involucrum originates from the inner branch of a primary vein, which is usually the case, it opens inwards or towards the mid-rib of the frond from which the vein is derived; and that when it arises from the lower or outer branch of a vein it opens outwards, or in an opposite direction, instances of which occur in several species of the genus, in some of those especially where the frond is simple. On the same law also depends the peculiar character of Scolopendrium, in which the involucra are produced in pairs, one of each pair originating from the lower branch of a vein, the other from the upper branch of the vein immediately below it; they consequently open in opposite directions and towards each other. This law, however, in Asplenium is only observed where the vein has but few branches, for when these are more numerous, and especially when, in consequence of their greater number, the vein has a manifest trunk or axis, the involucra of all its branches open towards this axis: the most remarkable instances of this occur in those species of the genus which authors have separated from it, under the name of Diplazium, where, however, another peculiarity exists, depending on the same law. This peculiarity consists in the inner branch of the vein, or that adjoining the mid-rib, appearing to have a relation not only to the axis of the vein but to that of the pinna or frond from which the vein originates; a relation indicated by its having two involucra, one of which bursts towards the axis of the vein, the other towards the adjoining mid-rib. This double involucrum constitutes the character of Diplazium, but as it is confined to the inner branch, all the others being simple, and opening towards the axis of the vein, there do not appear to be sufficient grounds for its separation from Asplenium. I consider the curved involucrum of Asplenium Filix Fæmina, which exists only on this inner branch of the vein, as somewhat analogous to the double involucrum of Diplazium; but in another point of view it may be regarded as an approach to the structure of Nephrodium, to which this plant has been improperly referred.
