1. Lepidodendron vasculare. (Botany School, Cambridge.)
  2. Lepidodendron fuliginosum. (From a specimen from Shore, Lancashire, in the Cambridge Botany School Collection).
  3. L. fuliginosum. (“Biseriate Halonia” of Weiss No. 257, Manchester University Museum.)
  4. L. fuliginosum. (Manchester Univ. Museum.)

The result of the activity of this cambium band is the production of secondary parenchyma and tracheal tissue. In [fig. 179], E, drawn from a portion of the section represented in [fig. 168], B, a projecting arm of primary xylem is seen at x; this is followed by 2–3 layers of parenchymatous cells, some of which have dark contents, and beyond this is seen a group of secondary elements, tr, cut across somewhat obliquely, which are evidently products of the cambial cells on the inner margin of the secretory zone, sc. The longitudinal section ([fig. 169], D) shows the cambial cells, a, next the secretory zone, sc, passing internally into crushed and imperfectly preserved elongated elements which are presumably miniature tracheae, and these are succeeded by older and more completely lignified xylem elements, x. In larger shoots the amount of secondary tissue is considerably greater; it may consist almost entirely of short-celled parenchyma ([fig. 168], C, from x to sc), or it may include a large proportion of radially disposed and vertically elongated tracheae ([fig. 168], D, x2, and [fig. 170], A, x2), or it may consist of parenchyma containing scattered groups of tracheae (fig. 169, A, x2)[326].

Fig. 169. Lepidodendron fuliginosum.

Fig. 169, A, is a diagrammatic sketch of the tissues—1 mm. wide—between the primary xylem, x, and the inner cortex. The primary xylem is succeeded by short parenchymatous cells followed by a zone of radially elongated elements passing occasionally into rows of narrow scalariform tracheae, some of which, owing to their sinuous longitudinal course ([fig. 171], C), are seen in oblique section, as at C, [fig. 169], A. At its outer edge this secondary tissue, x2, consisting of parenchyma and tracheae, passes into the cambial band ([fig. 169], B, a).

Fig. 170. Lepidodendron fuliginosum. (From sections in the Manchester Museum.)

The radial longitudinal section represented in [fig. 168], C, is taken from the fossil described by Weiss as a biseriate Halonia; it agrees sufficiently closely in structure with others referred to Lepidodendron fuliginosum to be classed as an example of this anatomical type. A complete transverse section of the stem measures 9 × 6·3 cm.; the breadth of the tissues between the edge of the primary xylem and the outer edge of the secretory zone is 2·5 mm. The middle cortical region, characterised by the sooty appearance, which led Williamson to choose the specific name fuliginosum, is traversed by the leaf-traces and is sharply differentiated from both the inner and outer cortex. The longitudinal section ([fig. 168], C) shows the outer edge of the primary xylem, x, abutting on a band of dark and small-celled parenchyma which passes into the broad zone of secondary tissue, m, the inner region of which consists of fairly thick-walled elements in radial series passing externally into the thin-walled cells of the cambial region, a, on the inner edge of the secretory zone, sc. This section shows also the interruption of the secretory zone by an outgoing leaf-trace, lt, the lower part of which, sc, is continued downwards into the secretory zone. The exit of a leaf-trace produces a gap in the secretory zone of the stem, but not in the xylem. If we applied the term phloem to the secretory zone—a course adopted by Prof. F. E. Weiss and some other authors, but which I do not propose to follow—we should speak of a phloem foliar-gap as a characteristic feature of a Lepidodendron shoot. This applies to other species of the genus as well as to L. fuliginosum.