316 Air. Knight on the Origin and Office of 
most in the summer, when their leaves have attained their 
full growth, and of course that much sap must ascend at 
this period; yet at this period the tubes of the alburnum 
appear dry, and to contain air only ; which induced Grew to 
suppose that the sap rose in the state of vapour ; a supposi- 
tion by no means admissible. Yet it is, I conceive, evident 
that the sap can not rise, as a liquid, through dry tubes, nor 
in any state through intersected tubes ; and therefore it ap- 
pears probable that it does not rise at all through the tubes of 
the alburnum, and that those tubes are intended to execute 
a different office. 
If the sap do not rise through the tubes of the alburnum, 
it must rise through the cellular substance ; yet the passage 
of any fluid through this has been denied by almost every 
naturalist, probably because coloured infusions have not been 
observed to penetrate it, and because many naturalists have 
considered it as mere compressed medulla. Mirbel, how- 
ever, contends that the fluid which generates the new bark 
exudes from it ; and although a fluid, capable of producing 
the same effects, exudes from the bark, when detached from 
the alburnum, I am much disposed to coincide with him in 
opinion, having observed a new bark to be generated on the 
surface of the cellular substance of pollard oaks, in detached 
spaces.* And if the sap in sufficient quantity to generate a 
new bark can pass through the cellular substance of an oak, 
it appears possible at least that the whole of the sap may ascend 
through it. Coloured infusions do not, I think, in any degree, 
pass through the bark of trees, yet it is evident that the 
^ap passes readily through it ; and therefore, should it be 
* Phil. Trans. 1807, p. 7, 
