£44< FORMATION' OF THE BARK OF TREKS. 



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 

 considerable portion of alburnum was generated, particularly 

 near their lower edges; the st p appearing in its passage 

 across the bark to have been given a considerable inclination 

 downwards: probably owing to an arrangement in the or- 

 ganization of the bark, that I have noticed in a former me- 

 moir*, which renders it better calculated to transmit the sap 

 towards the roots than in any other direction. 

 Bark repro- I have in very few instances been able to make the walnut- 



alburaunTof 16 trce re P ro ^ uce its bark from the alburnum, though under the 

 the sycamore same management I rarely failed to succeed with the syca- 

 app e. more and apple tree. Pieces of the bark of the apple-tree 

 will also live, and generate a small portion of alburnum, 

 though only attached to the tree at their lower extremities; 

 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 Ha- 

 mel, having perfectly satisfied me, that both the alburnum 

 and the 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 bark and Both the bark and the alburnum of trees are composed 

 alburnum con- . . .. - -:. , c , j , . ■ c , 



sist of tubes and principally of two substances; one ol which consists of long 



celluiarsub* tubes, and the other is cellular; and the cellular substance 



of the bark is in contact with the similar substance in the 



alburnum, and through these I have long suspected the true 



sap to pass Irom the vessels of the bark to those of the albur- 



numf. The intricate mixture of the cellular and vascular 



substances long baffled my endeavours to discover from which 



* Philosophical Transactions of 1804, or Philosophical Journal, vol. 

 X, p. 289. 



f Phil. Trans. 1805, p. 14, or Philos. Journal, vol. XIII, p. Cb2. 



of 



