Beyond the most recently formed tracheae we have a band of delicate parenchymatous cells (m, [figs. B and E, 181]; C, figs. 183, 184) forming the meristematic zone[359]. The longitudinal section represented in [fig. 184] shows some recently formed narrow tracheae, T, and beyond these the meristematic zone composed of thin-walled short cells, C, arranged in horizontal rows. It is this small-celled tissue to which the name phloem has been applied by some authors[360], a term which seems to me to be misleading and inappropriate. In passing through this zone of dividing cells the leaf-traces become surrounded by an arc of meristem from which elements are added to the radially placed rows of secondary tracheae. Beyond the meristematic region portions of the secretory zone are preserved, consisting of large sacs or spaces and small dark cells as seen in figs. [181], B, E, sc, F; [183], [184]. This tissue has the same structure as in L. vasculare and in L. fuliginosum: it is a striking fact that there are no indications of any additions to the secretory zone even in stems with such a large amount of secondary xylem as in the Dalmeny specimen ([fig. 182], x2). If the secretory zone were of the nature of phloem we should expect to see signs of additions made to it in the course of growth. In this connexion it is worth mentioning that in the recent fern Botrychium (Ophioglossaceae) secondary xylem is formed in the stem, but apparently no additions are made to the phloem. The structure of the secretory zone tissue as seen in the longitudinal section [fig. 184], S, is also a serious difficulty in the way of accepting the designation phloem as employed by Scott and Weiss. Between the secretory zone and the outer cortical region, no tissues have been preserved. The shell of bark consists chiefly of radial rows of elongated cells with rather thick walls characterised by the occurrence of small intercellular spaces and by tangentially placed bands of secretory cells and sacs ([fig. 181], D, s). Immediately internal to the secondary cortex or phelloderm occur groups of secretory tissue as shown in the section of L. Harcourtii ([fig. 179], B).
Fig. 183. Lepidodendron Wünschianum. From the specimen shown in [fig. 181], S, secretory zone; C, meristem; T, immature tracheae.
Fig. 184. Lepidodendron Wünschianum. Longitudinal section of the specimen represented in transverse section in [fig. 183].
The large tree shown in transverse section in [fig. 181], A, has lost its leaf-cushions; the bark, as seen in the lower part of the photograph, presents a fissured appearance like that with which we are familiar on an old Oak or Elm stem. A radial longitudinal section through the phelloderm revealed the existence of a crushed leaf-trace passing outwards in an approximately horizontal course accompanied by a strand of parenchymatous tissue[361] having the characteristic structure of a parichnos. It is probable that the surface of this partially decorticated stem differed in appearance from that of an old Sigillaria (cf. [fig. 198]) in the much smaller and less conspicuous parichnos strands.
In addition to the large stems of L. Wünschianum from Arran and Dalmeny numerous examples of smaller axes from the former locality are represented in the Williamson collection (British Museum). Some of the twigs are characterised by a solid stele (protostele) giving off numerous leaf-traces accompanied by short spirally thickened tracheids like those which occur at the outer edge of the primary xylem in the larger stem: these extend into the leaf where they are arranged round the vascular bundle like the transfusion tracheids[362] in many recent conifers. The surface of these smaller shoots bears large leaf-cushions which are seen in longitudinal section to have the form characteristic of Lepidophloios. It is worthy of note that a section of a bifurcating axis of this species from the Calciferous Sandstone of Craigleith (British Museum Collection[363]), although its diameter is 19 × 14 cm., shows no signs of secondary wood. This late appearance of secondary xylem and other anatomical features suggest the possibility of the specific identity of L. Wünschianum and L. Harcourtii[364].
In 1871 Binney[365] described a specimen of a heterosporous cone, Lepidostrobus Wünschianus, from Arran exhibiting the ordinary features of lepidodendroid strobili; this was probably borne by Lepidodendron Wünschianum.
7. Lepidodendron macrophyllum (Williamson). Fig. 186, C.
The diagrammatic sketch reproduced in [fig. 186], C, was made from the transverse section of a small twig, slightly less than 2 cm. in its longest diameter, originally figured by Williamson[366] in 1872. Earlier in the same year Carruthers[367] published a short account of the same form based on specimens collected by Mr Butterworth from the Coal-Measures of Lancashire near Oldham, but both authors refrained from instituting a new specific name. In a later publication Williamson spoke of the type as Lepidodendron macrophyllum[368]. Williamson’s species has nothing to do with Lycopodites macrophyllus of Goldenberg[369]. The most striking feature of this rare form is the large size of the leaf-cushions, which are of the Lepidophloios type, in proportion to the diameter of the shoot. The stele consists of a ring of xylem, all of which is primary in the sections so far described, enclosing a parenchymatous pith: a Stigmarian rootlet is shown at s.