THE CARBONIFEROUS FLORA. Ifil 



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-soalaritorm tissue proceeding to 

 the leaves. In some SigiUarim 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 mul- 

 tiporous 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 fur- 

 nished by the medullary rays and the radiating bundles of scalari- 

 form 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 con- 

 viction that it is not homologous with the scalariform ducts of ferns 

 and other acrogens, but that it is merely a modification of the dis- 

 eigerous wood-cells, with pores elongated transversely, and sometimes 

 separated by thickened bars, corresponding to the hexagonal areo- 

 lation of the ordinary wood-cells. A similar tissue exists in cycads, 

 and is a substitute for the spiral vessels existing in ordinary ex- 

 ogens. 



e. A large medulla, or pith, consisting of a hollow cylinder of 

 cellular tissue, from which proceed numerous thin diaphragms to- 

 wards the centre of the stem. 



These structures of the highest type of SigiUaria are on the 

 one hand scarcely advanced beyond those of Calamopitvs, as de- 

 scribed by Williamson, and on the other approach to those of 

 Cordaites, as seen in specimens presented to me by Renault. 



Finally, as to the fruit of SigiUarim, I have no new facts to 

 offer. The strobiles or spikes associated with these trees have been 

 variously described as gymnospermous (Renault) or cryptogamous 

 (Goldenberg and WiUiamson). I have never seen them in place. 

 Two considerations, however, have always weighed with me in refer- 

 ence to this subject. One is the constant abundance of Trigonocarpa 



hexagonal rim of thickened wall ; but in all cases these structures are 

 less pronounced than in Dadoxylon, and less regular in the walls of the 

 same cell, as well as in different layers of the tissues of the axis. 



