XV] LEPIDODENDRON 109 



sented in fig. 146, A, showing the vascular bundle It, the 

 two parichnos strands, p, composed of large thin-walled cells 

 (of. Isoetes, fig. 133, H, I), and the ligular pit near the upper 

 edge of the section enclosing the shrunken remains of the ligule 

 (fig. 147. B, 0- 



Fig. 147, B shows the form of the tangentially elongated 

 leaf-cushions of Lepidophloios and their spiral disposition. 



Fig. 146, F, represents a section similar to that shown in 

 figs. 147, A and B, but in this case the leaf-trace. It, and the 

 parichnos strands, p, lie in a cavity formed by the destruction 

 of some of the leaf-cushion tissue. It is worthy of notice that 

 the parichnos cells have resisted decay more successfully than 

 the adjacent tissue of the cushion. 



The diagrammatic sketches reproduced in fig. 146, H and I, 

 were made from a transverse section similar to one originally 

 figured by Williamson': fig. 146, H, corresponding in position to 

 the line gh in fig. 146, A, passes through the ligular pit, I, and 

 cuts across the parichnos in the act of branching; the leaf-trace 

 passes outwards beyond the Y-shaped parichnos strand. In the 

 other section, fig. 146, 1, the parichnos is shown in a horizontal 

 plane and the leaf-trace, It, appears in oblique transverse 

 section. In both sections and in fig. 146, G the shaded band at 

 the base represents the secondary cortical tissue external to the 

 phellogen. 



The transverse section represented in fig. 146, G shows in the 

 left-hand cushion, a, the exit of the two parichnos arms and the 

 leaf- trace between them: it illustrates also the various forms 

 assumed by lepidodendroid leaf-cushions when cut across at 

 different levels. 



iv. The Anatomy of Lepidodendron vasculare Binney^- 

 Figs. 148—155, 168, A. 



In the earlier literature dealing with the anatomy of Lepido- 

 dendron and Sigillaria the presence or absence of secondary 

 vascular tissue was made the criterion of generic distinction and 

 the distinguishing feature between the classes Pteridophytes and 



1 Williamson (93) PL iv. figs. 30—32. " Binney (62). 



