302 On the Origin and Office of the Alburnum of Trees, 



full growth, and of course that much sap must ascend at 

 this period; yet at this period the tubes of the alburnum ap- 

 pear 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, evi-» 

 dent that the sap cannot rise, as a liquid, through dry tubes, 

 nor in any state through intersected tubes; and therefore it 

 appears probable that it does not rise at all through the tube§ 

 of the alburnum, and that those tubes are intended to exe- 

 cute 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, 

 however, contends that the fluid which generates the new 

 bark exudes from it ; and although a fluid, capable of pro- 

 ducing the same effects, exudes from the bark, when de- 

 tached from the alburnum, I am much disposed to coincide 

 with him in opinion, having observed a new bark to be ge- 

 nerated 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 cel- 

 lular substance of an oak, it appears possible at least thai 

 the whole of the sap may ascend through it. Coloured in- 

 fusions do not, I think, in any degree, pass through the 

 bark of trees, yet it is evident that the sap passes readily 

 through it ; and therefore, should it be proved that such 

 infusions do not penetrate the cellular substance of the al- 

 burnum, the evidence which this circumstance would afford 

 would be very defective. 



Amongst other experiments that I made to ascertain whe- 

 ther the cellular substance of the alburnum would imbibe 

 Coloured infusions, I took off branches of two years old with 

 the annual shoots and leaves attached to them, in the sum- 

 mer, from trees of different species ; and I effectually closed 



* Philosophical Transactions for 1807, p. 7. 



the 



