The gaps gg, seen in [fig. 148], A and B, mark the position of the delicate meristematic zone or phellogen which arises close to the bases of the leaf-cushions; the phellogen has already produced a few rows of radially disposed elements, represented by short radial lines in the drawing, which constitute secondary cortical tissue.
Fig. 150. Lepidodendron vasculare. Leaf-cushions in longitudinal section. (From a specimen in Dr Kidston’s Collection.)
In older shoots the amount of the secondary cortical tissue developed on the inner side of the phellogen is considerable (cf. figs. [152], [153]).
The structure of the cortex of a shoot in which secondary growth, both in the stele and in the outer cortex, has progressed further than in the specimen shown in [fig. 148] is represented in [fig. 151].
Fig. 151. Lepidodendron vasculare. An older stem than that shown in [fig. 148]. (From a section in the Manchester Museum. No. 351.)
The section ([fig. 151], A) measures 7 × 3·8 cm. in diameter; the primary xylem is surrounded by a fairly broad cylinder of secondary wood ([fig. 151], E, x and x2). The almost smooth surface of the primary wood ([fig. 151], E, x) is succeeded by the secondary xylem, x2, characterised at its inner edge by the tapered ends of the radial rows of scalariform tracheids between which occur several delicate parenchymatous cells ([fig. 151], E, a). The occurrence of such isodiametric elements, often exhibiting a delicate spiral thickening band, is a characteristic feature of the boundary between primary and secondary wood in lepidodendroid stems. The secondary wood is penetrated by numerous medullary rays and in some of them are seen strands of narrow spirally thickened tracheae—the leaf-traces—which are in organic continuity with the exarch protoxylem of the primary wood. The leaf-traces are oval and mesarch. The space, c2, ([fig. 151], A) originally occupied by the delicate middle cortex, is succeeded by a shell of outer cortex composed chiefly of secondary tissue (phelloderm, pd) passing towards the inner boundary of this region into the primary outer cortex g ([fig. 151], A and C). The radially disposed elements which make up the bulk of the phelloderm are associated with concentric rows of secretory strands, represented by tangentially arranged dots in [fig. 151], A: on the outer edge of the phelloderm a few patches of primary cortex are still preserved, as at c, fig. A. One of these is shown on a larger scale in fig. B; at m the phelloderm is interrupted by a gap beyond which the cells have thinner walls and show signs of recent division; this is probably the position of the phellogen. The tissue b, [fig. 151], B, consists of secondary cortex succeeded beyond d by the parenchymatous tissue of the leaf-cushion, in which the remains of a ligule, l, are seen in the ligular pit. This section corresponds in position to a line drawn across [fig. 150] at the level of b. In this specimen we have two kinds of secondary cortical tissue: that formed external to the phellogen, from m to d in [fig. 151], B, is less in amount than that produced internal to the phellogen. We cannot make any satisfactory statement as to the nature of this secondary tissue, whether or not any of it agreed in composition with the cork which is usually formed external to the phellogen in recent plants. As the stem of a Lepidodendron grew in girth the leaf-cushions became separated by intervening depressions composed of the secondary cortex formed external to the phellogen, but at a later stage the cushions were thrown off, leaving the outer edge of the phelloderm as the superficial tissue. This exposed tissue became fissured as growth and consequent stretching continued, producing the appearance seen on the surface of the still older stem represented in [fig. 153].
The inner edge of the phelloderm seen at e in [fig. 151], C, passes suddenly into the inner primary region of the outer cortex ([fig. 151], A and C, g) which comprises two types of parenchymatous tissue, patches of isodiametric cells, g, g, alternating with radially arranged areas consisting of tangentially elongated elements (fig. C, f, f; fig. D) which extend as wedges into the phelloderm.
The longitudinal section represented in [fig. 152], B, shows an equal bifurcation of a stem in which no secondary xylem is present; in the lower part of the section the xylem and the outgoing leaf-traces are seen in radial section and at the upper end of each arm the leaf-traces alone, lt, are exposed, as in [fig. 148], B. It is interesting to notice the large amount of phelloderm which has been produced in the fork of the branch, at pd, where greater strength is required.