Fossil plants, Vol. 1

In the above account of a few common pith-casts it has been pointed out that there is occasionally satisfactory evidence for connecting certain casts with wood of a particular structure, and with sterile and fertile foliage-shoots of a definite type. It is, however, impossible in many cases to recognise with any certainty the leaf-bearing branches and strobili of the different casts of Calamites; it is equally impossible to determine what type of pith-cast or what type of foliage-shoots belongs to petrified stem-fragments in which it is possible to investigate the microscopical features. The scattered and piece-meal nature of the material on which our general knowledge of Calamitean plants is based, necessitates a system of nomenclature which is artificial and clumsy; but the apparent absurdity of attaching different names to fragments, which we believe to be portions of the same genus, is of convenience from the point of view of the geologist and the systematist. As our material increases it will be possible to further simplify the nomenclature for Calamarian plants, but it is unwise to allow our desire for a simpler terminology to lead us into proposals which are based rather on suppositions than on established fact. If it were possible to discriminate between pith-casts of stems having the different anatomical characters designated by the three sub-genera, Arthropitys, Arthrodendron and Calamodendron, the genus Calamites might be used in a much narrower and probably more natural sense than that which we have adopted. The tests made use of by some authors for separating pith-casts of Calamodendron and Arthropitys stems do not appear to be satisfactory; we want some term to apply to all Calamitean casts irrespective of the anatomical features of the stems, or of the precise nature of the foliage-branches. As used in the present chapter, Calamites stands for plants differing in certain features but possessing common structural characters, which must be defined in a broad sense so as to include types which may be worthy of generic rank, but which for convenience sake are included in a comprehensive generic name. The attempts to associate certain forms of foliage with Arthropitys on the one hand and with Calamodendron on the other, cannot be said to be entirely satisfactory; we still lack data for a trustworthy diagnosis of distinct Calamarian genera which shall include external characters as well as histological features. If we restricted the genus Calamites to stems with an Arthropitys structure and an Asterophyllitean foliage, we should be driven into unavoidable error. Within certain limits it is possible to distinguish generically or even specifically between petrified branches, and we already possess material enough for fairly complete diagnoses founded on internal structure; but it is not possible to make a parallel classification for pith-casts and foliage-shoots. For this reason, and especially bearing in mind the importance of naming isolated foliage-shoots and stem-casts for geological purposes, I believe it is better to admit the artificially wide application of the name Calamites, and to express more accurate knowledge, where possible, by the addition of a subgeneric term. In dealing with distinctions exhibited by Calamitean stems it may be advisable to make use of specific names, but we must keep before us the probability of the pith-cast and petrified stem-fragment of the same plant receiving different specific names. If the structural type is designated by a special sub-genus, this will tend to minimise the anomaly of using more than one binominal designation for what may be the same individual.

The following summary may serve to bring together the different generic and subgeneric terms which have been used in the foregoing account of Calamites.

IV. Conclusion.

A brief sketch of the main features of Calamites suffices to bring out the many points of agreement between the arborescent Calamite plants and the recent Equisetums. The slight variation in morphological character among the present-day Horse-tails, contrasts with the greater range as regards structural features among the types included in Calamites. The Horse-tails probably represent one of several lines of development which tend to converge in the Palaeozoic period; the Calamite itself would appear to mark the culminating point of a certain phylum of which we have one degenerate but closely allied descendant in the genus Equisetum. We shall, however, be in a better position to consider the general question of plant-evolution after we have made ourselves familiar with other types of Palaeozoic plants. Grand’Eury’s[833] striking descriptions of forests of Calamites in the Coal-Measures of central France, enable us to form some idea of the habit of growth of these plants with their stout branching rhizomes and erect aerial shoots.

By piecing together the evidence derived from different sources we may form some idea of the appearance of a living Calamite. A stout branching rhizome ascended obliquely or spread horizontally through sand or clay, with numerous whorls or tufts of roots penetrating into swampy soil. From the underground rhizome strong erect branches grew up as columnar stems to a height of fifty feet or more; in the lower and thicker portions the bark was fissured and somewhat rugged, but smoother nearer the summit. Looking up the stem we should see old and partially obliterated scars marking the position of a ring of lateral branches, and at a higher level tiers of branches given off at regular or gradually decreasing intervals, bearing on their upper portions graceful green branchlets with whorls of narrow linear leaves. On the younger parts of the main shoot rings of long and narrow leaves were borne at short intervals, several leaf-circles succeeding one another in the intervals between each radiating series of branches. On some of the leaf-bearing branchlets long and slender cones would be found here and there taking the place of the ordinary leafy twigs. Passing to the apical region of the stem the lateral branches given off at a less and less angle would appear more crowded, and at the actual tips there would be a crowded succession of leaf-segments forming a series of overlapping circles of narrow sheaths with thin slender teeth bending over the apex of the tree.

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.

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.

The fossils included under Archaeocalamites have been referred by different authors to various genera, and considerable confusion has arisen in both generic and specific nomenclature. The following synonomy of the best known species, A. scrobiculatus (Schloth.) illustrates the unfortunate use of several terms for the same plant.

For other lists of synonyms reference may be made to Binney[851], Stur[852], Kidston[853] and other authors.

Some of the best specimens of this species are to be seen in the Museums of Breslau and Vienna, which contain the original examples described by Göppert and Stur. An examination of the original specimens, figured by Göppert under various names, enables one to refer them with confidence to the single species, Archaeocalamites scrobiculatus. The generic name Archaeocalamites, which has been employed by some authors, was suggested by Schimper[854] in 1862, as a subgenus of Calamites, on account of the occurrence of a deeply divided leaf-sheath, attached to the node of a pith-cast, which seemed to differ from the usual type of Calamitean leaf. The specimens described by Schimper are in the Strassburg Museum; the leaf-sheath which he figures is not very accurately represented.

The example given in fig. 103 shews very clearly the continuous course of the ribs and grooves of the pith-cast. Each rib is traversed by a narrow median groove which would seem to represent the projecting edge of some hard tissue in the middle of each principal medullary ray of the stem. The specimen was found in a Carboniferous limestone quarry, Northumberland; there is a similar cast from the same locality in the Museum of the Geological Survey.

This genus agrees very closely with Calamites both in the anatomical structure of the stem and in the verticillate disposition of the leaves. The strobili appear to be Equisetaceous in character, and there is no satisfactory evidence of the existence of whorls of sterile bracts in the cone, such as occur in Calamostachys and in other Calamitean strobili. The continuous course of the vascular bundles of the stem from one internode to the next is the most striking feature in the ordinary specimens of the genus; but it sometimes happens that the grooves on a pith-cast shew the same alternation at the node as in Calamites. This is the case in a specimen in the Göppert collection in the Breslau Museum, and Feistmantel[855] has called attention to such an alternation in specimens from Rothwaltersdorf. In the true Calamites, on the other hand, the usual nodal alternation of the vascular strands is by no means a constant character[856]. Stur[857], Rothpletz[858], and other authors have pointed out the resemblance of Archaeocalamites to Sphenophyllum. The deeply divided leaves of some Sphenophyllums and those of Archaeocalamites are very similar in form; and the course of the vascular strands in Sphenophyllum may be compared with that in Archaeocalamites. But the striking difference in the structure of the stele forms a wide gap between the two genera. We have evidence that the Calamites and Sphenophyllums were probably descended from a common ancestral stock, and it may be that in Archaeocalamites, some of the Sphenophyllum characters have been retained; but there is no close affinity between the two plants.

On the whole, considering the age of Archaeocalamites and the few characters with which we are acquainted, it is probable that this genus is very closely related to the typical Calamites, and may be regarded as a type which is in the direct line of development of the more modern Calamite and the living Equisetum. Weiss[859] includes Archaeocalamites as one of his subgenera with Calamitina and others, and it is quite possible that the genus has not more claim to stand alone than other forms at present included in the comprehensive genus Calamites.

The student will find detailed descriptions of this genus in the works which have been referred to in the preceding pages.