38 On the Formation of the Bark of Trsts, 



upwards by capillary attraction, when the proper action of 

 the cortical vessels is necessarily suspended. 



The preceding experimeuts, and the authority of Du 

 Hamel, having perfectly satisfied nie, that both the albur- 

 num and bark of trees are capable of generating a new bark, 

 or at least of transmitting a fiuid capable of generating a cel- 

 lular substance, to which the bark in its more perfectly or- 

 ganised state owes its existence, my attention was directed 

 to discover the sources from which this fluid is derived. 

 Both the bark and the alburnum of trees are composed 

 principally of two substances ; one of which consists of long 

 tubes, and the other is cellular ; and the cellular substance 

 of the bark is in contact with the similar substance in the 

 alburnum, and through these I have long suspected the true 

 sap to pass from the vessels of the bark to those of the albur- 

 num*. The intricate mixture of the cellular and vascular 

 substances long baffled ray endeavours to discover from 

 which of them, in the preceding cases, the sap, and conse- 

 quently the new bark, proceeded ; but I was ultimately suc- 

 cessful. 



The cellular substance, both in the alburnum and hark of 

 old pollard oaks, often exists in masses of near a line in 

 •width, and this organisation was peculiarly favourable to my 

 purpose. I therefore repeated on the trunks of trees of this 

 kind, experiments similar to those above mentioned which 

 were made on the walnut-tree. 



Apparently owing to the small quantity of sap, which the 

 old pollard trees contained, their bark was very imperfectly 

 reproduced ; but I observed a fluid to ooze fr6m the cellular 

 substance, both of the bark and alburnum ; and on the sur- 

 face of these substances alone, in many instances, the new 

 bark was reproduced in small detached pieces. 



I have endeavoured to prove in former communicatipnsf, 

 that the true sap of trees acquires those properties which di- 

 stinguish it from the fluid recently absorbed, by circulating 

 through the leaf; and that it descends down the bark, where 

 part of it is employed in generating the new substance^ 



* Phil. Trans. 1805, pa.^e H. 

 t Id. for 1801, 1805, and 180G. 



annually 



