Page:Quarterly Journal of the Geological Society of London, vol. 27.djvu/270

152 and Cycads, or with two, three, or four rows of such pores sometimes inscribed in hexagonal areoles in the manner of Dadoxylon. This woody cylinder is traversed by medullary rays, which are short, and composed of few rows of cells superimposed. It is also traversed by oblique radiating bundles of pseudo-scalariform tissue proceeding to the leaves. In some Sigillarioe this outer cylinder was itself in part composed of pseudo-scalariform tissue, as in Brongniart's specimen of S. elegans; and in others its place may have been taken by multiporous tissue, as in a case above referred to; but I have no reason to believe that either of these variations occurred in the typical ribbed species now in question. The woody fibres of the outer cylinder may be distinguished most readily from those of Conifers, as already mentioned, by the thinness of their walls, and the more irregular distribution of the pores. Additional characters are furnished by the medullary rays and the radiating bundles of scalariform tissue when these can be observed.

(d) An inner cylinder of pseudo-scalariform tissue. I have adopted the term pseudo-scalariform for this tissue, from the conviction that it is not homologous with the scalariform ducts of Ferns and other Acrogens, but that is merely a modification of the discigerous wood-cells, with pores elongated transversely, and sometimes separated by thickened bars, corresponding to the hexagonal areolation of the ordinary wood-cells. A similar tissue exists in Cycads, and is a substitute for the spiral vessels existing in ordinary Exogens.

(e) A large medulla, or pith, consisting of a hollow cylinder of cellular tissue, from which proceed numerous thin diaphragms toward the centre of the stem.

The structures above referred to may undoubtedly exist in different proportions in different species, and also in the same species in different parts, and at different stages of growth. In the woody axis more particularly, there is evidence that in such forms as S. (Favularia) elegans, the scalariform, or pseudo-scalariform, tissues were predominant. In young stems also, and in roots, this would probably be the case; and in the latter the texture was much coarser than in the stem; and, further, Prof. Williamson has shown me specimens from the Lancashire coal-field, which I have no doubt are Sigillarioid trees of the type of S. vascularis of Binney, and which, instead of a Sternbergia pith, have scalariform cells and vessels in the centre, and in which the bundles of scalariform vessels traversing the wood are included in considerable masses of cellular tissue, elongated vertically, like medullary rays. This plant presents external markings of the Clathraria-type. Mr. Carruthers has also shown me a specimen ribbed externally, and apparently a Sigillaria or Syringodendron, which shows only a cylinder of large scalariform fibres similar to those of Stigmaria. These facts show how wide differences may exist in the structures of stems referred by their superficial markings to Sigillaria.

In the case of specimens showing structure merely, it will undoubtedly require much further investigation to enable us always to