LEPIDODENDRON. 



71 



differences dans les diverges couches dii tissu parenchyraateux qui se suivent depuis la 

 surface du cylindre vasculaire jusqu'a I'epiderme inclusivement. 



" II est a noter cependant que ce qui a ete dit sur la structure interne ne se rapporte 

 qu'a une seule espece, le Lep. Harcourtii, prise pour type, et il n'est pas du tout certain 

 que tons les fossiles qui, a la suite de leur organisation exterieure, se trouvent reunis dans 

 ce genre, aient aussi la meme organisation interieure ; cela est merae peu probable. 

 M. Binney, dans son memoire cite plus baut [Sigillaria and Lepidodendron, printed in the 

 'Proceedings of the Geological Soc. of London,' Jan., 1862), parle de deux plantes dont 

 le cicatrices foliaires coincident parfaitement avec celles des Lepidodendron, et dont I'une, 

 le Sigillaria vascularis, ofJ're la struct\ire des Sigillaires, tandis que I'autre, le Lepidoden- 

 dron vasculare, a son axe central entierement compose de gros vaisseaux scalariforraes et 

 de fins vaisseaux rayes, au lieu d'offrir un cylindre vasculaire occupe interieureraent par 

 un parenchyme medullaire, comme dans le L. Harcourtii. Ce type se rapprocherait done 

 davantage des Lycopodes, et le dernier des Psilotum et Tniesipteris." 



§ 4. Williamson. — Professor W. C. Williamson, F.R.S., in a paper read before the 

 Royal Society, June 15, 1871,^ "On the Organisation of the Fossil Plants of the Coal- 

 Measures, Part 2, Lepidodendron and Sigillaria," says, " The Lepidodendron selaginoides 

 described by Mr. Binney, and still more recently by Mr. Carruthers, is taken as the 

 standard of comparison for numerous other forms. It consists of a central medullary 

 axis composed of a combination of transversely barred vessels with similarly barred cells ; 

 the vessels are arranged without any special linear order. The tissue is closely surrounded 

 by a second and narrower ring, also of barred vessels, but of smaller size, and arranged in 

 vertical laminae which radiate from within outwards. These laminae are separated by 

 short vertical piles of cells, believed to be medullary rays. In the transverse section the 

 intersected mouths of the vessels form radiating lines, and the whole structure is regarded 

 as an early type of an exogenous cylinder ; it is from this cylinder alone that the vascular 

 bundles going to the leaves are given off. This woody zone is surrounded by a very thick 

 cortical layer, which is parenchymatous at its inner part, the cells being without definite 

 order ; but externally they become prosenchymatous, and are arranged in radiatnig lines, 

 which latter tendency is observed to manifest itself whenever the bark-cells assume the 

 prosenchymatous type. Outside the bark is an epidermal layer separated from the rest 

 of the bark by a thin bast-layer of prosenchyma, the cells of which are developed into a 

 tubular and almost vascular form ; but the vessels are never barred, being essentially of 

 the fibrous type. Externally to this bast-layer is a more superficial epiderin of 

 parenchyma supporting the bases of the leaves, which consist of similar parenchymatous 

 tissue. Tangential sections of these outer cortical tissues show that the so-called 

 'decorticated' specimens of Lepidodendron, and of other allied plants, are merely examples 



1 'Proceed. Roy. Soc.,' vol. xix, p. 500, &c. ; 'Nature,' No. 87, vol. iv, p. 173, June 29th, 1871. 



1] 



