Thus we may feebly attempt to picture to ourselves one of the many types of Calamite trees in a Palaeozoic forest, growing in a swampy marsh or on gently sloping ground on the shores of an inland sea, into which running water carried its burden of sand and mud, and broken twigs of Calamites and other trees which contributed to the Coal Period sediments. The large proportions of a Calamite tree are strikingly illustrated by some of the broad and long pith-casts occasionally seen in Museums; in the Breslau Collection there is a cast of a stem belonging to the sub-genus Calamitina, which measures about 2 m. in length and 23 cm. in breadth, with 36 nodes. In the Natural History Museum, Paris, there is a cast nearly 2 metres long and more than 20 cm. wide, which is referred to the sub-genus Calamodendron.
E. Archaeocalamites.
In the Upper Devonian and Culm rocks casts of a well-defined Calamitean plant are characteristic fossils; stems, leaf-bearing branches, roots and cones have been described by several authors, and the genus Archaeocalamites has been instituted for their reception. Although this genus agrees in certain respects with Calamites, and as recent work has shown this agreement extends to internal structure, it has been the custom to regard the Lower Carboniferous and Devonian plants as genetically distinct. The surface features of the stem-casts, the form of the leaves, and apparently the cones, possess certain distinctive characters which would seem to justify the retention of a separate generic designation.
We may briefly summarise the characteristics of the genus as follows:—
Pith-casts articulated, with very slightly constricted nodes; the internodes traversed by longitudinal ribs slightly elevated or almost flat, separated by shallow grooves. The ribs and grooves are continuous from one internode to another, and do not usually show the characteristic alternation of Calamites[834]. Along the nodal line there are occasionally found short longitudinal depressions, probably marking the points of origin of outgoing bundles. Branches were given off from the nodes without any regular order; a pith-cast may have branch-scars on many of the nodes, or there may be no trace of branches on casts consisting of several nodes. The leaves[835] are in whorls; in some cases they occur as free, linear, lanceolate leaves, or on younger branches they are long, filiform and repeatedly forked. The structure of the wood agrees with that of some forms of Arthropitys. The strobili consist of an articulated axis bearing whorls of sporangiophores, and each sporangiophore has four sporangia. Our knowledge of the fertile shoots is, however, very imperfect.
Renault[836] has recently described the structure of the wood in some small silicified stems of Archaeocalamites from Autun. A large hollow pith is surrounded by a cylinder of wood consisting of wedge-shaped groups of xylem tracheids associated with secondary medullary rays; at the apex of each primary xylem group there is a carinal canal. The primary medullary rays appear to have been bridged across by bands of xylem at an early stage of secondary thickening, as in the Calamite of fig. 83, D.
Fig. 103. Archaeocalamites scrobiculatus (Schloth.).
From a specimen in the Woodwardian Museum, Cambridge. From the Carboniferous limestone of Northumberland. ½ nat. size.
Our knowledge of the cones of Archaeocalamites is far from satisfactory. Renault[837] has recently described a small fertile branch bearing a succession of verticils of sporangiophores; each sporangiophore stands at right angles to the axis of the cone and bears four sporangia, as in Calamostachys. It is not clear how far there is better evidence than that afforded by the association of the specimen with pith-casts of stems, for referring this cone to Archaeocalamites, but the association of vegetative and fertile shoots certainly suggests an organic connection. The cone described by the French author agrees with Equisetum in the absence of sterile bracts between the whorls of sporangiophores. It is an interesting fact that such a distinctly Equisetaceous strobilus is known to have existed in Lower Carboniferous rocks.
Stur[838] has also described Archaeocalamites at considerable length; he gives several good figures of stem-casts and foliage-shoots bearing long and often forked narrow leaves. The same writer describes specimens of imperfectly preserved cones in which portions of whorls of forked filiform leaves are given off from the base of the strobilus[839]. Kidston[840] published an important memoir on the cones of Archaeocalamites in 1883, in which he advanced good evidence in support of the view that certain strobili, which were originally described as Monocotyledonous inflorescences, under the generic name Pothocites[841], are the fertile shoots of this Calamarian genus. Kidston’s conclusions are based on the occurrence on the Pothocites cones, of leaves like those of Archaeocalamites, on the non-alternation of the sporangiophores of successive whorls, and on the close resemblance between his specimens and those described by Stur. Good specimens of the cones, formerly known as Pothocites, may be seen in the Botanical Museum in the Royal Gardens, Edinburgh; as they are in the form of casts without internal structure it is difficult to form a clear conception as to their morphological features.