2i6 LEPIDODENDREAE. 



the epidermis-eovered tissue-layer which forms the leaf-cushions. Some- 

 times the growth in thickness of the stem falls either wholly or principally 

 to certain portions of the cortex only, and in that case the development of 

 the axile bundle is relatively small. But various forms of the group are so 

 far differently constituted, that they have in addition a secondary woody 

 body, the product of cambium, which forms a ring round the central bundle 

 and may in certain circumstances be of very considerable size. 



These various stems thus differing in structure cannot of course be 

 referred with perfect certainty to the species founded on the character of 

 the surface. The preservation of the leaf-cushions in our material never 

 suffices for this, and in the majority of cases even the genera cannot be 

 distinguished. When identifications of the kind are nevertheless attempted, 

 they are apt to appear somewhat arbitrary in character. This is the case 

 for example with a type of Lepidodendron extremely abundant in the 

 calcareous nodules of the Lancashire coal-field, which has been referred by 

 Carruthers 1 to Lepidodendron selaginoides, Stbg, itself a decidedly obscure 

 form. Williamson 2 has followed Carruthers in this, though there was 

 already another name for the species, L. vasculare, Binney, whereas he has 

 elsewhere cautiously applied to the types which differ in their anatomical 

 structure a peculiar nomenclature which runs parallel with the other. The 

 latter is in my opinion the more convenient plan, because the same structure- 

 type may contain distinct species which we cannot distinguish from one 

 another. 



The simplest case is presented to us in Lepidodendron rhodumnense, 

 Ren., which was found by Grand' Eury near Combres in the Department of 

 the Loire and has been described by Renault 3 . Young branches have the 

 leaves still attached to them ; these leaves spread almost at a right angle 

 with the branch and are then curved upwards and become hooked ; the 

 transverse section of their basal portion is transversely rhombic, that of the 

 upper part is flatly crescent-shaped, and they contain a single median 

 vascular bundle. The central bundle-strand of the stem is homogeneous 

 in character and consists entirely of scalariform tracheides ; its circum- 

 ference shows a number of small tooth-like projections, which answer to the 

 transverse sections of the points of attachment of the leaf-traces. The 

 section passes through the weak trace-bundles in the cortex at different 

 points in their course. The entire woody bundle is surrounded by a very 

 thin layer of delicate elongated parenchymatous cells, which should belong 

 to the bast, not, as Renault thinks, to the bundle-sheath. In the xylem 

 the tracheides of smallest size on the transverse section lie in groups on the 

 periphery, and these groups correspond to the attachment of the leaf-traces 

 and are explained by Renault to be protoxylem-elements. The rind 



Carruthers (13). ' Williamson (1), II. 3 Renault (1), t. 10. 



