

FUNCTION.] MODE OF LENGTHENING. 181 



while the functions of absorption go on through the spongioles, 

 which, being at the extremities of the roots, are placed 

 beyond the influence of the branches, and extend wherever 

 moisture is to be found. This property prevents a plant 

 from exhausting the earth in which it grows; for, as the 

 roots are always spreading further and further from the main 

 stem, they are continually entering new soil, the nutritious 

 properties of which are unexhausted. 



It is generally believed that roots increase only by their 

 extremities, and that, once formed, they never undergo any 

 subsequent elongation. This was first noticed by Du Hamel, 

 who passed fine silver threads through young roots at different 

 distances, marking on a glass vessel corresponding points 

 with some varnish : all the threads, except those that were 

 within two or three lines of the extremity, always continued 

 to answer to the dots of varnish on the glass vessel, although 

 the root itself increased considerably in length. Variations 

 in this experiment, which has also been repeated in another 

 way by Knight, produced the same result, and the whole 

 phenomenon appears to be one of those beautiful evidences of 

 design which are so common in the vegetable kingdom. If 

 plants growing in a medium of unequal resistance lengthened 

 by an extension of their whole surface, the nature of the 

 medium in which they grow would be in most cases such as 

 the mere force of their elongation would be unable to over- 

 come ; and the consequence would be, that they would have 

 a twisted, knotted, form, that would be unfavourable to 

 the rapid transmission of fluid, which is their peculiar office. 

 Lengthening, however, only at the extremities, and this by 

 the continual formation of new matter at their advancing 

 point, they insinuate themselves with the greatest facility 

 between the crevices of the soil ; once insinuated, the force of 

 horizontal expansion speedily enlarges the cavity ; and if they 

 encounter any obstacle which is absolutely insurmountable, 

 they simply stop, cease growing in that particular direction, 

 and follow the surface of the opposing matter, till they again 

 find themselves in a soft medium. 



It is curious, however, to remark that, although this pro- 

 perty of lengthening only by the ends of their roots seems 



