Page:EB1911 - Volume 20.djvu/579

PALAEOZOIC] entirely of radial bands of tracheides interspersed with medullary rays. The pitting of the tracheides is more or less scalariform in character, and is limited to the radial walls. In favourable cases remains of the cambium are found on the outer border of the wood, and phloem is also present in the normal position, though it does not seem to have attained any considerable thickness. In the old stems the primary cortex was replaced by periderm, giving rise to a thick mass of bark. The above description applies to the stems of Calamites in the narrower sense (Arthropitys of the French authors), to which the specimens from the British Coal Measures mostly belong. Archaeocalamites appears to have had a similar structure, but in some specimens from the Lower Carboniferous of Burntisland, provisionally named Protocalamites pettycurensis, centripetal wood was present in the stem. In Calamodendron (Upper Coal Measures) the wood has a more complex structure than in Calamites, the principal rays including radial tracts of fibrous tissue, in addition to the usual parenchyma. Arthrodendron (Lower Coal Measures) approaches Calamodendron in this respect. The longitudinal course of the vascular bundles and their relation to the leaves in Calamarieae generally followed the Equisetum type, though more variable and sometimes more complex. The attachment of the branches was immediately above the node, and usually between two foliar traces, as in the recent genus. Where the structure of the leaves is preserved it proves to be of an extremely simple type; the narrow lamina is traversed by a single vascular bundle, separated by a sheath from the surrounding palisade-parenchyma. Stomata of the same structure as in Equisetum have been detected in the epidermis. The roots (formerly described as a separate genus, Astromyelon) were borne directly on the nodes, not on short lateral branches as in Equisetum. They are of similar structure in all known Calamarieae, the main roots having a large pith, while the rootlets had little or none. The structure is in all respects that typical of roots, as shown by the centripetal primary wood, and the alternation of xylem and phloem groups observable in exceptionally favourable young specimens. A striking feature is the presence of large, radiating inter cellular cavities in the cortex, suggesting an aquatic habit. The young roots show a double endodermis, just as in the recent Equisetum.

A considerable number of Calamarian fructifications are known, preserved, some as carbonaceous impressions, others as petrified specimens, exhibiting the internal structure. In many cases the cones have been found in connexion with branches bearing characteristic Calamarian foliage. Almost all strobili of the Calamarieae are constructed on the same general lines as those of Equisetum, with which some agree exactly; in most, however, the organization was more complex, the complexity consisting in the intercalation of whorls of sterile bracts, between those of the sporangiophores. In several cases heterospory, unknown among recent Equisetaceae, has been demonstrated in their Palaeozoic representatives.

Four main types of structure may be distinguished among Calamarian strobili. 1. Calamostachys, Schimper. Here the whorls of peltate sporangiophores alternate regularly with those of sterile bracts, the former being inserted on the axis midway between the latter (fig. 2). The sporangiophores, which are usually half as numerous in each verticil as the bracts, have the same form as in Equisetum, but each bears four sporangia only. The spores are frequently found to be still united in tetrads. In some species, e.g. the British C. Binneyana, numerous specimens have been examined and only one kind of spore observed; here, then, there is a strong presumption that the species was homosporous. In other cases, however, e.g. C. Casheana, Will., two kinds of spore occur, in different sporangia, but on the same strobilus and even on the same sporangiophore. The megaspores, of which there are many in the megasporangium, have a diameter about three times that of the microspores. The abortion of certain spores, which is known to have taken place both in the homosporeus C. Binneyana and in the megasporangia of C. Casheana, may throw some light on the origin of the heterosporous condition. The bracts were sometimes coherent in their lower part (e.g. C. Binneyana), sometimes free (e.g. C. Ludwigi); in all cases their free extremities formed a protection to the fertile whorl above. In some continental species (e.g. C. Grand' Euryi, Ren.) radial membranous plates hung down from each verticil of bracts, forming compartments in which the subjacent sporangiophores were enclosed. The anatomy of the axis is essentially similar to that of a young Calamarian twig, with some variations in detail. Strobili of the Calamostachys type occur in connexion both with Annularia and Asterophyllites foliage.

2. Palaeostachya, Weiss. Here, as in the previous genus, sterile and fertile verticils are ranged alternately on the axis of the cone. The main difference is that in Palaeostachya the sporangiophores, instead of standing midway between the whorls of bracts, are inserted immediately above them, springing, as it were, from the axil of the sterile verticil (fig. 3, A). This singular arrangement has suggested doubts as to the correctness of the current interpretation of the Equisetaceous sporangiophore as a modified leaf (cf. Cheirostrubus below). In most other respects the two genera agree; there is evidence for the occurrence of heterospory in some strobili referred to Palaeostachya. The anatomy of the axis is that of a young branch of a Calamite. According to Grand' Eury, the Palaeostachya fructification was most commonly associated with Asterophyllites foliage. The external aspect of a Palaeostachya is shown in fig. 4 (Plate).

3. Equisetum type of strobilus. In certain cases the strobili of Palaeozoic Calamarieae appear to have had essentially the same organization as in the recent genus, the axis bearing sporangiophores only, without intercalated bracts. It is remarkable that fructifications apparently of this kind have been found by Renault in close association with the most ancient of the Calamarieae—Archaeocalamites. In these strobili the peltate scales, like the vegetative leaves of the plant, are in superposed verticils; each appears to have borne four sporangia (fig. 3, B). Other cones, however, namely, those known as Pothocites, have also been attributed on good grounds to the genus Archaeocalamites; they are long strobili, constricted at intervals, and it is probable that the succession of fertile sporangiophores was interrupted here and there by the intercalation of sterile bracts, which may also have been present, at long intervals, in Renault's species. Cones from the Middle Coal Measures, described by Kidston under the name of Equisetum Hemingwayi, but probably belonging to one of the Calamarieae, bear a striking external resemblance to those of a recent Equisetum.

4. Cingularia, Weiss. This form of strobilus, from the Coal Measures of Germany, is imperfectly known, and its relation to Calamarieae not beyond doubt. In the lax strobili the sporangiophores, which are not peltate, but strap-shaped, were borne, as C. E. Weiss first showed, immediately below the verticils of bracts, the position thus being the reverse of that in Palaeostachya.

The Palaeozoic Calamarieae, though so far surpassing recent Equisetaceae, both in stature and complexity of organization, clearly belonged to the same class of Vascular Cryptogams. There is no satisfactory evidence for attributing Phanerogamic