Professor Seward, however, has found a Zygosporites in situ, terminating
an apparently fungal hypha: he suggests a possible comparison
with the mould Mucor. Bodies closely resembling the perithecia of
Sphaeriaceous Fungi have often been observed on impressions of
Palaeozoic plants, and may probably belong to the group indicated.
Professor F. E. Weiss has obtained interesting evidence that the
symbiotic association between roots and Fungi, known as
“Mycorhiza,” already occurred among Carboniferous plants. The few
and incomplete data which we at present possess as to Palaeozoic
Fungi do not as yet justify any inferences as to the evolution of
these plants. The writer is not aware of any evidence for the
occurrence of Palaeozoic Lichens.
The important class of the Bryophyta, which, on theoretical grounds, is commonly regarded as more primitive than the Bryophyta. Pteridophyta, is as yet scarcely represented among known fossils of Palaeozoic age. In the Lower Carboniferous of Scotland Mr Kidston has found several specimens of a large dichotomous thallus, with a very distinct midrib; the specimens, referred to the provisional genus Thallites, much resemble the larger thalloid Liverworts. Similar fossils have been described from still older rocks. In one or two cases Palaeozoic plants, resembling the true Mosses in habit, have been discovered; the best example is the Muscites polytrichaceus of Renault and Zeiller, from the Coal Measures of Commentry. In the absence, however, both of reproductive organs and of anatomical structure, it cannot be said that there is at present conclusive evidence for the existence of either Hepaticae or Musci in Palaeozoic times.
Our knowledge of the Vascular Cryptograms of the Palaeozoic period, though recent discoveries have somewhat reduced their Pteridophyta. relative importance, is still more extensive than of any other class of plants, and in fact it is here that the evidence of Palaeontology first becomes of essential importance to the botanist. They extend back through the Devonian, possibly to the Silurian system, but the systematic summary now to be given is based primarily on the rich materials afforded by the Carboniferous and Permian formations, from which our detailed knowledge of Palaeozoic plants has been chiefly derived.
In addition to the three classes, Equisetales, Lycopodiales and Filicales, under which recent Pteridophytes naturally group themselves, a fourth class, Sphenophyllales, existed in Palaeozoic times, clearly related to the Horsetails and more remotely to the Ferns and perhaps the Club-mosses, but with peculiarities of its own demanding an independent position. We further find that, whereas the Ferns of the present day form a well-defined and even isolated class, this was not the case at the time when the primary rocks were deposited. A great group of Palaeozoic fossils, showing evident affinity to Ferns, has proved to consist of seed-bearing plants allied to Gymnosperms, especially Cycads. This important class of plants will be described at the beginning of the Spermophyta under the name Pteridospermeae. The arrangement which we shall adopt for the Palaeozoic Pteridophyta is therefore as follows:—
| I. | Equisetales. |
| II. | Sphenophyllales. |
| III. | Lycopodiales. |
| IV. | Filicales. |
We must bear in mind that throughout the Palaeozoic period, and indeed far beyond it, vascular plants, so far as the existing evidence shows, were represented only by the Pteridophyta, Pteridosperms and Gymnosperms. Although the history of the Angiosperms may probably go much further back than present records show, there is no reason to suppose that they were present, as such, amongst the Palaeozoic vegetation. Consequently, the Pteridophytes, Gymnosperms and their allies had the field to themselves, so far as regards the higher plants, and filled places in nature which have now for the most part been seized on by families of more modern origin. Hence it is not surprising to find that the early Vascular Cryptograms were, beyond comparison, more varied and more highly organized than their displaced and often degraded successors. It is among the fossils of the Palaeozoic rocks that we first learn the possibilities of Pteridophytic organization.
I. Equisetales. — This class, represented in the recent flora by the single genus Equisetum, with about twenty species, was one of the dominant groups of plants in Carboniferous times. The Calamarieae, now known to have been the chief Palaeozoic representatives of the Horsetail stock, attained the dimensions of trees, reaching, according to Grand' Eury, a height of from 30 to 60 metres, and showed in all respects a higher and more varied organization than their recent successors.
Their remains occur in three principal forms of preservation. (1) carbonaceous impressions of the leafy branches, the fructifications and other parts; (2) casts of the stem; these are usually internal, or medullary casts, as described above. Around the cast the organic tissues may be represented by a carbonaceous layer, on the outer surface of which the external features, such as the remains of leaves, can sometimes be traced. More usually, however, the carbonaceous film is thin, and merely shows the impress of the medullary cast within; (3) petrified specimens of all parts—stem, roots, leaves and fructifications—showing the internal structure, more or less perfectly preserved. The correlation of these various remains presents considerable difficulties. Casts surrounded by wood, with its structure preserved, have sometimes been found, and have established their true relations. The position of the branches is shown both on casts and in petrified specimens, and has helped in their identification, while the petrified remains sometimes show enough of the external characters to allow of their correlation with impressions. Fructifications have often been found in connexion with leafy shoots, and the anatomical structure of the axis in sterile and fertile specimens has proved a valuable means of identification.
In habit the Calamarieae appear to have borne, on the whole, a general resemblance to the recent Equisetaceae, in spite of their enormously greater bulk. The leaves were constantly in whorls, and were usually of comparatively small size and of simple form. In the oldest known Calamarian, however, Archaeocalamites (Devonian and Lower Carboniferous), the leaves were repeatedly forked. There is evidence that in some, at least, of the Calamarieae the leaves of each verticil were united at the base to form a sheath. The free lamina, however, was always considerably more developed than in the recent family; in form it was usually linear or narrowly lanceolate. Different genera have been founded on leaf-bearing branches of Calamarieae; apart from Archaeocalamites, already mentioned, and Autophyllites (Grand' Eury), in both of which the leaves were dichotomous, we have Annularia, Asterophyllites and Calamocladus (in Grand' Eury's limited sense), with simple leaves. In some species of Annularia the extremely delicate ultimate twigs, bearing whorls of small lanceolate leaves, give a characteristic habit, suggesting that they may have belonged to herbaceous plants; other Annulariae, however, have been traced with certainty into connexion with the stems of large Calamites. In Asterophyllites, the generic distinction of which from Annularia is not always clear, the narrow linear leaves are in crowded whorls, and the ultimate branches distichously arranged; in the Calamocladus of Grand' Eury—characteristic of the Upper Coal Measures—the whorls are more remote, and the twigs polystichous in arrangement. In all these groups a leaf-sheath has been recognized.
The distribution of the branches on the main stem shows considerable variations, on which genera or sub-genera have been founded by C. E. Weiss. In Archaeocalamites, which certainly deserves generic rank, the branches may occur on every node, but only in certain parts of the stem; the ribs of successive internodes do not alternate, but are continuous, indicating that the leaves were superposed. Using Calamites as a generic name for all those Calamarian stems in which the ribs alternate at the nodes, we have, on Weiss's system, the following sub-genera: Stylocalamites, branches rare and irregularly arranged; Calamitina, branches in regular verticals, limited to certain nodes, which surmount specially short inter nodes; Eucalamites, branches present on every node. These distinctions can be recognized on petrified specimens, as well as on the casts, but their taxonomic value is somewhat doubtful. In many Calamites there is evidence that the aerial stem sprang from a horizontal rhizome, as in the common species C. (Stylocalamites) Suckowi; in other specimens the aerial stem has an independent, rooting base.
The anatomical structure of all parts of the plant is now known, in various Calamarieae, thanks more especially to the work of Williamson in England and of Renault in France. The stem has a structure which may be briefly characterized as that of an Equisetum with secondary growth in thickness (fig. 1, Plate). The usually fistular pith is surrounded by a ring of collateral vascular bundle, (see Anatomy of Plants, and Pteridophyta), each of which, with rare exceptions, has an intercellular canal at its inner edge, containing the disorganized spiral tracheae, just as in the recent genus. The cortex is often preserved; in certain cases it was strengthened by hypodermal strands of fibres, as in Equisetum. It is only in the rare cases where a very young twig is preserved that the primary structure of the stem is found unaltered. In all the larger specimens a broad zone of wood, with its elements in radial series, had been added. This secondary wood, in the true Calamites (Arthropitys, Goeppert), has a simple structure comparable to that of the simplest Coniferous woods; it is made up
