the Ascent of the Sap in Trees. $51 
stating a few of the conclusions that I have ventured to draw 
from the foregoing, and many similar experiments. As I have not 
been able to find the spiral tubes any where, except immediately 
surrounding the medulla in different parts, in the seed, and in 
the leaf, and as they every where terminate at short distances, I 
conclude that the sap is not raised by their agency ; nor by the 
central vessels, to which they are appendages ; for these extend 
no greater length downwards than the spiral tubes, and termi- 
nate with them, at the external surface of that annual layer of 
wood to which they belong ; and they have not any apparent 
communication with the similar vessels of the succeeding year. 
In the lower parts of hollow trees, they must long have ceased 
to exist at all ; and, in all trees, except very young ones, they 
are (as it were) ossified within the heart-wood; and those in the 
annual shoots and buds are often a hundred and fifty feet distant 
from the roots, from which they are supposed to raise the sap. 
The common tubes of the alburnum, (which do not appear to 
me to have been properly distinguished from the central vessels, 
by the authors that I have read,) extend from the points of the 
annual shoots to the extremities of the roots; and up these 
tubes the sap most certainly ascends, impelled, I believe, by the 
agency of the silver grain. At the base of the buds, and in the 
soft and succulent part of the annual shoot, the alburnum, with 
the silver grain, ceases to act, and to exist ; and here, I believe, 
commences the action of the central vessels, with their appen- 
dages, the spiral tubes. By these, the sap is carried into the 
leaves, and exposed to the air and light ; and here it seems 
to acquire (by what means I shall not attempt to decide) the 
power to generate the various inflammable substances that are 
