The question of the recognition of the pith-casts of stems possessing the structure of any of the three subgenera of Calamites is referred to in a later section of this chapter.
b. Leaves
Leaves of Calamites and Calamitean foliage-shoots, including an account of (α) Calamocladus (Asterophyllites) and (β) Annularia.
Our knowledge of the structure and manner of occurrence of Calamite leaves is very incomplete. There are numerous foliage-shoots among the fossils of the Coal-Measures which are no doubt Calamitean, but as they are nearly always found apart from the main branches and stems, it is generally impossible to do more than speak of them as probably the leaf-bearing branches of a Calamite. The familiar fossils known as Asterophyllites, and in recent years often referred to the genus Calamocladus, are no doubt Calamitean shoots; but they are usually found as isolated fragments, and it is seldom that we are able to refer them to definite forms of Calamites. Another common Coal-Measure genus, Annularia, is also Calamitean, and at least some of the species are no doubt leafy shoots of Calamites. Although it is generally accepted that the fossils referred to as Asterophyllites or Calamocladus are portions of Calamites, and not distinct plants, it is convenient, and indeed necessary, to retain such a term as Calamocladus as a means of recording foliage-shoots, which may possess both a botanical and a geological value.
Some of the Calamite casts, especially those referred to the subgenus Calamitina, are occasionally found with leaves attached to the nodes. In some stems the leaves are arranged in a close verticil, and each leaf has a narrow linear form and is traversed by a single median vein. Figures of Calamite stems with verticils of long and narrow leaves may be found in Lindley and Hutton[636], and in the writings of many other authors[637]. In the specimen shown in fig. 85 the leaves are preserved apart from the stem, but from their close association with a Calamite cast, and from the proofs afforded by other specimens, it is quite certain they formed part of a whorl of leaves attached to the node of a true Calamite, and a stem having that particular type known as Calamitina[638] (figs. 99, 100). It is probable that in some Calamites, and especially in younger shoots, the leaves had the form of narrow sheaths split up into linear segments. This question has already been referred to in dealing with certain Palaeozoic fossils referred to Equisetites[639].
Fig. 85. Linear leaves of a Calamite (Calamitina). After Weiss, slightly reduced.
A few years ago the late Thomas Hick[640], of Manchester, described the structure of some leaves which he believed to be those of a Calamite. He found them attached to a slender axis which possessed the characteristics of a young Calamite branch. There can be little doubt that his specimens are true Calamite leaves. The sketches of fig. 86 have been made from the sections originally described by Hick. Fig. 86, 1 shows a leaf in transverse section; on the outside there is a well-defined epidermal layer with a limiting cuticle. Internal to this we have radially elongated parenchymatous cells forming a loose or spongy tissue, the cells being often separated by fairly large spaces (fig. 86, 5), especially in the region of the blunt lateral wings of the leaf. Some of these cells contain a single dark dot, which in all probability is the mineralised nucleus. These pallisade-like cells probably contained chlorophyll and constituted the assimilating tissue of the leaf. In the centre there is a circular strand of cells limited by a layer of larger cells with black contents, enclosing an inner group of small-celled parenchyma and traversed by a few spiral or scalariform tracheids constituting the single median vein. It is hardly possible to recognise any phloem elements in the small vascular bundle; there appear to be a few narrow tracheids surrounded by larger parenchymatous elements (fig. 86, 2). At one point in the epidermis of fig. 86, 1, there appears to be a stoma, but the details are not very clearly shown (fig. 86, 4); the two cells, s, s, bordering the small aperture are probably guard-cells.
Fig. 86. A leaf of Calamites.