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Mr. Knight concerning the State in which 
when the roots were considerably elongated, alburnous tubes 
formed ; and as soon as they had acquired some degree of 
firmness in their consistence, they appeared to enter on their 
office of carrying up the aqueous sap, and the leaves of the 
plumula then, and not sooner, expanded. 
The leaf contains at least three kinds of tubes : the first is 
what, in a former Paper, I have called the central vessel, through 
which the aqueous sap appears to be carried, and through which 
coloured infusions readily pass, from the alburnous tubes into 
the leaf-stalk. These vessels are always accompanied by spiral 
tubes, which do not appear to carry any liquid: but there is 
another vessel which appears to take its origin from the leaf, 
and which descends down the internal bark, and contains the 
true or prepared sap. When the leaf has attained its proper 
growth, it seems to perform precisely the office of the cotyledon; 
but being exposed to the air, and without the same means to 
acquire, or the substance to retain moisture, it is fed by the al- 
burnous tubes and central vessels. The true sap now appears to 
be discharged from the leaf, as it was previously from the cotyle- 
don, into the vessels of the bark, and to be employed in the for- 
mation of new alburnous tubes between the base of the leaf and 
the root. From these alburnous tubes spring other central vessels 
and spiral tubes, which enter into and possibly give existence to, 
other leaves ; and thus by a repetition of the same process the 
young tree or annual shoot continues to acquire new parts, 
which apparently are formed from the ascending aqueous sap. 
But it has been proved by Du Hamel that a fluid, similar to 
that which is . found in the true sap vessels of the bark, exists 
also in the alburnum, and this fluid is extremely obvious in the 
fig, and other trees, whose true sap is white, or coloured. The 
