ON THE FORMATION OF THE BARK OF TREES. 139 



noticed in a former memoir*, which renders it better calculated to 

 transmit the sap towards the roots than in any other direction. 



I have in very few instances been able to make the walnut-tree repro- 

 duce its bark from the alburnum, though under the same management I 

 rarely failed to succeed with the sycamore and apple-tree. Pieces of the 

 bark of the apple-tree will also live, and generate a small portion of 

 alburnum, though only attached to the tree at their lower extremities ; 

 probably owing to a small part of the true sap being carried upwards by 

 capillary attraction, when the proper action of the cortical vessels is 

 necessarily suspended. 



The preceding experiments, and the authority of Duhamel, having per- 

 fectly satisfied me, that both the alburnum and bark of trees are capable 

 of generating a new bark, or at least of transmitting a fluid capable of 

 generating a cellular substance, to which the bark in its more perfectly 

 organised 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 alburnumf. The intricate 

 mixture of the cellular and vascular substances long baffled my endeavours 

 to discover from which of them, in the preceding cases, the sap, and con- 

 sequently the new bark, proceeded ; but I was ultimately successful. 



The cellular substance, both in the alburnum and bark 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 from the cellular substance, both of the 

 bark and alburnum ; and on the surface of these substances alone, 

 in many instances, the new bark was reproduced in small detached 

 pieces. 



I have endeavoured to prove in former communications*, that the true 

 sap of trees acquires those properties which distinguish it from the fluid 

 recently absorbed, by circulating through the leaf; and that it descends 



* See above, No. IV. f See above, No. V. p. 118. 



t See above, Nos. II. V. and VII. 



