219 



than Deobaii at the top of the outer ridge (elevation 9300'). 



The amount of moisture in the soil may be largely due to Also on 



percolation from rivers, canals, or lakes, to which fact is largely Percolation. 



due the characteristic vegetation of river-banks and of areas 



near large sheets of water. 



An important Indian tree which requires a very heavy 

 rainfall is Ficus elastica, the Caoutchouc tree, and another 

 tree which is usually confined to areas near rivers or to swampy 

 ground is Lagerstroemia Flos-Reginae, one of the most im- 

 portant timber trees of Assam and Burma. On the other 

 hand, Prosopis spicigera is a tree of some importance which 

 can thrive in districts with a very small rainfall. 



Under this head, also, must be considered the importance Action of 

 of water as an agent for distributing seeds. The sea is a ^. a * e . r , m , . 



i T -i ciii rrn Distributing 



great obstacle to the distribution 01 land plants. Ine geed?. 

 fruit of the Cocoa Nut tree is provided with buoyant 

 tissue which enables it to float, and its seeds are able to 

 germinate after prolonged immersion in sea water. Many other 

 plants have similar contrivances; their seeds are often 

 distributed great distances by marine currents, and they are thus 

 able to establish themselves on distant shores. 



Water is largely responsible for the distribution of the 

 spores of fungi, while rivers and streams play an important 

 part in distributing the seeds of many plants. The seed of 

 Sissoo (Dalbergia Sissoo), for instance, is chiefly distributed by 

 water, and that of Khair (Acacia Catechu) frequently so. 



Finally must be noted the denuding action -of water which, injurious 

 by washing away the soil, exposes the roots of plants, thus Denuding 

 injuring the roots more or less, even if the plants are not 

 actually up-rooted. The shrub Rhabdia lycioides, common 

 in river-beds, with its creeping, rooting branches, is well 

 adapted to withstand the action of torrential streams. 



199. On bare rocks an insignifi- (2) SOIL. 



cant vegetation, consisting chiefly of lichens, alone can exist, Depth of 

 and on very shallow soil, capable of supporting a good growth Soil, 

 of grasses, the majority of forest trees cannot thrive, hence the 

 depth of the soil is of primary importance. 



It has been rioted in Part V above that, for healthy plant Supply of 

 development, the essential mineral salts must be present in the Mineral 

 soil, and these must exist in a suitably dilute solution, see page Salts> 

 205. As the solution of salts in the soil becomes concentrated 

 beyond a certain limit, plants experience increasing difficulty in 

 obtaining their necessary supply of water from the soil. Many 

 substances, also, exercise a directly poisonous effect on plants 



