329 ‘ On the inverted Action 
existed previously in their cotyledons; and as the radicles 
increase only in length by parts successively added to their 
apices, or points most distant from their cotyledons, it fol- 
lows of necessity that the first motion of the true sap, at this 
period, is downwards. And as no alburnous tubes exist im 
the radicles of germinating seeds during the earlier periods 
of their growth, the sap in its descent must either pass 
through the bark or the medulla. But the medulla does not 
apparently contain any vessels calculated to carry the de- 
scending sap, whilst the cortical vessels are during this pe~ 
riod much distended and full of moisture; and as the me- 
dulja certainly does not carry any fluid in stems or branches 
of nore than one year old, it can scarcely be suspected that 
it, at any period, conveys the whole current of the descend- 
ing sap, 
As the leaves grow, and enter on their office, cortical] 
vessels, imevery respect apparently similar to those which 
descended from the cotyledons, are found to descend from 
the bases of the leaves; and there appears no reason, with 
which I am acquainted, to suspect that both do not carry 
a similar fluid, and that thé course of this fluid is, in thé 
fitst instance, always towards the roots. : 
The ascending sap, on the contrary, rises wholly through 
the alburnum, and central vessels; for the destruction of 4 
portion of the bark, in a circle round the tree, does not im- 
tuediately, in the slightest degree, check the growth of its 
leaves and branches; but the alburnous vessels appear, from 
the-experiments I have related in a former paper *, and from 
those I shall now proceed to relate, to be also capable of an 
inverted action, when that becomics necessary to preserve the 
existence of the plant. | 
Ags soon as the leaves of the oak were nearly full grown 
in the last spring, TI selected in several instances two poles 
of the same age, and springings from the same roots in a 
coppice, whicli had been felled about six years preceding ; 
and making two circular incisions at the distance of three 
inches from each other through the bark of onc of the poles 
* Phil. Trans, for 1804, 
on 
hg ee 
sc oh) 
