382 ZONES AND REGIONS [Pt. Ill, Sect. I 



tropics, in particular in South India, which is covered to about one-third 

 of its area by the fertile black soil — regur — that also occurs further north ; 

 similar soil is found also in fiat, densely wooded tracts of country, where 

 the flow of water is slower and shade retards the process of decomposition. 

 Peat is never produced, except in mountains over 1,200 meters in height. 



iv. SILICEOUS SOIL. 



A very permeable soil rich in silica and gravel is a substratum unfavour- 

 able to the growth of trees, and therefore situations with soil of such 

 a nature always exhibit a characteristic form of vegetation. Some species 

 of plants withstand these unfavourable conditions better than others and 

 form more or less pure woods. This is to a great extent the case in 

 India with the sal-tree (Shorea robusta) l , which forms forests of great 

 extent in the long valleys (duns) between the outer chains of the 

 Himalaya Mountains (Fig. 206), then again in a southern very extensive 

 tropical area that is separated from its northern habitat by the Ganges 

 valley. The sal-forest always occurs on a loose soil that is very permeable 

 to water, and is absent whenever the soil becomes firm. In general it is 

 not the climate, but the soil alone, that determines its presence. Hence the 

 sal-tree is absent from the western half of the Indian peninsula, where trap 

 is the prevailing rock, whilst it forms extensive forests in the eastern half, 

 which has a very similar climate. 



In all probability the occurrence of natural bamboo woods (Fig. 207) is also 

 connected with peculiarities of the soil that are less favourable to other trees, since 

 they usually appear only locally, except in the Burmese mountains, where they 

 sometimes cover extensive tracts. Kurz assigns, as the substratum for bamboo- 

 woods, rocky or shallow alluvial soil in the case of certain species, and deep alluvial 

 soil in the case of others. 



Bamboo-woods merit the term ' pure ' better than any others, for they consist 

 only of one or two species of bamboo and are devoid of any other plants. According 

 to Kurz, in the very dense forests of certain species of bamboo a few mosses 

 (Hypnum, Fissidens) and lichens appear only here and there on the soil and on 

 the base of the stems. 



Bamboo-woods often owe their origin to cultivation. As in such cases they 

 frequently cannot be distinguished with any certainty from others that are pro- 

 duced natural^, it is evidently difficult to shed light upon the obscure problem 

 regarding the conditions determining their appearance. 



v. SWAMPY GROUND. 

 According to the still defective observations regarding the oecology of 

 tropical vegetation, a persistent and great amount of water in soil is 



1 Brandis, op. cit. 



