Introduction to the Study of Palæontological Botany

by John Hutton Balfour

For a fuller account of Acrogenous plants, see Balfour's Class Book of Botany, p. 954.

These orders are represented in the Palæozoic flora. Many of the fossil species assume a large size, and show a greater degree of development than is seen in their recent congeners. The most important coal plants belong to the Ferns, Lycopods, and Horse-tails. The examination of the structure and conformation of the plants of the present flora assists much in the determination of the fossil carboniferous flora.

Fig. 22.

Fig. 22. Marsilea Fabri, a species of Pepperwort or Rhizocarp, with a creeping stem, quadrifoliate stalked leaves on one side, and roots on the other. The fructification, s, is at the base of the leaves, and consists of sporangia, called sporocarps.

In the lower Palæozoic strata the plants which have been detected are few. In the Silurian and Cambrian systems, we meet with the remains of ancient marine plants, as well as a few terrestrial species. Even in the still older Laurentian rocks, if the remarkable structure known as Eozoon canadense be considered, as it generally is, an animal, the existence of contemporary plants may be inferred, inasmuch as without vegetable life animals could not obtain food. In the Lower Silurian or Grauwacke, near Girvan, Hugh Miller found a species resembling Zostera in form and appearance. In the Lower Old Red Sandstone of Scotland he detected Fucoids, a Lepidodendron, and Lignite with a distinct Coniferous structure resembling that of Araucaria,[1] besides a remarkable pinnate frond. In the middle Old Red of Forfarshire, as seen in the Arbroath pavement, he found a fern with reniform pinnæ and a Lepidodendron. In the Upper Old Red, near Dunse, a Calamite and the well-known Irish fern Cyclopteris Hibernica occur.[2] This fern, Palæopteris Hibernica of Schimper ([Plate I. Figs. 1 to 4]), along with Sigillaria dichotoma, is very abundant in beds of the same age in the south of Ireland, from which the specimens described by Edward Forbes were obtained. The fructification has recently been discovered. This shows that the fern belongs to the Hymenophylleæ, and is consequently nearly related to the equally famous Killarney fern, Trichomanes radicans.

Mr. Carruthers states that the frond-stalk of this fern is thick, of considerable length, and clothed with large scales, which form a dense covering at the somewhat enlarged base. The well-defined separation observed in several specimens probably indicates that the frond-stalks were articulated to the stem or freely separated from it, and some root-like structures which occur on the slabs with the ferns may be their creeping rhizomes. The pinnæ are linear, obtuse, and almost sessile. The pinnules are numerous, overlapping, of an ovate or oblong-ovate form, somewhat cuneate below, and with a decurrent base. The veins are very numerous, uniform, repeatedly dichotomous, and run out to the margin, where they form a slight serration. Single pinnules rather larger than those of the pinnæ are placed over the free spaces of the rachis, as was pointed out by Brongniart. Carruthers has not met with any recent fern in which this occurs; but it has been observed in several fossil species, as in the allied American Palæopteris Halliana (Sch.), in Sphenopteris erosa (Morris), and others. The pinnules are sometimes entirely, but only partially fertile. The ovate-oblong sori are generally single and two-lipped, the slit passing one-third of the way down the sorus. The vein is continued as a free receptacle in the centre of the cup or cyst, as in existing Hymenophylleæ, in which it is included, not reaching beyond its entire portion. In some specimens the receptacle is broad or thick, indicating the presence of something besides itself in the cup, and giving the appearance that would be produced if it were covered with sporangia; there is no indication on the outer surface which might have been expected from the separate sporangia. The compression of the specimens in the rock, which has made the free receptacle appear like a vein on the wall of the cup, together with the highly altered condition of the rock in which the fossils are contained, accounts for the imperfect preservation of the minute structures. The interpretation here given of the fructification of this interesting fossil exhibits so close a resemblance to what we find in the living genus Hymenophyllum, that, were it not for the vegetative portions, it would be placed in that genus. Several ferns have been described by Bunbury from Devonian rocks at Oporto. A still more extensive and varied land flora of Devonian age (or Erian, as he calls it) has been described and illustrated by Principal Dawson from the rocks of that period occurring in Canada; and during a recent visit to Britain he has correlated many of the fragments collected by Miller, Peach, and others, with the American species he has described. The following are some of the fossil plants from beds older than the Carboniferous system:[3]—Prototaxites Logani, Dadoxylon Ouangondianum, Calamites transitionis, Asterophyllites parvulus, Sphenophyllum antiquum, Lepidodendron Gaspianum, Lepidostrobus Richardsoni, L. Matthewi, Psilophyton princeps, P. robustius, Selaginites formosus, Cordaites Robbii, C. angustifolius, Cyclopteris Jacksoni.

From the microscopic examination of the structure of specimens of fossil trunks described under the name of Prototaxites Logani, and which Principal Dawson believes to be the oldest known instance of Coniferous wood, Mr. Carruthers has come to the conclusion that they are really the stems of huge Algæ, belonging to at least more than one genus. They are very gigantic when contrasted with the ordinary Algæ of our existing seas, nevertheless some approach to them in size is made in the huge and tree-like Lessonias which Dr. Hooker found in the Antarctic Seas, and which have stems about 20 feet high, with a diameter so great that they have been collected by mariners in these regions for fuel, under the belief that they were drift-wood. They are as thick as a man's thigh. Schimper regards the Psilophyton of Dawson ([Plate IV. Fig. 5]) as allied to Pilularia, one of the Rhizocarps (Fig. 22), and Carruthers places it among the true Lycopodiaceæ.

[Flora of the Carboniferous Epoch.]

The Carboniferous period is one of the most important as regards fossil plants. The vegetable forms are numerous, and have a great similarity throughout the whole system, whether exhibited in the Old or the New World. The important substance called Coal owes its origin to the plants of this epoch. It has been subjected to great pressure and long-continued metamorphic action, and hence the appearance of the plants has been much altered. It is difficult to give a definition of Coal. The varieties of it are numerous. There is a gradual transition from Anthracite to Household and Parrot Coal; and the limit between Coal and what is called bituminous shale is by no means distinct. Coal may be said to be chemically-altered vegetable matter inter-stratified with the rocks, and capable of being used as fuel. On examining thin sections of coal under the microscope, we can detect vegetable tissues both of a cellular and vascular nature. In Wigan cannel coal, vegetable structure is seen throughout the whole mass. Such is likewise the case with other cannel, parrot, and gas coals. In common household coal, also, evident traces of organic tissue have been observed. In some kinds of coal punctated woody tissue ([Plate III. Fig. 5]) has been detected, in others scalariform tissue ([Plate III. Fig. 6]), as well as cells of different kinds. Sporangia are also frequently found in the substance of coal, as shown by Mr. Daw in that from Fordel ([Plate III. Figs. 1 to 3]); and some beds, like the Better bed of Bradford, are composed almost entirely of these sporangia imbedded in their shed microspores, as has been recently shown by Huxley. The structure of coal in different beds, and in different parts of the same bed, seems to vary according to the nature of the plants by which it has been formed, as well as to the metamorphic action which it has undergone. Hence the different varieties of coal which are worked. The occurrence of punctated tissue indicates the presence of Coniferæ in the coal-bed, while scalariform vessels point to ferns, and their allies, such as Sigillaria and Lepidodendron. The anatomical structure of the stems of these plants may have some effect on the microscopic characters of the coal produced from them. In some cannel coals structure resembling that of Acrogens has been observed. A brownish-yellow substance is occasionally present, which seems to yield abundance of carburetted hydrogen gas when exposed to heat.