The Botryopteridece, 59 
stele and parts of which become included in the leaf-traces (Figs. 
6 and 7). 
Fig. 7. Zygopteris corrugata. A double lobe of the stele showing the 
large metaxylem tracheids forming curved bands, and the central thin-walled 
parenchyma with scattered thicker-walled elements (tracheids), which pass 
into the lobes. Univ. Coll. Collection, K 32a. x about 35. 
The leaf-trace departs as an isodiametric strand which divides 
tangentially as it passes through the cortex, the inner half becoming 
the stele of the axillary shoot, while the outer becomes the petiolar 
strand. The latter at first has the form of a tangentially extended 
band, as in Tubicaulis or Grammatopteris, but soon it spreads out 
at the ends and acquires the characteristic H-shaped form from 
which the name of the genus is derived (Figs. 8, 9, 10). The side- 
pieces of the H, which have their smaller elements (protoxylem ?) 
on their outer edges, are sometimes straight, but very often 
they are bent inwards so as to give the section of the strand the 
form of a double anchor. The xylem is often narrowed on each 
side at the points where the side-pieces join the central bar, so 
that they become almost (or in places quite) separated into four 
independent strands (Fig. 10). It is from these strands that the 
bundles supplying the pairs of branches on each side of the rachis 
of the frond depart. The two branches of each pair are evidently 
situated in the plane at right angles both to the ordinary plane of 
symmetry (vertical plane) and to the plane of the lamina of a 
