of the allurnous Vessels of Trees. 128 
on each stool, I destroyed the bark between the incisions, 
and thus cut off the communication between the jeaves and 
the lower parts of the stem and roots, through the bark, 
Much growth, as usual, took place above the space from 
which the bark had been taken off, and very little below it. 
Examining the state of the experiment in the succeeding 
winter, I found it had not suceeeded according to my hopes; 
for a portion of the alburnum, in almost every instance, was 
lifeless, and almost dry, to a considerable distance below the 
space from which the bark had béen removed. Jn one in- 
stance the whole of it was, hawever, perfectly alive; and in 
this I found the specific gravity of the wood above the de- 
corticated space to be 1114, and below it 11113 and the 
wood of the unmutilated pole at the same distance from the 
ground to be 1112, each being weighed as soon as it was 
detached from the root. E 
-Had the true sap in this instance wholly stagnated above 
the decorticated space, the specific gravity of the wood there 
ought to have been, according to the result of former expe- . 
riments *, comparatively much greater: but I do not wish 
to draw any conclusion from a single experiment; and in- 
deed I see very considerable difficulty in obtaining any very 
satisfactory or decisive facts from any experiments on plants 
in this case, in which the same roots and stems collect and 
convey the sap during the spring and summer, and retain 
within themselves that which is, during the autumn and 
winter, reserved to form new organs of assimilation in the 
succeeding spring. In the tuberous-rooted plants, the roots 
and stems which collect and convey the sap in one season, 
and those in which it is deposited and reserved for the suc- 
eeeding season, are perfectly distinct organs; and from one 
of these, the potatoe, I obtained more interesting and de+ 
cisive results, 
My principal object was, to prove that a*fluid descend 
from the leaves and stem to form the tuberous roots of this 
plant; and that this fluid will, in part, escape down the al- 
burnous substance of the stem when the continuity of the 
* Phil, Trans. for 1805. 
cortical 
