242 LYCOPODIALES [CH. 



instead of a band with a crenulated inner margin as figured by- 

 Renault. 



An interesting agreement between the French and English 

 specimens is the occurrence in the cortex of groups of reticulate 

 elements: in Weiss's section these are short and wide and 

 occur in the middle cortex ; in Renault's plant they are more 

 fusiform and occur in the secondary cortical tissue. These 

 elements appear to have been arranged as an interlacing 

 network in the middle cortex and were in close connexion with 

 the rootlet-bundles, comparable, as Weiss points out, with the 

 transfusion tracheids accompanying Lepidodendron leaf-traces. 



It is probable that these short and wide tracheal elements 

 served for water-storage and thus afford another indication of the 

 xerophilous character of the Carboniferous Lycopods, a feature 

 possibly connected with a salt-marsh habitat. 



The presence of conspicuous medullary rays gives the 

 secondary xylem of Stigmaria the appearance of being divided 

 into several more or less distinct groups (fig. 210, E, St). In 

 tangential longitudinal section the xylem assumes the form of 

 a broad reticulum with lenticular meshes filled with medullary- 

 ray tissue through which strands of xylem are cut across in a 

 transverse direction as they pass outwards from the inner edge 

 of the wood to supply the rootlets. In addition to these broader 

 or primary medullary rays, there were numerous secondary 

 rays composed of narrow plates of parenchymatous cells one 

 or several elements in depth. As Williamson pointed out, the 

 medullary-ray tissue consists in part of radially elongated 

 tracheal elements with spiral or scalariform thickening bands like 

 those described in the same position in Lepidodendron stems. 



Our knowledge of the minute structure of the tissues 

 abutting on the secondary xylem is far from complete. 



The xylem is succeeded by a zone of delicate cells which was 

 the seat of meristematic activity. It is noteworthy that in a 

 section figured by Williamson^ there is the same disparity in 

 size between the outermost elements of the xylem and the 

 adjacent cells of the meristematic zone as in Lepidodendron 

 stems. Beyond this region an imperfectly preserved lacunar 

 ' Williamson (87) A. PI. iv. fig. 20. 



