1039 



office of these being only to convey the fluid elaborated in the leaves 

 to the ducts of the wood), this apparatus of cellules may be inferred to 

 perform some function connected with the especial organization of 

 new wood and bark, as yet unknown. 



" 46. Perhaps the spiral fibre of the vessels of leaves, besides tend- 

 ing to prevent the dissipation of their fluid by evaporation — and thus 

 keeping up a greater degree of distension than would be required in 

 the ordinary cells, — may confer also upon these vessels a resiliency and 

 elasticity advantageous in regulating the pressure upon their contents, 

 and the passage of the fluid along them. 



"47. That the vessels of the leaves do elaborate a fluid, and con- 

 duct it from thence into the branches, stem, and roots, for the pro- 

 duction of new wood and new bark, is proved by the fact of these 

 structures completely ceasing to be formed, where the descent of the 

 elaborated fluid is prevented by ligature, whilst they are produced in 

 the ordinary quantity in the stem above the constriction ; for, if the 

 intercellular tissue, or the ordinary cells, and not the vessels, had been 

 the elaboratories and conductors of the descending sap, these two 

 structures, the intercellular tissue, and the cells situated above the 

 ligature being so much, and so closely connected with the same 

 structures below it, would still have allowed of the descent of a por- 

 tion of the elaborated sap, as one of them, the intercellular tissue, al- 

 lows the free ascent of the crude sap, and the growth of the stem 

 would not have been so completely arrested below the wire whilst it 

 was going on uninterruptedly above it. The layer of wood and bark 

 of the sucker, which had been cut at the end of the first year, being 

 much thinner below than above the ligature, was the consequence of 

 the constriction of the newly formed vessels at the girt part, by the 

 pressure of the gradually increasing layer of new wood on one side, and 

 the wire on the other. And the circumstance of there being, in the 

 other sucker, no addition whatever made to the wood and bark below 

 the ligature during the second year of its application, proceeded from 

 the entire closure of these vessels by the continued increase of this 

 pressure, so that now no sap could descend below the constricted 

 part. And the fact observable in the same sucker, of the sap elabo- 

 rated in the leaves of 1846 not being permitted to descend below the 

 ligature applied in 1844, proves that there are no collateral or anas- 

 tomosing channels between the layers of wood of separate years, ca- 

 pable of conducting the descending sap, although there is no inter- 

 ruption whatever to the passage and lateral diffiision of the ascending 

 sap. It shows also that the descent and the ascent of the sap must 



