56* COURSE OF THE SAP. 



that gives us any adequate or satisfactory no- 

 tion. In those organs the sap is exposed to 

 the action of light, air and moisture, three 

 powerful agents, by which it is enabled to 

 form various secretions, at the same time that 

 much superfluous matter passes off by per- 

 spiration. These secretions not only give pe- 

 culiar flavours and qualities to the leaf itself, 

 but are returned by another set of vessels, as 

 Mr. Knight has demonstrated, into the new 

 layer of bark, which they nourish and bring 

 to perfection, and which they enable in its 

 turn to secrete matter for a new layer of al- 

 burnum the ensuing year. It is presumed 

 that one set of the returning vessels of trees 

 may probably be more particularly destined 

 to this latter office, and another to the secre- 

 tion of peculiar fluids in the bark. See Phil, 

 Trans, for 1801, p. 337- In the bark princi- 

 pally, if I mistake not, the peculiar secretions 

 of the plant are perfected, as gum, resin, &c, 

 each undoubtedly in an appropriate set of 

 vessels. From what has just been said of the 

 office of leaves, we readily perceive why all 

 the part of a branch above a leaf or leaf-bud 

 dies when cut, as each portion receives nou- 



