314 Mr. Knight on the Origin and Office of 
thus deposited, and shall proceed to enquire into the origin and 
office of the alburnous tubes. 
The position and direction of these tubes have induced almost 
all naturalists to consider them as the passages through which 
the sap ascends ; and at their first formation, when the sub- 
stance which surrounds them is still soft and succulent, they are 
always filled with the fluid, which has apparently secreted from 
the bark. They appear to be formed in the soft cellular mass, 
which becomes the future alburnum, 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 sur- 
rounding 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. 
Many different hypothesis 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 
adequate to the effects produced. Saussure, jun. 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 have 
suggested that the expansion and contraction of the com- 
pressed cellular, or laminated substance (the tissue cellulaire 
of Duhamel andMiRBEL) which expands and contracts with 
change of temperature * after the tree lias ceased to live, might 
produce similar effects by occasioning nearly a similar motion 
and compression of the tubes, the coats of which are, I believe, 
universally admitted not to be membranous. But both these 
* Phil. Tians. 1801, p. 345. 
