Parenchymatous cells occur in contact with the outer edge of the xylem-cylinder some of which are distinguished by an irregular reticulate pitting. The tangential section represented in [fig. 148], B, illustrates the appearance of a shoot of L. vasculare in which no secondary xylem is present: the central strand of tissue consists of the parenchyma abutting on the xylem with several leaf-traces (lt) passing upwards in an almost vertical course from the outer edge of the stele.
The secondary xylem ([fig. 148], A, x2) consists of radially arranged scalariform tracheae with associated rows of parenchymatous cells which form medullary rays ([fig. 149], mr). Leaf-traces pass through the medullary rays in the secondary xylem cylinder in a direction at right angles to the primary xylem stele from which they are given off, but at the outer edge of the secondary xylem they bend suddenly upwards and for a time follow a steep and almost vertical course.
In well-preserved longitudinal sections the outermost secondary xylem tracheae are seen to be succeeded by a few narrow and vertically elongated elements ([fig. 149], A, a), which represent young unlignified tracheae: these are followed by shorter parenchymatous cells (m) forming part of a meristematic zone from which the secondary xylem receives additions.
Returning to [fig. 148], A; the zone of secondary wood, x2, composed of scalariform tracheids and medullary rays, is succeeded by a few layers of parenchymatous cells and beyond this is a broader zone, sc, to which the term secretory zone has been applied[272]; this is made up of small parenchymatous cells varying in size and of larger spaces which appear to have been formed by the disorganisation of thin-walled elements. The whole zone presents a characteristic appearance due to the association of small cells, large clear spaces, and a certain amount of dark-coloured material suggestive of tissue disorganisation and secreted products. The anatomical characters of the secretory zone are shown in the photograph, [fig. 168], A, sc. Several leaf-traces are seen in transverse section in the secretory zone (black dots in [fig. 148], A, sc; [fig. 154], C, lt): each trace consists of a strand of narrow tracheae accompanied by a few encircling layers of small parenchymatous cells. As a trace continues its steeply ascending course through the secretory zone, it becomes associated with a strand of that tissue and assumes the form of a collateral vascular bundle, the outer part of which does not consist of typical phloem but of shorter elements derived from the secretory zone. Beyond the secretory zone we find a more homogeneous tissue composed of parenchymatous elements slightly extended tangentially ([figs. 148], A, c1; [fig. 168], A, c); this is spoken of as the inner cortical region. In the great majority of sections of L. vasculare as of other species of the genus, the broader middle cortex ([fig. 148], c2) is occupied by mineral matter, introduced subsequent to decay of the tissue; or it is represented by patches of delicate tissue composed of loosely arranged parenchymatous cells varying considerably in size and shape, some being small, oval or polygonal elements while others have the form of sinuous hypha-like tubes.
In this middle cortical region may be seen leaf-traces passing outwards in an almost horizontal course ([fig. 148], A, lt): after leaving the inner cortex the leaf-traces bend somewhat abruptly outwards to follow a more direct path through the middle and outer cortex. The ring of tissue, s, seen in the middle cortex of [fig. 148], A, belongs to a Stigmarian rootlet.
The outer cortex ([fig. 148], A and B, c3) consists of homogeneous parenchyma which is stronger and more resistant to decay than the looser middle cortex. The leaf-traces, as shown in [fig. 148], B, pass through this region in a rather steeply ascending direction: each is seen to be enclosed by a space originally occupied by a strand of middle cortical tissue which accompanies lepidodendroid leaf-traces on their under side and has already been described as the parichnos, ([pp. 97], [100–103]; [figs. 146, 147]).
The surface of the stem shown in section in [fig. 148], A, is composed of broad leaf-cushions. A single leaf-trace with its parichnos passes into each cushion, but in the neighbourhood of the base of a cushion the parichnos bifurcates (cf. [fig. 146], H, I) and the arms diverge slightly to the right and left finally passing beyond the cushion into the lamina of the leaf, their position being shown, as already explained, by the two small lateral scars on the leaf-scar area.
The diagrammatic sketch of a radial longitudinal section through a leaf-cushion represented in [fig. 150] illustrates the relation of the leaf-trace to the leaf-cushion. The trace consists of xylem, x, above and a strand of the secretory zone, st, below; the parichnos tissue was originally present on the under side of the leaf-trace at a. The external surface, bc, marks the limit of the leaf-scar through the middle of which passes the vascular strand lt.
The lower gap a has been formed by the tearing of thin-walled cells of the phellogen, the meristematic tissue from which a considerable amount of secondary cortical tissue or phelloderm has been produced at pd. On the outside of the cushion, c, the cells are somewhat crushed and distinguished by their darker colour from the bulk of the parenchymatous tissue d.
This section also illustrates another characteristic feature of Lepidodendron, namely the presence of a ligule and a ligular pit: the former is represented by a carbonised patch of tissue and the latter extends from the surface of the cushion at b, just above the leaf-scar, almost to the level of the leaf-trace, lt. A comparison of this section with [figs. 146 and 147] will make clear the relation of the several parts of the cushion and leaf-scar.