Part 9

Coniferæ. Taxaceæ. Gnetaceæ. Cycadaceæ.

Among Petaloid Monocotyledons--

Smilaceæ. Orchidaceæ. Zingiberaceæ. Musaceæ. Liliaceæ. Palmæ. Pandanaceæ. Araceæ. Typhaceæ. Naiadaceæ. Restiaceæ.

Among Glumiferous Monocotyledons--

Cyperaceæ. Gramineæ.

Among Acrogenous Acotyledons--

Filices. Marsileaceæ. Lycopodiaceæ. Equisetaceæ. Musci. Hepaticæ.

Among Thallogenous Acotyledons--

Lichenes. Characeæ. Algæ. Fungi.

PERIODS OF VEGETATION AMONG FOSSIL PLANTS.

On taking a general survey of the known fossil plants, Brongniart thought that he could trace three periods of vegetation, characterised by the predominance of certain marked forms of plants. In the ancient period there is a predominance of Acrogenous Cryptogamic plants; this is succeeded by a period in which there is a preponderance of Gymnospermous Dicotyledons; while a third period is marked by the predominance of Angiospermous Dicotyledons. There is thus--1. The reign of Acrogens, which includes the plants of the Devonian, Carboniferous, and Permian periods. During these periods there seems to be a predominance of Ferns, and a great development of arborescent Lycopodiaceæ, such as Lepidodendron and Sigillaria, and with them are associated some Gynmosperms, allied to Araucaria, and some anomalous plants, as Noeggerathia. 2. The reign of Gymnosperms, comprehending the Triassic and Jurassic periods. Here we meet with numerous Coniferæ and Cycadaceæ, while Ferns are less abundant. 3. The reign of Angiosperms, embracing the Cretaceous and the Tertiary periods. This is characterised by the predominance of Angiospermous Dicotyledons, a class of plants which constitute more than three-fourths of the present vegetable productions of the globe, and which appear to have acquired a predominance from the commencement of the Tertiary formations. These plants appear sparingly even at the beginning of the chalk formation in Europe, but are more abundant in this formation as developed in North America.

FLORA OF THE PRIMARY OR PALÆOZOIC PERIOD.

REIGN OF ACROGENS.

In the present day, acrogenous plants are represented by cellular and vascular Cryptogams. In considering fossil plants our attention is specially directed to the latter. In the recent Floras, vascular Acrogens are represented by such plants as Ferns, Lycopods, and Equisetums. Some of them have an arborescent habit, but the greater number are shrubby and herbaceous. Many of them have creeping rhizomes, which are either subterranean, or run along the surface of the ground. One of these arborescent forms is seen in Tree-ferns (Fig. 9). Another form with a rhizome is seen in Fig. 10. The trunks of ferns are marked by scars, which indicate the parts where the bases of the fronds were attached, and where the vascular tissue passes out from the interior (Fig. 11, a and b). A transverse section of the stem (Fig. 12) shows a continuous cylinder of scalariform vessels (Fig. 13), enclosing a large mass of cellular tissue frequently penetrated by small scalariform bundles. The cylinder is pierced by meshes, from the inner sides of which rise the vascular bundles going to the leaves, while some of the free bundles of the axis pass through the mesh, carrying with them a portion of the cellular tissue into the petiole. The fructification consists of spore-cases (sporangia), often with an elastic ring round them, containing spores in their interior (Fig. 14).

Among Acrogens of the present day there are also plants belonging to the natural order Lycopodiaceæ or Club-mosses (Fig. 15), having creeping stems, which give rise to leafy branches. The leaves are small, sessile, and moss-like, and the fructification consists of two kinds of cellular bodies, small spores or microspores (Fig. 16), and large spores or macrospores (Fig. 17). They consist of cellular and vascular tissues, the latter occurring in the form of woody, annular, and scalariform vessels, which occupy the axis or central part of the stem. They differ from ferns in the distribution of their vascular bundles. The order is represented also by such plants as Selaginella, Psilotum, Phylloglossum, and Isoetes. In the plant called Isoetes (Quillwort) there is a peculiar short stem which does not increase in height. It produces additions laterally, so that the stem increases in thickness. The leaves continue to multiply, and bear fructification at their bases. They have both large and small spores.

Fig. 14. Sporangia of a Fern, supported on stalks, p, each of which ends in an elastic cellular ring, s, partially surrounding the spore-case, and opening it when mature.

Fig. 15. Lycopodium clavatum, a common Club-moss. The leafy branch, l, ends in a stalk bearing two spikes of fructification, f.

Fig. 16. A kidney-shaped 2-valved case, containing small spores (microspores) of Lycopodium.

Fig. 17. Two-valved case, containing large spores (macrospores) of Selaginella.]

Another important order of vascular Acrogens is the Equisetaceæ or Horse-tails (Fig. 18). These are Cryptogams, having rhizomes, bearing hollow, striated branches, which secrete in their epidermis a considerable amount of silex. These branches are jointed and have membranous sheaths at the articulations, which are whorls of leaves reduced to a very rudimentary condition. The fructification consists of cone-like bodies (Fig. 18, f) bearing peltate polygonal scales, under which are spore-cases (Fig. 19), enclosing spores with four hygrometric club-shaped filaments called elaters (Figs. 20 and 21). At the present day some of these plants in tropical regions have stems of 15 or 16 feet high.

Fig. 20. Spore of Equisetum, surrounded by two filaments with club-shaped extremities. The filaments are represented as coiled round the spore.

Fig. 21. Spore of Equisetum, with the filaments (elaters) expanded.]

Among vascular Acrogens is included the natural order Marsileaceæ or Rhizocarpeæ, the Pepperworts (Fig. 22). The order consists of aquatic plants, with creeping stems, bearing leaves, which are either linear, or divided into three or more wedge-shaped portions not unlike clover. The fructification is at the base of the leaf-stalks, and consists of sacs (sporocarps) containing spores of two kinds, microspores and macrospores. The order contains Marsilea, Pilularia, Azolla, and Salvinia.

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.

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, 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. 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:--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.