XXXIII] MESOXYLON 273 



and a narrower peripheral zone of solid parenchyma (fig. 483, A, a). 

 The secondary wood of the stele is composed of tracheids with 

 2 — 3 contiguous alternate rows of bordered pits on the radial 

 walls, but none on the tangential walls. In the stem shown in 

 fig. 483, A, the secondary wood is preserved only in patches. 

 Numerous blunt teeth varying in prominence project into the 

 pith ; these consist chiefly of serially disposed centrifugal tracheids 

 distinguished by their spiral and scalariform structure and by 

 the medullary rays which are broader than those in the more 

 external xylem. Further reference is made to these perimeduUary 

 strands in the description of the leaf- traces. The medullary 

 rays are uniseriate and usually 1 — 6 cells in depth: beyond the 

 secondary wood is a cambium and a cyUnder of secondary phloem 

 (fig. 483, D, ph^) consisting of tubular elements, presumably 

 sieve-tubes, and elongated secretory sacs. The pericycle is com- 

 posed of several rows of rather large and short cells and has an 

 ill-defined outer boundary. A succession of arcs of periderm-like 

 tissue and phellogen, which may invade the pericycle and phloem, 

 forms a prominent feature in the cortex; radially placed bands 

 of fibres similar to those in Lyginopteris and other genera occur 

 in the outer cortex. At the edge of the pith the more prominent 

 projections of xylem are arranged in pairs (fig. 483, B) and as 

 each pair travels downwards the component strands gradually 

 fuse^ Each bundle of a double trace consists internally of an 

 arc of centripetal xylem, the elements of which are arranged in 

 rows (fig. 483, B, cp), with a single protoxylem group in the 

 middle of the inner face, px. It is not clear whether any primary 

 centrifugal tracheids are present, but there are indications that 

 such are occasionally represented. In most cases the primary 

 xylem of the leaf-traces is exarch, but the existence of mesarch 

 bundles is not improbable. The bulk of each foliar bundle is 

 formed of a fan-shaped mass of secondary centrifugal xylem 

 (fig. 483, B, cf) and an island of parenchyma occurs next the 

 protoxylem. There is no clearly defined boundary between the 

 outer or centrifugal xylem of the leaf-traces and the tracheids of 

 the stem- wood; the latter may consist exclusively of tracheids 



1 Maslen (11) PI. xxxin. figs. 3—5. 

 S. Ill 18 



