138 ON THE FORMATION OF THE BARK OF TREES. 



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 from contact with 

 the alburnum, provided they remain united to the tree at their upper 

 ends, much more readily generate a new bark, than the alburnum does 

 under similar circumstances : 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 

 organised ; and the vessels which contain the true sap in the bark, are 

 still wanting ; and I have found, that these may be made, by appropriate 

 management, to traverse the new cellular substance in almost any direc- 

 tion. When I cut off all communication above, and on one side, between 

 the old bark and that substance, I observed 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 

 remained united to the bark at its upper end only, the vessels descended 

 nearly perpendicularly down it ; but they did not readily ascend into it, 

 when it was connected with the bark at its lower extremity only ; the result 

 of similar experiments, when made on different species of trees, was, how- 

 ever, subject to some variations. 



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

 four long, having been detached from contact 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 

 generated 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 retained its natural position ; 

 which, I think, affords very decisive evidence of the descent of the sap 

 through the bark. Similar pieces of bark, under the same mode of treat- 

 ment, 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 extre- 

 mities ; and a considerable portion of alburnum was generated, particu- 

 larly near their lower edges ; the sap appearing in its passage across the 

 bark to have been given a considerable inclination downwards : probably 

 owing to an arrangement in the organisation of the bark, that I have 



