Nature, Longevity^ and Size of Trees, 21 



change. Again, if the buds be cut off from a branch prior to 

 the commencement of the annual process of vegetation, no 

 sap will pass into the branch during the entire spring and 

 summer, although the other branches, not thus mutilated, 

 will be filled with it. Once more, if, at a later period in the 

 season, the leaves be stripped from off a branch, the flow of 

 sap through it will speedily, if not immediately, cease. 



What may be the nature of the agency thus exerted in the 

 growing buds and leaves which causes the sap to circulate, 

 and regulates the quantity of it passing through the old 

 stems and roots, it is not easy to say, and for our present 

 purpose unnecessary to inquire. But the old stems and roots 

 may be no farther concerned in it, than as being the channels 

 through which the nourishing fluid passes upwards from the 

 soil. And no facts yet known to physiologists demonstrate 

 that they have any other share in it. 



' But, in the course of the season, there is a descending, as 

 well as an upward, movement of the sap. And the former 

 must be regarded as being equally of a vital nature as the 

 igitter, and equally due to vital agency. Does not that move- 

 ment, at least, argue vitality and vital action in the old stems 

 and roots % I apprehend not ; and for this reason, that while 

 the ascending current seems referable to the processes going 

 on in the buds and leaves, the movement in question appears 

 to be connected with the formation of the woody layer all 

 over the exterior of the tree, and referable to the process by 

 which that structure is evolved. Whether the woody layer 

 be of the nature supposed by M. Thouars, or of that insisted 

 by other physiologists, is immaterial. It is distinct from the 

 woody layer of previous years, and is of the same year's for- 

 mation with the existing leaves and flowers. And it requires 

 for its organization equally as these do for theirs, a supply 

 of prepared or elaborated sap. But the sap is elaborated 

 only in the leaves ; and as the woody layer extends from the 

 base of these downwards to the extremities of, and even be- 

 yond, the roots of last year, so that sap can only be supplied 

 from above, and must descend, in order to the formation of the 

 tissue in question. This descent, however, may be solely con- 

 nected with that formation, and there is no proof that it has 



