234 



fix and protect these " flood-plains," as they are sometimes 

 called, against the action of subsequent floods, while the soil 

 is gradually enriched by the accumulation of organic debris. 

 Sissoo, being a light demander, the forests of this species thin 

 out and become open with increasing age, thus leaving room 

 for various miscellaneous species which soon establish them- 

 selves and eventually, ousting the light- demanding Sissoo, 

 give rise to a mixed forest of inferior species. As time goes on 

 however and the soil is still further improved by the accumula- 

 tion of humus and by the continued shelter from climatic in- 

 fluences, the struggle for existence between the competing trees 

 becomes more severe, more trees finding on it congenial condi- 

 tions for their development, and cases may often be seen on the 

 older alluvial deposits where the Sal, partly owing to its capa- 

 city of withstanding the injurious effects of a considerable 

 degree of shade, has been able not only to obtain a footing in 

 the mixed forest but to entirely oust other miscellaneous species 

 and to form pure forests of its own. 



Action of 214. From these two instances the 



Man in important part played by plants themselves in forming soil 



Ordinary an< ^ * n a ^ering its physical and chemical properties is clearly 



Course of recognized, and we see that some plants are able to create 



Natural ^ ne conditions necessary for the existence of others ; this 



necessarily P ar ^ ^ * ne subject, however, has been more fully discussed 



Restricted, in Chapter I above under Existence of other Plants, see pages 225, 



226. Cases such as these also show us that in nature a particular 



type of vegetation is not always able to maintain possession of 



an area for an indefinite period, and that an area carrying a 



good forest of a particular species may not necessarily be able, 



in the natural course of events, after the removal of that crop, 



to at once produce a second crop of the same species. Further 



we must remember, as already stated above, that although 



man can to a certain extent interfere with and alter the 



process of natural development yet his power in this respect 



is limited. 



(3) Action of 215. Several cases are on record 



Man. i n India in which man, by destroying the forest growth on 



orSesert! hill-sides, has reduced woodland to desert, and the Forester is 

 Grassland, often now employed in the difficult task of inducing vege- 

 and Forest tation to re-establish itself on the barren, rocky slopes and 

 depends on ^ transforming the desert once more into woodland. In many 

 the Action of cases, however, the destructive influence of man has not pro- 

 Ma 11 - ceeded so far, and his action has resulted in establishing 

 grassland where flourishing forests once existed. Throughout 



