io 6 Mr. Knight on the Formation 
broad, and four long, having been detached from contact with 
the alburnum, except at their upper ends, and covered with a 
plaister composed of bees-wax and turpentine, in some in- 
stances, and with clay only in others, readily generated the 
cellular substance of a new bark ; and between that and the 
old detached bark, very nearly as much alburnum was depo- 
sited as in other parts of the tree, where the bark retained its 
natural position ; which, I think, affords very decisive evi- 
dence of the descent of the sap through the bark. Similar 
pieces of bark, under the same mode of treatment, but united 
to the tree at their lower ends only, did not long remain 
alive, except at their lower extremities ; and there a very little 
alburnum only was generated. Other pieces of bark of the 
same dimensions, which were laterally united to the tree, 
continued alive almost to their extremities ; and a considera- 
ble portion of alburnum was generated, particularly near 
their lower edges ; the sap appearing in its passage across the 
bark to have been given a considerable inclination down- 
wards : probably owing to an arrangement in the organiza- 
tion of the bark, that I have noticed in a former memoir,* 
which renders it better calculated to transmit the sap towards 
the roots than in any other direction. -f 
I have in very few instances been able to make the walnut- 
tree reproduce its bark from the alburnum, though under the 
same management I rarely failed to succeed with the syca- 
more and apple-tree. Pieces of the bark of the apple-tree 
will also live, and generate a small portion of alburnum, 
*hough only attached to the tree at their lower extremities; 
* Philosophical Transactions of 1804. 
