The absence, then, of any true medullary ray excludes the Stigmaria from the Dicotyledons, and places it beyond doubt among the Cryptogams ; and this is further confirmed by the fact that the whole of the vascular tissue is composed of scalariform vessels, which are not known to make up the whole of the vascular tissue of any plants except among the Cryptogamia.
As the internal structure of the root is the same as that of the stem, we ought to find in the latter an absence of true medullary rays, and the existence only of a vascular horizontal system communicating with the axial appendages.
Brongniart's elaborate memoir on Sigillaria elegans still supplies the most detailed account of the internal structure of the stem of this genus. It consists of a central cellular axis, or medulla, surrounded by a vascular cylinder ; and this, again, is invested by a thick cellular cortical layer, the outer portion of which is composed of fusiform cells of less diameter than those of the inner portion. So far this is precisely what is found in Stigmaria ; but the vascular cylinder is, according to Brongniart, broken up by numerous delicate medullary rays, whose tissue, however, is destroyed. He was unable to detect the least trace of the cells ; yet he assumes without any hesitation or doubt that the openings between the radiating lines of the vascular tissue are certain indications of medullary rays, and points out that the difference between Lepidodendron Harcourtii, which Lindley and Hutton had described, and his own Sigillaria elegans lay entirely in the possession of medullary rays by his plant. When, however, the figures of Brongniart are compared with the numerous specimens which have been discovered since, many of which have been figured, and especially with those published by Binney under the name of Sigillaria vascularis (which, however, are species of Lepidodendron) , it is easy to see that he wrongly interpreted the series of cracks in the small fragment he was examining, just as I have shown he did in Robert Brown's specimen of Stigmaria. The beautiful longitudinal section cut parallel to the supposed medullary rays (pl. iii. fig. 2) is without any trace of the muriform structure ; and it ought to have been seen here if it ever existed. The supposed ends of the medullary rays shown in fig. 203 of plate iv. are much more like the results of desiccation or decay than the sections of rays ; and the aspect of the adjoining scalariform vessels does not agree with what would have been produced by a medullary ray upon the vascular tissue forming the margins of the mesh through which it passes.
Brongniart's interpretation of these spaces is repeated by him in his ' Tableau, ' and is adopted by all subsequent writers. It forms the real difficulty in the way of Dr. Hooker's referring without hesitation the genus to Lycopodiaceoe.
In Dr. Dawson's valuable contribution to our knowledge of the Coal-flora, he describes the medullary rays as existing in Sigillaria, in addition to the vascular bundles belonging to the axial appendages ; but neither in his letterpress nor his plates can I determine whether this arises from the already adopted diagnosis of the genus, or
VOL. XXV. PART I. T
