364 CONSERVATIVE ORGANS. [LECT. VII. 



the discoveries of the immortal Harvey demon- 

 strated them to be blood-vessels. The opinion 

 that they contain air only, is still maintained by 

 Mr. Knight, who supposes them to be appendages 

 to another set of vessels, which he denominates 

 central, and through which he supposes the sap 

 ascends as soon as it arrives at the bud or suc- 

 culent shoot, " aided by the spiral vessels *." 

 I have however demonstrated to you that the 

 spiral vessels are unaccompanied by any others, 

 at least in all those trees which I have examined, 

 and are even at some distance from the alburnous 

 vessels in the annual shoot of many plants. 

 Mr. Knight, it is true, in another essay-}-, en- 

 deavours to explain the meaning of his term cen- 

 tral Vessels, by saying that they are the same 

 as the tubular tissue of M. Mirbel ; but, as the 

 spiral vessels form part of Mirbel's tubular tissue, 

 which besides comprehends all the other species 

 of vegetable vessels, this attempt at explanation 

 only renders the subject more confused. Mr. 

 Keith, in criticising Mr. Knight's explanation, has 

 fallen into a curious mistake regarding Mirbel's 

 opinion of the vessels which carry the sap. He 

 says, " If we regard their (the central vessels) re- 

 " spective functions, they can correspond only to 

 " the small tubes, as it is by them alone, accord- 



* Phil. Trans. 1801. f Ibid. 1807. 



