OP THE ALBURNUM OF TREES. 151 



deep enough to cover the decorticated spaces. At the end of twenty 

 hours, or somewhat longer periods, these branches were examined, and 

 the coloured infusion was found to have insinuated itself between the 

 alburnous tubes, in many instances apparently through the cellular sub- 

 stance. This was most obvious in the walnut-tree, the young wood of 

 which is very white. The principal object 1 had in view in making this 

 experiment, was to detect the passages through which I conceived the 

 sap to pass from the bark into the alburnum*. 



From the preceding circumstances, I am disposed to infer that the sap 

 secretes through the cellular substance of the alburnum ; and through 

 this I conceived that it must ascend when the tubes were intersected in 

 the preceding experiments, and in those seasons of the year when the 

 alburnous tubes are empty, though the sap must be rising with great 

 rapidity : and I shall endeavour to show that the presence of the sap m 

 the alburnous tubes, during that part of the year in which trees, when 

 wounded, bleed abundantly, does not afford any decisive evidence of the 

 ascent of the sap through those tubes. 



In the last spring, when the buds of the sycamore first began to pre- 

 pare for unfolding, I found that the sap abounded in the points at the 

 annual branches ; and at the same time it flowed abundantly from 

 incisions made into the alburnum near the root. But when similar 

 incisions were made at the distance of eight or ten feet from the ground, 

 not the least moisture flowed ; and the tubes of the alburnum appeared 

 to contain air only. 1 also observed that the sap flowed as abundantly 

 from the upper as from the under side of the lower incisions, if not more 

 abundantly, and so it continued to flow to the end of the bleeding 

 season. 



The sap must therefore have been, by some means, thrown into the 

 tubes above the incisions, for the quantity discharged from them exceeded 

 more than a hundred times that which the tubes could have contained at 

 the time the incisions were made, even had every tube been filled to the 

 extremity of the most distant branch. And, as it has been shown that 

 the sap can pass up when all the alburnous tubes are intersected, there 

 appears, I think, sufficient evidence that it must in this case have been 

 raised by some other agent than those tubes. 



Through the cellular substance I therefore venture to conclude that 

 the sap ascends ; and it is not, I think, difficult to conceive that this sub- 

 stance may give* the impulse with which the sap is known to ascend in 

 the spring. I have shown that the bark more readily transmits the 



* See above, p. 139. 



