207 
of the Bark of Trees . 
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 Du 
Hamel, having perfectly 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 organized 
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 con- 
tact 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 alburnum.* The intricate 
mixture of the cellular and vascular substances long baffled 
my endeavours to discover from which of them, in the pre- 
ceding cases, the sap, and consequently the new bark, pro- 
ceeded ; 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 organization 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 ouze from the cellular 
• Phil. Trans. 1805, page 14. 
P 2 
