ON THE FORMATION OF THE BARK OF TREES. 143 



evidently passes through the central vessels*, which surround the 

 medulla. In both cases a cellular substance, similar to that which was 

 generated in the preceding experiments, is first formed, and this cellular 

 substance in the same manner subsequently becomes vascular ; whence it 

 appears, that the true sap, or blood of the plant, produces similar effects, 

 and passes through similar stages of organisation, when it flows from 

 different sources, and that the power of generating a new bark, properly 

 speaking, belongs neither to the bark nor alburnum, but to a fluid which 

 pervades alike the vessels of both. 



I shall, therefore, not attempt to decide on the merits of the theory 01 

 Malpighi, or of Hales, respecting the reproduction of the interior bark ; 

 but I cannot by any means admit the hypothesis of Malpighi and other 

 naturalists, relative to the transmutation of bark into alburnum ; and I 

 propose, in my next communication, to state my reasons for rejecting 

 that hypothesis. 



X. ON THE INCONVERTIBILITY OF BARK INTO ALBURNUM. 



[Read before the ROYAL SOCIETY, Feb. 4, 1808.] 



IN a letter which I had the honour to address to you in the end of the 

 last year^f", I endeavoured to prove that the matter which composes the 

 bark of trees previously exists in the cells both of their bark and albur- 

 num, in a fluid state, and that this fluid, even when extravasated, is 

 capable of changing into a pulpous and cellular, and ultimately a vascular 

 substance ; the direction taken by the vessels being apparently dependent 

 on the course which the descending fluid sap is made to takej. The 

 object of the present memoir is to prove, that the bark thus formed 

 always remains in the state of bark, and that no part of it is ever trans- 

 muted into alburnum, as many very eminent naturali&ts have believed. 



Having procured, by grafting, several trees of a variety of the apple 

 and crab tree, the woods of which were distinguishable from each other by 



* See above, No. V. Mirbel has called the tubes, which I call the central vessels, the 

 " tissu tubulaire" of the medulla. 



t See the preceding paper. 



I I had observed this circumstance in many successive seasons ; but I was not by any means 

 prepared to believe that such an arrangement could take place in the coagulum afforded by an 

 extravasated fluid ; and I am indebted to Mr. Carlisle for having pointed out to me many circum- 

 stances in the motion and powers of the blood of animals, which induced me to give credit to 

 the accuracy of my observations ; and to that gentleman, and to Mr. Home, I have also subse- 

 quently to acknowledge many obligations. 



