202 CONSERVATIVE ORGANS. [LECT. V. 



of their mode of growth ; for, as has been just 

 stated, a root, in extending, is not lengthened 

 through all its parts, but by additions made to the 

 extremity, and as these, in the early stages of its 

 growth, are made more rapidly than the alburnous 

 depositions, which add to its diametrical dimen- 

 sions, its shape must necessarily be tapering. 

 That the longitudinal extension of the root is ef- 

 fected by the deposition of new matter at its ex- 

 tremity, was first ascertained by Du Ham el *, who 

 having passed small pieces of silver thread trans- 

 versely through a vegetating root, at distances 

 which were accurately measured, found that the 

 upper threads, those nearest to the stem, or herb- 

 age, retained their original and relative situation, 

 whilst that one only which was very near to the 

 end of the root was carried down ; and this is the 

 case both in succulent and ligneous roots. The 

 lateral extension, on the contrary, in succulent 

 roots, appears to depend on the deposition of ad- 

 ditional particles, throughout the whole of their 

 substance ; and in perennial and ligneous roots, 

 by the formation of an annual new alburnum, in 

 the same manner, as I shall explain to you in its 

 proper place, occurs in the stems and branches of 

 trees and shrubs. When the extremity of a root 

 is destroyed, or when the root has attained the 



* Phys. des Arbres, liv. I. c, v. 



