42 PTERIDOSPERMEAE [CH. 



united in the perimedullary zone. The cylinder of secondary 

 wood is partially interrupted at r by the bending outwards of 

 the stele of an adventitious root cut across transversely as it 

 bends down after emerging from the outer cortical region. In 

 more or less close association with the outer surface of the secondary 

 xylem are four pairs of leaf-trace bundles and one larger trace 

 at d containing two widely separated protoxylem strands and 

 faced externally with an arc of secondary xylem: this is a leaf- 

 trace which shows by the slight constriction on the outer edge 

 of its primary xylem that it is beginning to divide into a pair 

 of equal strands. A precisely similar strand is shown on a larger 

 scale in fig. 404, D. The twin bundles seen at b, fig. 403, 

 represent a divided leaf-trace at a slightly higher level than the 

 partially severed trace at d, and the arcs of secondary xylem 

 are narrower. The appearance of the double leaf-trace at a 

 still higher level is shown at c: the two strands are farther 

 apart and the secondary xylem has almost disappeared, while 

 those at e, nearer their entrance into the leaf-stalk, consist exclu- 

 sively of primary xylem and phloem. At a the two strands of 

 a leaf-trace, still nearer to the petiole, are inclined towards one 

 another preparatory to reunion after reaching the leaf-stalk. 

 A slender root is seen in transverse section at r' immediately 

 outside the two leaf-bundles. As Williamson and Scott 1 have 

 pointed out, there are always five leaf-traces beyond the xylem 

 cylinder of a Lyginopteris stem as seen in transverse section, and 

 these traces in the pericycle, separated from one another by f 

 of the circumference, alternate in position with the lower portions 

 of leaf -traces in the perimedullary region of the same stem. The 

 phyllotaxis is thus seen to be f . 



The secondary wood is succeeded by a cambium of normal 

 structure passing gradually into a narrow band of secondary 

 phloem which in well-preserved stems is seen to consist of sieve- 

 tubes and parenchyma with medullary rays rather broader than 

 those in the xylem. Beyond the phloem is the comparatively 

 broad pericycle consisting of parenchyma with nests of scleren- 

 chyma like those in the pith and scattered secretory cells. In 

 the outer layers of the pericycle a phellogen was formed at an 

 1 Williamson and Scott (95). 



