246 PROCESS OF DEVELQPEMENT. CHAP. IV. 



SUBSECTION I. 



The Root. The Root. From the foregoing observations and 

 experiments it appears that the roots of plants, or at 

 least of woody plants, are augmented in their width by 

 the addition of an annual layer, and in their length 

 by the additon of an annual shoot, bursting from 

 the terminating fibre. But how is the developement 

 of the shoot effected ? Is it by the intro-susception of 

 additional particles throughout the whole of its ex- 

 tent ; or only by additions deposited at the ex- 

 tremity ? In order to ascertain the fact, with regard 

 Elongated to the elongation of the root, Du Hamel instituted 

 tremity 6 *" ^ e ^^ ow ^ n g experiment : Having passed several 

 onl y threads of silver transversely through the root of a 

 plant, and noted the distances, he then immersed 

 the root in water. The upper threads retained 

 always their relative and original situation, and the 

 lowest thread which was placed within a few lines 

 of the end was the only one that was carried down. 

 Hence he concluded that the root is elongated 

 merely by the extremity.* Mr. Knight who from 

 a similar experiment obtained the same result de- 

 duced from it also the same conclusion.-}- We may 

 regard it then as certain that the mode of the elon- 

 gation of the root is such as is here represented, 

 though in the progress of its developement it may 

 affect a variety of directions. 



* Phys. des Arb. liv. i. chap. v. | Phil. Trans. 180& 



