227 



Enough, however, has now been said to show that the ( 6 ) ExIS ' 

 presence of an injurious plant parasite, or competitor, or the / OTHER 

 absence of a useful symbiont, may suffice to prevent the exist- PLANT. 

 ence of a given plant in a particular locality, and hence it will be 

 seen how necessary it is to avoid generalisations and to study in 

 detail the case of each individual species in different localities. 

 The valuable Sandal tree, we have seen above, depends to a, 

 large extent on other plants for its supply of water and mineral 

 salts, and the existence of such plants is undoubtedly an im- 

 portant factor influencing the distribution of this tree. 



204. In India examples of forests (7)Exis- 



which have been practically ruined by excessive grazing are not ^ANIMAI 

 uncommon. In such areas species which are readily eaten by injurious 

 cattle are rapidly exterminated, all young growth being destroy- and Useful 



j IT. iu 4-i, Action of 



ed and there being, in consequence, no young stems to replace the ^^^ 

 mature trees which must sooner or later die. Goats are parti- 

 cularly destructive, and in areas continually browsed by them, 

 the forest is frequently reduced to a scrub of scattered thorny 

 shrubs and small trees, which alone are able to survive. Some 

 of our important trees are not readily eaten by goats, and this 

 fact has considerably affected the distribution of such trees as 

 Pinus excelsa, Terminalia tomentosa, and Butea frondosa. Xylia 

 dolabriformis is also an important tree which is, as a rule, little 

 eaten by cattle. On the other hand, insects and birds are 

 largely instrumental in helping plants to exist in certain localities 

 by pollinating their flowers, while birds and other animals are 

 often the most important agents for scattering seeds. Many 

 species of plants may thus have their areas of distribution con- 

 fined to that of definite insects, birds, or other animals. Red 

 Clover is a well-known instance ; this plant, being exclusively 

 pollinated by humble bees, is only able to form fertile seed 

 where humble bees exist. In India the flowers of Butea 

 frondosa are largely pollinated by the rose-coloured starling 

 (Pastor roseus) ; the seeds of species of Loranthus, Ficus, Morus, 

 and of the Sandal tree are chiefly distributed by birds, while 

 jackals are believed to be largely responsible for distributing the 

 seed of species of Zizyphus. The seeds of Babul also are said to 

 germinate best after they have passed through goats. From 

 such instances it is obvious that the presence of an injurious 

 parasitic, or the absence of a useful symbiotic, animal may suffice 

 to explain the absence of a particular plant in a certain locality* 



205. If grasslands are burnt many (8) Sire. 

 of the most valuable fodder grasses, which are delicate annuals, 



are killed out and only the coarser species remain. Fires also, 



