122 LYCOPODIALES [CH. 



secondary xylem has been added ; the other stele, s, possesses 

 no fully developed secondary elements. 



Fig. 154, B and C, illustrates the anatomical features immedi- 

 ately external to the primary xylem of the smaller stele, s. The 

 comparatively broad band of radially disposed parenchyma, m, 

 is connected with the outermost elements of the xylem by a 

 few rather dark and small crushed parenchymatous cells. The 

 band m, which we may speak of as the meristematic zone, 

 clearly consists of cells in a state of division ; it is in this region 

 that the secondary xylem is produced. Beyond the leaf-trace, 

 (fig. 154, C It), occurs a portion of the secretory zone, some of the 



I 



I 



I 



mr 



Fig. 155. Lepidodendron vasculare. Outer edge of secondary xylem : m, 

 meristematic zone; mr, medullary ray. (Drawn from the section 

 shown in fig. 168, A). 



smaller cells of which show signs of disorganisation ; but most of 

 this tissue has been destroyed (fig. 154, B, sc). The outer edge of 

 the secretory zone is shown in fig. 154, D abutting on the cells 

 of the inner cortex, c'. The leaf-trace shown in the inner cortex 

 in fig. 154, B illustrates the more oval or tangentially extended 

 form of the xylem in this region, in contrast to the more circular 

 outline which it exhibits on the inner side of the secretory 

 zone. 



The transverse section, part of which is reproduced in 

 fig. 168, A, illustrates a characteristic feature, namely the 

 juxtaposition of the outermost tracheae of the secondary xylem 



