Fig. 171. Lepidodendron fuliginosum. (From sections in the Manchester Museum.)
Fig. 171, A, shows more clearly the broad zone of secondary parenchyma with the thinner-walled cambial region, a; the latter is represented on a larger scale in [fig. 171], B. The section shown in [fig. 168], D, and in [fig. 170], A, affords an example of a stem in which the secondary tissue consists largely of narrow scalariform tracheae, x2; the primary stele has a diameter of 1 cm.; the secondary xylem, x2, forms a fairly broad zone of parenchyma and tracheal elements through which leaf-traces pass vertically, a fact of some interest in comparison with the horizontal course which they pursue through the medullary rays in the normal secondary wood of L. vasculare and L. Wünschianum. The secondary tracheae pass gradually into thin-walled cambial cells (a, [fig. 168], D; 170, A) with parallel tangential walls. Fig. 171, C, shows the sinuous course of the secondary tracheae as seen in longitudinal section, and a few small groups of parenchymatous cells, mr, which may be of the nature of medullary rays, enclosed between the winding scalariform tracheae.
Fig. 172. Lepidodendron fuliginosum. From a section (4 × 3·4 cm.) in the Williamson Collection, British Museum (No. 379), figured by Williamson, Phil. Trans. R. Soc. 1881, Pl. 52.
The secretory zone of Lepidodendron fuliginosum agrees essentially with that of other species; it usually presents the appearance shown in [fig. 168], B, sc; [fig. 169], B and C; fig. 170, B (longitudinal section); [fig. 171], D, sc. The comparatively large clear spaces which characterise this tissue, as seen in [fig. 168], B, appear to owe their origin to groups of small cells which gradually break down and give rise to spaces containing remnants of the disorganised elements, as in [fig. 171], D, and [fig. 169], B, b. The secretory tissue seen in [fig. 170], B, consists of large and small parenchymatous cells without any of the broad sacs or spaces such as are shown in [fig. 169], C.
Fig. 172 represents a diagrammatic sketch of a transverse section (4 × 3·4 cm. in diameter) of a young shoot from the Lower Coal-Measures of Lancashire figured by Williamson[327] in 1881 as Lepidodendron Harcourtii. It shows the features characteristic of L. fuliginosum and is of importance as affording an example of a shoot giving off a branch from the stele to supply a lateral axis of the type characteristic of Halonia. The exit of the branch-stele forms a gap in the main stele; a ramular gap as distinguished from a foliar gap. The outgoing vascular strand is at first crescentic, but becomes gradually converted into a solid stele. The primary xylem of the main stele (black in the figure) consists of a ring six tracheae in breadth; this is succeeded by a few layers of dark parenchymatous cells and a band of radially elongated elements, a, which abuts on the secretory zone. The middle lacunar cortex, c2, with Stigmaria rootlets, s, is fairly well preserved. In the outer cortex occur several leaf-traces, lt, accompanied by spaces originally occupied by the parichnos strand, p. A band of secondary cortex, consisting chiefly of phelloderm, is seen at pd. The prominent leaf-cushions, some of which show the parichnos, p, appear to be of the Lepidophloios type.
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It remains to consider the external characters of Lepidodendroid shoots possessing the anatomical features represented by the comprehensive species Lepidodendron fuliginosum.
Certain sections exhibiting this type of structure were described by Binney in 1872 as Halonia regularis[328] on evidence supplied by Mr Dawes, who stated that they were cut from a specimen bearing Halonia tubercles. The section represented in [fig. 172] is no doubt from an Halonia axis. In 1890 Cash and Lomax[329] stated that they had in their possession a stem of the L. fuliginosum type with the external features of Lepidophloios; this identification has been confirmed by Kidston[330] and Weiss[331]. It is, however, equally clear that certain species with the elongated leaf-cushions of Lepidodendron must be included among examples of shoots with the anatomical characters of L. fuliginosum.
