746 PROFESSOR W. C. WILLIAMSON AND DR. D. H. SCOTT ON THE 
The central cylinder, with its Jarge axial mass of primary wood, traversed in all 
directions by anastomosing bands of parenchyma, by which the trachez are separated 
into irregular groups, presents a most characteristic appearance. We recognize at 
once that the smaller elements of the primary wood lie near the periphery, where we 
can distinguish definite groups of trachez, recalling the perimedullary xylem-strands 
of Lyginodendron (see Plate 26, figs. 21 and 24; also W1LLTAMsoNn, Part IV., Plate 28, 
fig. 30; Part XVIL, Plate 14, fig. 14). The whole structure at first suggests a Lycopo- 
diaceous stem a suggestion which, as we shall see, is altogether fallacious. A better 
parallel will be found in certain stems of Ferns. 
In many stems the primary structure has remained unaltered (fig. 21; WILLrIaM- 
son, “ Organization,” Part XVII. Plate 14, fig. 14); in others a certain amount of 
secondary wood has been added. At the exterior of the secondary wood the cam- 
bium and phloém are found in favourable cases (see fig. 24), though not so well 
preserved as in Lyginodendron or in the next species of Heterangium. 
The cylinder is surrounded by a zone of rather large clear cells, very well marked 
in many cases, which we regard as constituting a pericycle (see Plates 26 and 27, 
figs. 21, 26, 28, pe.; WiLLtaMson, “Organization,” Part IV., Plate 28, fig. 30, g.). 
The inner cortex is of great width, and consists of parenchyma, in which are imbedded 
large and conspicuous nests of sclerotic tissue (fig. 21, sc.). In longitudinal section 
we see that these nests form horizontal bands—a most characteristic feature of both 
stem and petiole (figs. 22, 23, 28; WuitLiamson, “Organization,” Part IV., Plate 29, 
fig. 82; Plate 31, figs. 45 and 47; Part XVII, Plate 15, figs. 17 and 18). 
The outer cortex is of the ‘‘ Dictyoxylon” type, but with narrower parenchymatous 
meshes than in Lyginodendron (WitL1aMsoN, Part LV., Plate 28, fig. 30; Plate 29, 
fig. 35). In young specimens the parenchymatous bands are very inconspicuous 
(fig. 21). Leaf-trace bundles are scattered throughout the inner cortex, and can 
readily be traced from the pericycle out into the bases of the leaves (see WILLIAMSON, 
“Organization,” Part iV., Plate 30). 
The outer surface of the stem was distinctly ribbed, the ribs, no doubt, correspond- 
ing to the decurrent bases of petioles. The “ restoration” of a portion of the stem,. 
represented in WILLIAMSON, “ Organization,” Part IV., Plate 31, fig. 49, still appears 
to us, after our renewed investigations, to be accurate in all essential points. 
Apart from the structure of the stele, the chief differences from Lyginodendron which 
strike us in a general survey of the organization of the stem are in the distribution of 
the sclerotic nests and in the course of the vascular bundles. 
In Heterangiwm, the former distinctly belong to the cortex, while in Lyginodendron 
they are usually limited to the pericycle. The vascular bundles in Heterangium 
leave the pericycle early in their outward course and pass gradually outwards through 
the cortex, while in Lyginodendron they keep within the pericycle until they 
approach their exit from the stem. 
