CR. JI. 



COURSE OF THE SAP. 



65 



selves into the latest layers of bark, if the stem of a 

 living oak-tree is cut across ; at least, there is a white 

 line across these layers of bark, opposite the end of 

 each medullary ray. 



Some physiologists have supposed a lateral com- Lateral as 



well as 



mumcation of the sap between each and all the annual longitudi- 



nal flow of 



layers of wood by means of these medullary rays, or 

 silver grain. But to show what guess-work vegetable ™'^* 

 physiology consists of, others suppose that these rays 

 are merely conductors of atmospheric air between the 

 bark and the pith. That there is a lateral transmission 

 of sap throughout the wood by some means or other, I 

 think may be argued from the existence of ringed 

 branches. Indeed, were it not for this lateral commu- 

 nication, whether branches were ringed or not, their 

 buds, leaves, fruits, and shoots could only be supplied 

 with upward sap from the last year's growth of wood 

 on which they are placed ; and the upward sap of 

 every annual growth of wood, except the last, would 

 be confined within the limits of the cone formed by 

 each annual growth. 



I have a bough of a pear-tree which I ringed for 

 the space of an inch in June 1832, and which I cut off 

 the tree in December 1843. It bore fruit for the last 

 ten of these eleven years, though the rest of the tree 

 had never borne fruit up to the last-named year. The 

 branch was alive when I cut it off. The woody part 

 above the ring is, owing to its annual growth in girth- 

 ing, double the size of the ringed part. 



