346 CONSERVATIVE ORGANS. [LKCT. V1L 



are the organs in which this matter is elaborated 

 from the sap ; or, in other words, that the albur- 

 num is generated from the cambium, which is 

 part of the proper juice of the plant, formed by 

 the exposure of the sap to the light and air in the 

 leaf, and returned from it by the vessels that pass 

 down from the leaf into the interior bark, by which 

 it is deposited. To determine this point, he re- 

 moved narrow circles of bark from shoots of Apple 

 trees, " leaving a leaf between the places where 

 " the bark was taken off; and on examining them 

 " frequently during the autumn," he found that 

 the diameter of the shoot between the insertion of 

 the leaf-stalk and the lower incision was as much 

 increased as in any other part of the tree ; but 

 when no leaf was left " on similar portions of in- 

 " sulated bark, on other branches of the same 

 " age, no apparent increase in the size of the wood 

 " was discoverable*." No other inference, than 

 that the leaf is the essential agent in producing 

 the increased diameter of the wood, could be 

 drawn from this experiment: and Mr. Knight 

 further found that where the deposition of the 

 proper juice returned from the leaf is greatest, 

 that is, at the points where the returning vessels 

 enter the inner bark, there the formation of al- 

 burnum is observed to take place in ridges cor- 



* Phil. Trans. 1801, P. I. p. 2, p. 335. 



