350 CONSERVATIVE ORGANS. [LECT. VII. 



points which have been overlooked by Mr. Knight, 

 and to which it is necessary to direct your at- 

 tention. 



In explaining his theory, Mr. Knight seems to 

 .imply that the whole of the change which the sap 

 undergoes is effected in the leaf, and that the 

 liber is the mere medium of transmission of the 

 alburnous matter. But if, as we must admif is 

 most probable, the whole of the secretions of the 

 plant are produced from the same proper juice 

 elaborated from the sap, by its exposure to light 

 and air in the leaf; and consider the great diver- 

 sity of these ; it is more likely that some altera- 

 tion takes place in the bark, previously to the 

 cambium being thrown out by its vessels ; and, con- 

 sequently, we must admit the force of Mr. Keith's 

 suggestion, that it is " possible the proper juice 

 " may receive its final degree of modification in 

 " the bark itself*." It is by such an admission 

 only that we can satisfactorily explain the fact 

 observed by Mr. Knight in one of his experiments, 

 that a small quantity of wood was generated even 

 at the lower lip of an insulated portion of bark, on 

 which there was neither bud nor leaf -f~. It is also 

 necessary for the sake of those unacquainted with 

 physiological reasonings, to remark that it is not 



* System of physiological Botany -, vol.ii. p. 229. 

 f Phil Trans. 1803. 



