XVIl] STIGMARIA 243 



tissue occurs like that which I have called the secretory zone in 

 Lepidodendron stems ; but information as to the structure of 

 this part of Stigmaria is much more incomplete than in the 

 case of the aerial shoots. The middle cortex was of the same 

 lacunar type as in the stems, and the fact that it is never well 

 preserved in large Stigmarian axes suggests that it may have 

 been even more richly supplied than in the aerial stems with an 

 aerating system of spaces. The outer cortex, consisting in young 

 examples of large-celled parenchyma, became at an early stage 

 of growth the seat of cambial activity which resulted in the 

 production of radially placed series of secondary elements (fig. 

 210, H, p). The outer and older elements of this secondary 

 cortex are more tangentially stretched than the inner cells, a 

 necessary result of the position of the phellogen on the internal 

 edge of the tissue and of the increasing girth of the axis. 



In comparatively young Stigmarian axes the outer cortex 

 already possesses a band of secondary radially disposed cells 

 characterised by the greater tangential extension of the more 

 external elements ; usually this tissue terminates abruptly on 

 the inner edge and the line of separation no doubt marks the 

 position of the phellogen. Occasionally some delicate secondary 

 elements are preserved internal to the phellogen, and these in 

 young specimens form a narrow cylinder composed in part of 

 radially elongated cells showing signs of recent tangential 

 divisions. In its earlier stage of activity the phellogen seems to 

 form a greater amount of secondary tissue on the outside, but 

 this is clearly not of the nature of cork, the tissue which occupies 

 a corresponding position in recent plants. The primary cortex 

 shows no signs of shrinkage or collapse as would be the case 

 were it cut off from the vascular system by a zone of imper- 

 meable cork. 



Fig. 210, G, represents a piece of the external tissue of a 

 specimen in which the slightly flattened xylem cylinder 

 measures 1*4 x 1 cm.; the inner cortex has disappeared and 

 fragments only of the middle cortex are preserved. The outer 

 cortex, with an average breadth of 2 mm., consists superficially of 

 primary parenchyma with a somewhat uneven surface and with 

 a rootlet attached here and there ; a short distance below the 



