28 ON THE ALBURNUM OF TREES. 



mim, as receptacles of this fluid, to which they may either 



afford a passage upwards, or simply retain it as reservoirs, 



till absorbed, and carried off, by the surrounding cellular 



substance. The former supposition is, at first view, the 



most probable ; but the latter is much more consistent with 



the circumstances, that I shall proceed to state. 



Ascent of the Many different hypotheses have been offered by naturalists 



***** to account for the force, with which the sap ascends in the 



spring; of these hypotheses two only appear in any degree 



Sausaure's hy- adequate to the effects produced. Saussurejun. supposes, 



** that the tubes contract as soon as they have received the sap 



in the root, and that this contraction, commencing in the 



root, proceeds upwards, impelling the sap before it: and I 



and one of Mr. have suggested, that the expansion and contraction of the 



Kn^ts, compressed cellular, or laminated substance (the tissu cel- 



lulaire of Duhamel and Mirbel) which expands and con-* 



tracts with change of temperature* after the tree has ceased 



to live, might produce similar effects, by occasioning nearly 



a similar motion and compression of the tubes, the coats of 



which are, 1 believe, universally admitted not to be mem- 



inconsistent branous. ' But both these hypotheses are inconsistent with 



with facts. the facts, that I have now the pleasure to communicate to 



you. 

 Tubes of an- Selecting parts of the stems of young trees, from which 

 conrtned u>\he annual branches had sprung in the preceding year, I ascer- 

 extemal an- tained by injecting coloured infusions into the stems, through 

 w^wdo^he the anrma l snoots, that the tubes, which descended from the 

 same side, latter, were, at their bases, confined to that side of the stem 

 from which they spring, and to the external annual layer of 

 Thecommunl- wood. Deep incisions were then made into the stems of 

 cation cut off other trees immediately beneath the bases of similar annual 

 some e S ■' shoots, by which I am quite confident, that all communica- 

 tion through the alburnous tubes, with the stem, was wholly 

 cut off: yet the sap passed into the annual shoots in the suc- 

 ceeding spring, all of which lived, and some grew with con- 

 d th t b siderable vigour. I, at the same time, selected many lateral 

 cut through in branches, about three lines in diameter, in a nursery of apple 

 ethers. trees, which I could easily secure to the stems of the adjoin- 



* ThiJ. Trans. *8o*, p. 345, 



nig 



