xv] 



LEPIDODENDRON 



117 



The structure of the cortex of a shoot in which secondary 

 growth, both in the stele and in the outer cortex, has pro- 

 gressed further than in the specimen shown in fig. 148 is 

 represented in fig. 151. 



The section (fig. 151, A) measures 7 x 3-8 cm. in diameter; 

 the primary xylem is surrounded by a fairly broad cylinder 

 of secondary wood (fig. 151, E, x and x% The almost smooth 

 surface of the primary wood (fig. 151, E, x) is succeeded by the 

 secondary xylem, a?, characterised at its inner edge by the 

 tapered ends of the radial rows of scalariform tracheids between 



Fio. 151. Lepidodendron vasculare. An older stem than that shown in fig. 148. 

 (From a section in the Manchester Museum. No. 351.) 



which occur several delicate parenchymatous cells (fig. 151, E, a). 

 The occurrence of such isodiametric elements, often exhibiting 

 a delicate spiral thickening band, is a characteristic feature of 

 the boundary .between primary and secondary wood in lepido- 

 dendroid stems. The secondary wood is penetrated by numerous 

 medullary rays and in some of them are seen strands of narrow 

 spirally thickened tracheae — the leaf-traces — which are in 

 organic continuity with the exarch protoxylem of the priraarj^ 

 wood. The leaf-traces are oval and mesarch. The space, c^, 

 (fig. 151, A) originally occupied by the delicate middle cortex, 

 is succeeded by a shell of outer cortex composed chiefly of 



