XV] LEPIDODENDRON 115 



size and shape, some being small, oval or polygonal elements 

 while others have the form of sinuous hypha-like tubes. 



In this middle cortical region may be seen leaf-traces passing 

 outwards in an almost horizontal course (fig. 148, A, It): after 

 leaving the inner cortex the leaf-traces bend somewhat abruptly 

 outwards to follow a more direct path through the middle and 

 outer cortex. The ring of tissue, s, seen in the middle cortex 

 of fig. 148, A, belongs to a Stigmarian rootlet. 



The outer cortex (fig. 148, A and B, (?) consists of homo- 

 geneous parenchyma which is stronger and more resistant to 

 decay than the looser middle cortex. The leaf-traces, as shown 

 in fig. 148, B, pass through this region in a rather steeply 

 ascending direction: each is seen to be enclosed by a space 

 originally occupied by a strand of middle cortical tissue which 

 accompanies lepidodendroid leaf-traces on their under side and 

 has already been described as the parichnos, (pp. 97, 100 — 103 ; 

 figs. 146, 147). 



The surface of the stem shown in section in fig. 148, A, is 

 composed of broad leaf-cushions. A single leaf-trace with its 

 parichnos passes into each cushion, but in the neighbourhood 

 of the base of a cushion the parichnos bifurcates (cf. fig. 146, 

 H, I) and the arms diverge slightly to the right and left finally 

 passing beyond the cushion into the lamina of the leaf, their 

 position being shown, as already explained, by the two small 

 lateral scars on the leaf-scar area. 



The diagrammatic sketch of a radial longitudinal section 

 through a leaf-cushion represented in fig. 150 illustrates the 

 relation of the leaf-trace to the leaf-cushion. The trace consists 

 of xylem, x, above and a strand of the secretory zone, st, below; 

 the parichnos tissue was originally present on the under side of 

 the leaf-trace at a. The external surface, he, marks the limit 

 of the leaf-scar through the middle of which passes the vascular 

 strand It. 



The lower gap a has been formed by the tearing of thin- 

 walled cells of the phellogen, the meristematic tissue from 

 which a considerable amount of secondary cortical tissue or 

 phelloderm has been produced at pd. On the outside of the 

 cushion, c, the cells are somewhat crushed and distinguished 



