Page:Philosophical Transactions - Volume 095.djvu/114

100 100 Mr. Knight concerning the State in which

when the roots were considerably elongated, albumous tubes formed; and as soon as they had acquired some degree of firmness in their consistence, they appeared to enter on their office of carr)ing 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 fonner 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 albumous 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 acqirire, or the* substance to retain moisture, it is fed by the al- bumous 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 albumous tubes between the base of the leaf and the root. From these alburnous tubes springother central Vessels and spiral tubes, which enter into and possibly ^ve 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 apparendy 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