of the alhurnous Vessels of Trees. 297 
this I found the specific gravity of the wood above the decor- 
ticated space to be 1114, and below it 1111 ; 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. 
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 experi- 
ments,* comparatively much greater ; but I do not wish to 
draw any conclusion from a single experiment ; and indeed 
I see very considerable difficulty in obtaining any very satis- 
factory, 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- 
ceeding season, are perfectly distinct organs ; and from one 
of these, the potatoe, I obtained more interesting and decisive 
results. 
My principal object was to prove that a fluid descends 
from the leaves and stem to form the tuberous roots of this 
plant ; and that this fluid will in part escape down the albur- 
nous substance of the stem when the continuity of the cortical 
vessels is interrupted : but I had also another object in view. 
Every gardener knows that early varieties of the potatoe 
never afford, either blossoms or seeds ; and I attributed this 
* Phil. Trans, for 1805. 
O q 2 
rvj .1 
