

FORMATION OF THE BARK OF TREES. $&3 



matter, which entere into the composition of this cellular 



substance, is evidently derived from the alburnum. 



These facts are therefore extremely favourable to the 



theory of Hales ; but other facts may be adduced, which are 



scarcely consistent with that theory. 



The internal surface of pieces of bark, when detached Internal sar- 

 • , , i. • i i l l i face of the bark 



from contact with the alburnum, provided they remain united i tS elf generates 



to the tree at their upper ends, much more readily generate it more readily, 

 a new bark, than the alburnum does under similar circum- 

 stances: a similar fluid exudes from the surfaces of both, 

 and the same phenomena are observable in both cases. The 

 Cellular substance, however, which is thus generated, though 

 it presents every external appearance of a perfect bark, is 

 internally very imperfectly organized; and the vessels which 

 contain the true sap in the bark are still wanting; md I Course of the 

 have found, that these may be made, by appropriate manage- "arTabhT* * 

 ment, to traverse the new cellular substance in almost any 

 direction. When I cut off all communication above, and 

 on one side, between the old bark and that substance, I ob- 

 served, that the vessels proceeded across it, from the old bark 

 on the other side, taking always in a greater or less degree an 

 inclination downwards; and when the cellular substance re- 

 mained united to the bark at its upper end only, the vessels 

 descended nearly perpendicularly down it; but they did not 

 readily ascend into it, wtten k was connected with the bark at its 

 lower extremity only; the result of similar experiments, when 

 made on different species of trees, was, however, subject to 

 some variations. 



Pieces of bark of the walnut-tree, which were two inches Experimentoti 

 broad, and four long, having been detached from contact the walnut * 

 with the alburnum, except at their upper ends, and covered 

 with a plaster composed of bees-wax and turpentine, in 

 some instances, and with clay only in others, readily gene- 

 rated the cellular substance of a new bark; and between that 

 and the old detached bark, very nearly as much alburnum 

 was deposited as in other parts of the tree, where the bark re- 

 tained its natural position; which. I think, affords very deci- 

 sive evidence of the descent of the sap through the bark. The ?ap de . 



Similar pieces of bark, under the same mode of treatment, scendsthrough 



r» r* t. ^ the bark. 



R2 but 



