SECT. II* ASCENT OF THE SAP. MQ 



them do the central vessels correspond? If we 

 regard their respective functions they can corres- 

 pond only to the small tubes, as it is by them alone, 

 according to M. Mirbel, that the sap ascends. 



And yet after all the elucidation that has been Function 

 thrown on the subject, the function of the spiral a i tubST 

 tubes is as much involved in obscurity as ever. ^ 



Grew, who, together with Malpighi, regarded them a ? d Mal- 

 originally as being destined to the transmission of 

 air, is known to have retracted his opinion, or at 

 least to have very much modified it; so that, instead 

 of regarding them as being solely air-vessels, he 

 afterwards regarded them as being also sap-vessels, 

 and as being even the sole sap-vessels of the wood 

 or alburnum. But this opinion is evidently con- 

 tradicted by the fact that no trachese are to be found 

 in the wood or alburnum, except in the annual 

 shoot immediately surrounding the pith ; for they 

 are not generated in the succeeding and annual 

 layers by which the stem and trunk are augmented 

 in width, and are obliterated by age in the vicinity 

 even of the pith itself. It is impossible, therefore, 

 that they should be the channel of the sap's as- 

 cent through the wood or alburnum of an aged 

 trunk. 



And yet this opinion seems to have been adopted According 

 even by Dr. Smith, upon the authority as it appears Kni^hT,'"' 

 of Dr. Darwin and Mr. Knight, whom he re pre- andSmith< 

 sents as having proved in the most satisfactory 

 manner that the spiral vessels are the channel 



