on the Descent of the Sap in Trees . 287 
the bark, possibly by discharging some of its component parts 
through the pores described and figured by Malpighi. 
I also suspected, since my former Paper was written, that the 
young bark, in common with the leaf, possessed a power, in 
proportion to the surface it exposes to the air and light, of pre- 
paring the sap to generate new wood ; for I found that a very 
minute quantity of wood was deposited by the bark, where it 
had not any apparent connection with the leaves. Having made 
two incisions through the bark round annual shoots of the 
apple-tree, I entirely removed the bark between the incisions, 
and I repeated the same operation at a little distance below, 
leaving a small portion of bark unconnected with that above 
and beneath it. By this bark, a very minute quantity of wood, 
in many instances, appearedto be generated, at its lower extre- 
mity. The buds in the insulated bark were sometimes suffered 
to remain, and in other instances were taken away ; but these, 
unless they vegetated, did not at all affect the result of the ex- 
periment. I could therefore account for the formation of wood, 
in this case, only by supposing the bark to possess in some 
degree, in common with the leaf, the power to produce the 
necessary changes in the descending sap ; or that some matter 
originally derived from the leaves, was previously deposited in 
the bark : or that a portion of sap had passed the narrow space 
above, from which the bark had been removed, through the 
wood. Repeating the experiment, I left a much greater length 
of bark between the intersections ; but no more wood than in 
the former instances was generated. I therefore concluded, that 
a small quantity of sap must have found its way through the 
wood, from the leaves above ; and I found, that when the upper 
incisions were made at ten or twelve lines distance, instead of 
P p 2 
