XXIX] LYGESrOPTBRIDBAB 45 



cycle, has been invaded in the stem shown in fig. 403 by numerous 

 rootlets of Stigmaria and Lyginopteris, some of which are seen 

 interrupting the continuity of the outer cortex. The greater 

 width of the cortical region at /, fig. 403, is due to the decurrent 

 base of a petiole the meristele of which is not included in the 

 section. The lighter and broader bands between the cross-sections 

 of the stereome-network in the outer cortex are occupied by re- 

 mains of tangentially stretched parenchymatous cells, and beyond 

 this zone in a yoimger stem there are a few layers of parenchyma 

 forming the superficial tissue, but there appears to be no well- 

 defined epidermal layer. 



Young stems have been recognised in which there is very 

 little secondary xylem and phloem: in these the stereome .bands 

 in the outer cortex are closer together than in the stretched hypo- 

 dermal tissue of older shoots and the scattered sclerous nests are 

 represented by unthickened cells. In addition to young stems 

 Williamson and Scott described a distinct type of small stem in 

 which the primary xylem forms an almost complete ring^ com- 

 parable with the primary xylem of some adult Sigillarian stems 

 (vol. II. p. 220) but distinguished by its mesarch structure and by 

 the reticidate pitting of the centripetal xylem. 



A characteristic feature of the stem is the occurrence of 

 emergences from the outer cortex which have the structure either 

 of spinous processes, broadly linear or flask-shaped, or of stalked 

 glands^. A portion of a glandular emergence is shown in fig. 405, B : 

 the group of small cells immediately below the blunt apex is in 

 this instance still intact though showing signs of disorganisation 

 in the centre ; but in many cases the secretory tissue has not been 

 preserved and the head of the emergence is occupied by a space. 

 A single stoma is seen at s in longitudinal section. Further 

 reference to the emergences is made in the description of the leaf. 

 It occasionally happens that a meristematic layer is formed 

 in the parenchymatous tissue immediately internal to some of 

 the perimedidlary xylem strands of a Lyg.inopteris stem from 

 which either secondary parenchyma is produced or a zone of 

 secondary xylem and phloem, the phloem facing the centre of 

 the pith. An example of such internal xylem was figured by 

 1 Williamson and Scott (95) p. 720. ' Williamson (90) PI. xn. fig. 6. 



