146 LYCOPODIALES [C^. 



s (to which the term secretory zone is applied), the compact 

 parenchyma between the two parts of the bundle, and surround- 

 ing the whole a narrow sheath sharply contrasted by the 

 smaller and more uniform size of the cells from the middle 

 cortex, a few cells of which are seen in the photograph. The 

 middle cortex shows a well-defined junction with the more 

 compact outer cortical region, which consists of primary 

 parenchyma passing externally into a zone of phelloderm com- 

 posed of thick-walled and more elongated cells. A noticeable 

 feature in many Lepidodendron shoots is the occurrence of a 

 circle of strands of secretory cells often surrounding fairly large 



Fig. 166. Lepidodendron fuliginosum. Leaf-trace: p, paricbnos. 

 (Binuey Collection, Cambridge.) 



ducts just internal to the edge of phelloderm : similar strands 

 form irregularly concentric circles, as was pointed out in the 

 case of L. vasculare, in the phelloderm itself 



Fig. 166 shows a leaf- trace in the outer cortex accompanied 

 by its crescent-shaped parichnos, j), derived from the middle 

 cortex and by means of which the outer cortex and the lamina 

 of the leaves are connected with the inner region of the shoot. 

 This lacunar middle cortex and parichnos doubtless constitute an 

 aerating tissue-system which after leaf-fall is exposed directly 

 to the air at the ends of the parichnos arras on the leaf-scars. 



