220 PROCESS OF DEVELOPEMENT. CHAP. IV. 



the bark and tin-foil, but none between the tin-foil 



and the interior layer. This experiment was com- 



pletely decisive of the point in question ; and yet 



there is an experiment of Dr. Hope's on the same 



subject, which is, if possible, still more convincing : 



Having made a longitudinal incision in the trunk of 



a Willow of three or four years old so as to penetrate 



through the bark, he laid bare a portion of the stem 



by slipping the bark to the one side, which was, 



however, still attached to the stem at the upper and 



lower extremities of the decorticated part ; the de- 



tached portion of bark was then bent into the form 



of a hollow cylinder, by uniting its edges as closely 



as possible, and the whole well secured from the 



action of the atmosphere. 3 * The plant was then 



allowed to remain undisturbed for several years, 



when the result of dissection was, that new layers 



of wood were generated within the lateral cylinder 



of bark, while the decorticated portion of the stem re- 



mained unaugmented; the portions above and below 



being augmented, as in other ordinary cases of vege- 



tation. It is evident, therefore, that the additional 



layer by which the plant increases in diameter is 



generated from the bark. 



Not by But it was not yet accurately ascertained whether 

 version'of tne new ly formed layer of wood was merely in- 



Tnto ^ uratec * liber, as was the opinion of Malpighi; or a 

 wood, production formed from it, as was the opinion of 

 Grew. This Du Hamel thought he might ascertain 



* Smith's Introduction, p. 35. 



