150 ON T THE ORIGIN AND OFFICE 



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 

 supposition by no means admissible. Yet it is, I conceive, evident 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 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, 

 however, 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. Co- 

 loured infusions 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 there- 

 fore, should it be proved that such infusions do not penetrate the cellular 

 substance of the alburnum, the evidence which this circumstance would 

 afford would be very defective. 



Amongst other experiments that I made to ascertain whether 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 summer, from trees of different species ; and 

 I effectually closed the alburnous tubes with a composition formed of 

 calcined oyster shells and cheeset ; and this was covered with a mixture 

 of bees-wax and turpentine, so as to effectually exclude all moisture. 

 A part of the bark was taken off each branch, in a circle round it, a 

 few lines distant from its lower end, where the tubes had been closed ; 

 and each branch was then placed in a decoction of logwood, in a vessel 



* See above, p. 139. 



f I have found this composition, and this only, to be capable of instantaneously stopping the 

 effusion of sap from the vine, or other tree, in the bleeding season. 



