Page:Proceedings of the Royal Society of London Vol 1.djvu/331

Rh Since it seems thus established, that the foetus is nourished without a placenta, the source of the gelatinous matter provided for its nourishment becomes a natural object of inquiry. In birds the albumen of the egg, which corresponds with this jelly, is formed in a tube called the oviduct, and the lateral canals, which open into the cavity of the uterus of the wombat and kanguroo near its neck, bear a strong resemblance in their form to the oviducts of birds; and in the kanguroo they were found, in the impregnated state, to be enlarged, and to have a very free communication with the uterus; circumstances which induce the author to believe their sole use is to form the jelly, and to deposit it in the uterus.

Mr. Knight having, by his former experiments on this subject, established to his satisfaction that the bark of trees is not converted into alburnum, but that the albunlous matter is deposited by a ﬂuid which descends from the leaves, and is subsequently secreted through the bark, proceeds to inquire into the origin and office of the alburnous tubes.

In the succulent shoot of the horse-chestnut it may be seen, that the alburnous tubes are arranged in ridges beneath the cortical vessels, and the number of these ridges at the base of each leaf corresponds with the number of apertures which pass from the leaf-stalk. The position and direction of these tubes have induced naturalists to consider them as passages through which the sap ascends; but Mr. Knight intends to show that they are reservoirs of ﬂuid secreted by the bark, which they retain till it is absorbed by the surrounding cellular substance. Having ascertained, by injections, that the alburnous tubes which descend from the base of an annual shoot are conﬁned to one side of the stem, and to the external annual layer of wood, Mr. Knight made deep incisions at the bases of many annual shoots of young trees, in such a manner as certainly to intercept all communication with the stem by means of the alburnous tubes; but the shoots lived, and in the succeeding spring grew with considerable vigour. In some small twigs, two incisions were made on opposite sides, one a few lines higher than the other, so that all the alburnous tubes might, in some part of their course, be interrupted; yet the sap continued to pass into the branches, and their buds unfolded in the succeeding spring.

All naturalists agree in stating that trees perspire most during summer, when the leaves have attained their full growth; but as the alburnous tubes at that time appear dry and ﬁlled with air, it is evident that the tubes do not convey the sap, but are intended to execute a different ofﬁce.

But if the sap does not rise through the alburnous tubes, says