ORGANIC STRUCTURE. 89 



earth or in pure water, roots invariably descend till 

 they are out of the influence of dry air. This, from 

 its withering effect, prevents all ejection of the deli- 

 cate and sensitive spongioles ; and to escape from it, 

 in ordinary circumstances, their course must be down- 

 ward. To prove that there is no constitutional 

 tendency in roots to obey the law of gravitation (as 

 has been supposed) they will take an entirely oppo- 

 site direction in quest of moisture, as may be seen 

 within the overhanging- banks of roads or rivers 

 when the bank has been undermined. 



Although fibrous roots are only ordinarily pro- 

 duced below the collet, yet the power of exserting 

 them is not wholly confined to that part of the system ; 

 they are readily ejected from the stem, branches, 

 and even the leaves also, under certain circumstances ; 

 but this will be adverted to when we describe that 

 member of the plant whence roots usually proceed. 



It is observed of radicles that they have the faculty 

 of extending themselves towards humid heat, or to 

 their food, whether that be simple water or nutritious 

 gas. Is this a spontaneous and inherent power by 

 which their organic action protrudes the fibres out- 

 ward, or are these sensitive bodies attracted by the 

 qualities to which they trend ? This phenemenon 

 can only be attributed to the universal law of attrac- 

 tion, as mentioned at page 22. 



The incidents which show this sensitive inclination 

 of roots, are the following. If a tree be planted on 

 the bank of a river, or on a spot of inferior soil near 

 a bed of superior quality, a majority of the roots in 



