31 B Mr. Knight on the Origin and Office of 
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 shew that the presence of the sap in 
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 prepare 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 dis- 
tance of eight or ten feet from the ground, not the least mois- 
ture flowed ; and the tubes of the alburnum appeared to con- 
tain air only. I also observed that the sap flowed as abun- 
dantly 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 shewn 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 con- 
