In the following account special attention is directed to the nature and origin of the secondary stelar tissue and to the secretory zone, as difference of opinion exists as to the interpretation of these features. Among the best examples of shoots of Lepidodendron fuliginosum without secondary tissue or in which it is feebly developed are those originally described by Binney. The stele includes a large parenchymatous pith, the cells of which frequently show signs of recent division, a feature observed also in the pith of the large stem of L. Wünschianum, represented in figs. [181], [182]. The primary xylem cylinder has an irregularly crenulate outer edge like that of L. Wünschianum and L. Harcourtii and the protoxylem elements occupy an exarch position. Isodiametric reticulately-pitted elements are met with both on the inner and outer edge of the xylem.
Fig. 162. Lepidodendron fuliginosum. Part of the stele in transverse section. (Binney Collection, Sedgwick Museum, Cambridge.)
Fig. 163. Lepidodendron fuliginosum. Longitudinal section. (Binney Collection, Cambridge.)
Figs. [162] and [163] illustrate the structure of the outer portion of the xylem and adjacent tissues in a section of a shoot 3·8 cm. × 2·5 cm. in diameter, which is in the act of branching, as shown by the occurrence of two steles of equal size. A figure of the complete section will be found in Binney’s memoir[324], and additional illustrations were published in 1899[325].
Fig. 164. Lepidodendron fuliginosum. Leaf-trace. (Binney Collection, Cambridge.)
The primary xylem (figs. [162], [163], x) is succeeded by 2–3 rows of polygonal cells with dark contents and associated with isodiametric tracheae: these pass into clearer parenchymatous tissue, a, characterised by the arrangement of the cells in vertical series, to which the term meristematic zone has been applied. The secretory zone, s, abutting on the meristematic zone, consists of more or less disorganised parenchymatous cells and broader and more elongated spaces; it is interrupted here and there by an outgoing leaf-trace, as at lt 1 and lt 2 in [fig. 162]. The secretory zone is succeeded by a homogeneous inner cortex like that described in L. vasculare; part of this region is seen at the upper edge of [fig. 162]. The broad middle cortex, which is separated from the inner cortex by a sharply defined boundary, is composed of rather small lacunar parenchymatous tissue consisting of sinuous tubular elements interspersed among isodiametric cells of various sizes ([fig. 166], p). In the middle cortical region the leaf-traces pursue an almost horizontal course; one is shown in [fig. 164], in oblique longitudinal section, in a reversed position; the xylem, x, should be on the inner side of the secretory tissue, s. The clear space between the two parts of the vascular bundle was originally occupied by a few layers of parenchymatous cells, as seen in the transverse sections, figs. [165] and [166]. In some specimens the leaf-traces pass through the middle cortex in a much more vertical course, as shown by the section represented in [fig. 165]. This section illustrates the structure of a typical leaf-trace with unusual clearness; it shows the tangentially elongated group of xylem, the strand of tissue which occupies the position of phloem, s (to which the term secretory zone is applied), the compact parenchyma between the two parts of the bundle, and surrounding the whole a narrow sheath sharply contrasted by the smaller and more uniform size of the cells from the middle cortex, a few cells of which are seen in the photograph. The middle cortex shows a well-defined junction with the more compact outer cortical region, which consists of primary parenchyma passing externally into a zone of phelloderm composed of thick-walled and more elongated cells. A noticeable feature in many Lepidodendron shoots is the occurrence of a circle of strands of secretory cells often surrounding fairly large ducts just internal to the edge of phelloderm: similar strands form irregularly concentric circles, as was pointed out in the case of L. vasculare, in the phelloderm itself.
