ir>2 



1'1U)CEKI>IX08 OF THE GKOLOOICAL SOClTrTT. 



ami Cycads, or with two, three, or four rows of such pores Bome- 

 times iiiHcribed in hexagonal ureoles in the manner of Datloaiilon. 

 This woody cylinder is traversed by medullary rays, which arc 

 short, and comiwhcd of few rows of cells superimposed. It is also 

 traversed by obli(iue rudiatinf; bundles of pscudo-scalariform tissue 

 proceeding; to the lenves. In some Si^lllario' this outer cylinder 

 was itself in ])art conii)osed of psendo-scalariform tissue, as in Brong- 

 niart's specimen of tS. tle(/aii8 ; and in others its jdace may liave been 

 taken by nnilti\)orous tissue, as in a case above referred to ; but I 

 have no reason to believe that eitlier of these variations occurred in 

 tho typical ribbed species now in (luestion. 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 

 arc furnished by the medullary rays and tho radiating bundles of 

 Bcalariform tissue when these can be obseiTed. 



(d) An inner cylinder of pscudo-scalariform tissue. I have 

 adopted tho term pscudo-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- 

 gcrous wood-cells, with pores elongated trjmsversely, and sometimes 

 separated by thickened bars, corresponding to the hexagonal arco- 

 lation of tho ordinary wood-cells. A similar tissue exists in 

 Cycads, and is a substitute for the spiral vessels existing inordinary 

 Exogens. 



(e) A largo medulla, or pith, consisting of a hollow cylinder of 

 cellular tissue, from which proceed numerous thin diai)hragm8 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 sjjccics 

 in different parts, and at diflerent stages of growth. In the woody 

 axis more particularly, there is evidence that in such forms as i^. 

 (Faimlarla) elajam, the scalariform, or pscudo-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 tho stem; and, further. Prof. Williamson has 

 shown me specimens from the Lancashire coal-field, which I have 

 no doubt are Higillarioid trees of the tvpe of S. vascularis of Bin- 

 ney, and which, instead of a iSternherf/id pith, have scalariform cells 

 and vessels m the centre, and in which the bundles of scalariform 

 vessels traversing tho wood are included in considerable masses of 

 cellular tissue, elongated vert cally, like medullary rays. This 

 plant presents external markings of the CJathraria -tyxie. Mr. 

 tarruthers has also shown me a specimen ribbed externally, and 

 apparently a S^giUarla ot ^,,nmjodendron, which shows only a 

 cylinder of krge scalariform fibres similar to those of Stujmaria. 

 Ihesc facts show how wide differences may exist in the structures 

 of stems referred by their superficial markings to SigUlaria. 



in the case of specimens showing structure merely, it will un- 

 doubtedly require much further investigation to enable us always to 



