152 PKOCEEDIKGS OP THE GEOLOGICAL SOCIETY. 



and. Cycads, or with two, three, or four rows of such pores some- 

 times 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 SigiUarice this outer cylinder 

 was itself in part composed of pseudo-scalariform tissue, as in Brong- 

 niart's specimen of S. elegans ; and in others its jDlace 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 t}'pical 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 con- 

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

 and other Acrogens, but that is merely a modification of the disci- 

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

 separated by thickened bars, corresponding to the hexagonal areo- 

 lation of the ordinary wood-ceUs. 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 to- 

 ward the centre of the stem. 



The structures above referred to may undoubtedly exist in dif- 

 ferent 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 /&', 

 (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 Ein- 

 ney, 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 ClatJiraria-type. Mr. 

 Carruthcrs 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 un- 

 doiibtedly require much further investigation to enable us always to 



