PLANTS. 167 



substances useless and injurious to them, the soil 

 where they grow may in time become so impreg- 

 nated with such substances as to render it incapable 

 of supporting the same species of plant any longer ; 

 or, at least, until such time as the rejected matter 

 shall have been decomposed. The next purpose 

 which the roots of all land, and of the majority of 

 aquatic plants, serve, is obviously to fix them firmly 

 in their places. On tin's account, we find, in many 

 cases, that a certain proportion exists between the 

 size of the stem and the root. This, however, is 

 subject to exceptions. But, on the other hand, in all 

 cases, an obvious relation may be perceived between 

 the form of the root and the kind of soil in which the 

 plant grows. Thus, if two specimens of the same 

 plant some of the grasses, for example be found 

 growing, the one in clayey, the other in a sandy soil, 

 it will be seen, on examination, that the root of the 

 one growing in the sand is much more minutely sub- 

 divided, and contains many more small fibres, than 

 the one which grows in clay ; and the reason of this 

 is obvious. We have already seen that the spon- 

 gioles are the only absorbent parts of the root ; that 

 they exist only at the extremities of the smallest 

 fibres ; and, moreover, that they can take up nothing 

 but what is presented to them in the form of solu- 

 tion. Now, in the clayey soil, from its retentive 

 nature, the soluble parts are not allowed to drain 

 away ; and hence the plant is supplied with food 

 near at hand, and, consequently, a few short fibres 

 are sufficient. On the other hand, plants growing in 

 sand are frequently deprived of. all fluid near them 

 by the sinking of the soluble matters through the 

 loosely aggregated soil ; in which case the plant 

 would inevitably perish from starvation, were it not 

 for the wise law of nature, which provides against 

 such calamities by endowing the roots of plants 

 placed under such circumstances with the power of 

 shooting forth innumerable minute fibres in all direc- 



