Page:Proceedings of the Royal Society of London Vol 1.djvu/311

Rh posed bark. And it was evident, that in each instance a new layer, both of cortex and of albumum, was generated.

Dir. Knight’s attention was next directed to the progressive formation of albumum in the young shoots of an oak coppice; but he could discover nothing like transmutation of bark into alburnum, although the commencement of albumous layers in this tree is peculiarly conspicuous, by a circular row of very large tubes. These tubes he found, at their ﬁrst formation, passing through a soft gelatinous substance, much less tenacious than the surrounding preexistent bark; and there was nothing in the bark at all corresponding to the circular row of tubes contained in the albumum. The interior surface of the bark is at the same time well deﬁned, and its own peculiar vessels are distinctly visible, and by no means exhibit any appearance of progressive transmutation.

Mr. Knight remarks also, that the qualiﬁes of different kinds of wood are not in any degree indicated by the bark which covers them. He instances the wych-elm and the ash, the woods of which, for agricultural implements, are frequently substituted for each other, although the textures of their barks are extremely dissimilar; inasmuch as one is brittle, and the other so tough as frequently to be used for ropes.

Another circumstance, very unfavourable to the theory of conversion, is the ﬁrm adhesion which subsists between the layers of bark to each other, in comparison to their adhesion to their albumum.

Two experiments of Du Hamel are, however, cited by Mirbel in support of that theory.

In the ﬁrst, pieces of silver wire, inserted into the bark, were frequently found in the alburnum; but the evidence is defective, as it was not rightly ascertained that the pieces of wire did not, at their ﬁrst insertion, pass between the bark and albumum, and thus be liable to be covered by a new deposition of either one or the other.

In the second experiment, the bud of a peach.tree, with a piece of bark attached to it, was inserted into aplum-stock: a layer of wood was afterwards found beneath the inserted bark, perfectly similar to the peach ; but it is easier to conceive a layer of alburnum, generated by deposition from ﬂuids that have circulated through the inserted bud, than that a part of its bark should be converted into a layer of albumum more than twice as thick as the inserted bark.

Mr. Knight also remarks, that when the bud is destroyed, the bark deposits no albumum; but, being small, it becomes ultimately covered by the successive albumous layers of the stock, and may be found many years afterwards to haVe made no progress towards conversion into wood.