants, depend mainly on the moisture Mature 

 in the surface soil. Typical grassland, therefore, can thrive if the produce 

 rainfall is sufficient to keep the surface soil moist in the Desert, 

 vegetative season. The majority of trees on the other hand a ' or 



depend mainly on the water in the subsoil and therefore 

 require a rainfall sufficiently great to keep the subsoil per- 

 manently moist. No more remarkable instance of the 

 dependence of vegetation on moisture can perhaps be given 

 than that quoted below : 



' The station of Jacobabad is a striking example of the effect 

 of water supply in that climate. It was founded in 1844 by 

 General Jacob, in the midst of a barren, treeless desert. A 

 canal was led to it from the Indus, and now the plain is a dense 

 forest of babool and other trees, upwards of sixty feet high, 

 sheltering the houses and gardens of the inhabitants. A ride 

 of a few miles takes you into the desert which skirts the hills 

 -of Beloochistan, a level plain of splendid, fertile, alluvial soil, 

 but hard, naked, and barren, like a threshing floor, without 

 shrub, herb, or grass, except in the vicinity of the canals, 

 where vegetation is rich and luxuriant. " * 



It has been noted above that in mountains the greatest 

 rainfall usually occurs at a comparatively low elevation and 



* Sir Dietrich Brandis op. cit., p. 11. 



