XV] STIGMAEIA 159 



invaded by a narrow tongue of outer cortical tissue. The stele 

 is characterised by a large pith filled -with parenchyma; in Stig- 

 maria ficoides^ the general absence of pith-tissue has led to the 

 inference that the stele was hollow. The xylem is represented by 

 a ring of bundles separated by broad medullary rays; each bundle 

 contains a few small, apparently primary, elements on its inner 

 edge but is mainly composed of radial rows of secondary tracheae 

 x'^, fig. 177, B. On the outer face of the secondary xylem occur a 

 few smaller and thinner walled cells, c, having the appearance 

 of meristematic tissue; from these additional tracheae were 

 added to the xylem. This meristematic zone occurs, as in the 

 stems of Lepidodendron, immediately internal to the secretory 

 tissue, sc; at C^, fig. 177, B, is seen the inner cortical tissue. 



In surface- view a specimen figured by Hick= shows a number 

 of circular scars agreeing in shape and arrangement with the 

 rootlet scars oi Stigmaria ficoides. At b in fig. 177, A, the basal 

 portion of a rootlet is shown in organic connexion with the outer 

 cortex. The rootlet bundles are given off from the stele as in 

 other examples of Stigmaria; each bundle consists of a triangular 

 strand of xylem with an endarch protoxylem at the narrow end 

 accompanied by a portion of the secretory tissue as in the 

 leaf-traces. As in Stigmaria ficoides the rootlets are attached 

 to the outer cortex above a cushion of small cells. It is inter- 

 esting to find that rootlet-bundles, as seen in tangential section 

 of the main axis, are associated with a parichnos strand, but 

 this is on the xylem side of the vascular strand, whereas in the 

 case of leaf-traces the parichnos is on the other side of the 

 bundle. 



Fig. 178, A, represents a transverse section of a rootlet 

 (6 mm. in diameter) associated with Stigmaria radiculosa and 

 probably belonging to this species. The xylem strand a; is com- 

 posed of a group of tracheae with a single protoxylem strand, 

 px, at the pointed end and with small metaxylem elements at 

 the broad end next the space originally occupied by the so- 

 called phloem. A parenchymatous sheath, c', surrounds the 

 bundle, and beyond this is the broad middle cortex a small 

 portion of which is shown on a larger scale in fig. 178, B; as Weiss 

 1 See p. 240. ^ Hick (93) PL xvi. fig. 1. 



