44 
N. Bancroft. 
in vascular evolution within the Lyginopterideas. Lyginopteris 
oldhamia (Fig. 2) from the English Coal Measures, possesses a 
stem which is in all cases, according to Scott, of monostelic 
structure. But here the centre of the stele is occupied by a large 
pith instead of by tracheides and conjunctive parenchyma as in 
Heterangium. The pith is surrounded by from five to eight or nine 
distinct strands of elements mostly possessing multiserate bordered 
pits; these strands constitute the primary xylem of the stele. 
According to Scott, they are mesarch in structure, like the 
peripheral strands associated with protoxylem in Heterangium and 
are thus comparable with the petiolar bundles of Cycads. 1 There 
Fig. 2. —Lyginopteris oldhamia. Transverse section of stem. Surrounding 
the pith are eight primary xylem-strands ; beyond these is a wide zone of 
secondary wood, and then the phloem, ph, phloem; pd, periderm, marking 
the outer limit of the pericycle ; It 1 to lt b , the five leaf-traces. (Adapted from 
Williamson and Scott, 1895). 
is more centripetal xylem than centrifugal, and a few parenchy¬ 
matous elements accompany the protoxylem on the inner side. 
Immediately outside the ring of primary xylem strands, there is in 
all except the youngest stems, a broad continuous zone of secondary 
wood consisting of radial series of tracheides and medullary rays. 
The rays are sufficiently developed to indicate that the wood was 
originally of a somewhat parenchymatous nature, similar to that of 
recent Cycad woods. In well-preserved specimens, cambium and 
primary and secondary phloem may be seen. 
The vascular strands surrounding the pith form the leaf-traces, 
which follow a nearly vertical course until they are free from the 
pericycle, when each trace divides into two and bends out sharply 
through the cortex to enter the leaf-bases. On leaving the stele, 
the leaf-traces have secondary wood which is generally lost by the 
1 Cf. the view of Chodat, quoted below, p. 49. 
