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.

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.

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,[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.

It appears that in general each bed of coal is accompanied by the remains of a somewhat limited amount of species. Their number, particularly in the most ancient beds, is scarcely more than eight or ten. In other cases the number is more considerable, but rarely more than thirty or forty. In the same coal-basin each layer often contains several characteristic species which are not met with either in the beds above or below. Thus, there are sometimes small local or temporary floras, each of which has given birth to layers of coal. The quantity of carbonaceous and other matter required to form a bed of coal is immense. Maclaren has calculated that one acre of coal three feet thick is equal to the produce of 1940 acres of forest.[4] The proportion of carbon varies in different kinds of coal. Along with it there is always more or less of earthy matter which constitutes the ashes. When the earthy substances are in such quantity that the coaly deposit will not burn as fuel, then we have what is called a shale. The coal contains plants similar to those of the shales and sandstones above and below it. Underneath a coal-seam lies the Underclay, containing roots only, and representing the ancient soil; then comes the Coal, composed of plants whose roots are in the clay, with others which have grown along with and upon them, in a manner precisely similar to the growth of peat at the present day; while above the coal is the Shale, marking how mud was laid down on the plants, and bearing evidences of vigorous vegetation on neighbouring land, from which currents brought down the fine sediment, mingled with broken pieces of plants.

The total thickness of coal in the English coal-fields is about 50 or 60 feet. In the Mid-Lothian field there are 108 feet of coal. Coal-beds are worked at 1725 feet below the sea-level, and probably extend down to upwards of 20,000 feet. They rise to 12,000 feet above the sea-level, and at Huanuco, in Peru, to 14,700.[5] It is said that the first coal-works were opened at Belgium in 1198, and soon after in England and Scotland; it was not till the fifteenth century that they were opened in France and Germany.

The following calculations have been made as to the extent of the coal formation in different countries, and the amount of coal raised:—[6]

The total quantity of coal annually raised over the globe appears thus to be about 100 millions of tons, of which the produce of Great Britain is more than two-thirds, and would be sufficient to girdle the earth at the equator with a belt of 3 feet in thickness and nearly 5 feet in width. The coal-fields of the United States are nearly forty times larger than those of Great Britain.

Roscoe gives the following estimated quantities of coal in the principal countries:—

Unger enumerates 683 plants of the coal-measures, while Brongniart notices 500. Of the last number there are 6 Thallogens, 346 Acrogens, 135 Gymnosperms, and 13 doubtful plants. This appears to be a very scanty vegetation, as far as regards the number of species. It is only equal to about ¹/₂₀th of the number of species now growing on the surface of the soil of Europe. Although, however, the number of species was small, yet it is probable that the individuals of a species were numerous. The proportion of Ferns was very large. There are between 200 and 300 enumerated. Schimper thinks there are 7 species congeneric with Lycopodium found in the coal-measures. The following are some of the Cryptogamous and Phanerogamous genera belonging to the flora of the Carboniferous period:—Cyclopteris, Neuropteris, Odontopteris, Sphenopteris, Hymenophyllites, Alethopteris, Pecopteris, Coniopteris, Cladophlebis, Senftenbergia, Lonchopteris, Glossopteris, Caulopteris, Lepidodendron (Lepidostrobus, Lepidophyllum, Knorria), Flemingites, Ulodendron, Halonia, Psaronius, Sigillaria and Stigmaria, Calamites (Asterophyllites and Sphenophyllum), Noeggerathia, Walchia, Peuce, Dadoxylon, Pissadendron, Trigonocarpum.

Ferns are the carboniferous fossil group which present the most obvious and recognisable relationship to plants of the present day. While cellular plants and those with lax tissues have lost their characters by the maceration to which they were subjected before fossilisation took place, ferns are more durable, and retain their structure. It is rare, however, to find the stalk of the frond completely preserved down to its base. It is also rare to find fructification present. In this respect, fossil Ferns resemble Tree-ferns of the present day, the fronds of which rarely exhibit fructification. Hooker states that of two or three kinds of New Zealand Tree-fern, not one specimen in a thousand bears a single fertile frond, though all abound in barren ones. Only one surface of the fossil Fern-frond is exposed, and that generally the least important in a botanical point of view. Fructification is sometimes evidently seen, as figured by Corda in Senftenbergia. In this case the fructification is not unlike that of Aneimidictyon of the present day. Carruthers has recently detected the separate sporangia of Ferns full of spores in calcareous nodules in coal (Plate I. Fig. 5). These have the elastic ring characteristic of the Polypodiaceæ, and in their size, form, and method of attachment, they are allied to the group Hymenophylleæ. The absence of fructification presents a great obstacle to the determination of fossil Ferns. Circinate vernation, so common in modern Ferns, is rarely seen in the fossil species, and we do not in general meet with rhizomes. Characters taken from the venation and forms of the fronds are not always to be depended upon, if we are to judge from the Ferns of the present day. There is a great similarity between the carboniferous Ferns of Britain and America; and the same species, or closely allied species of the same genera as those found in Britain have been met with in South Africa, South America, and Australia. In the English coal-measures the species are about 140. The Palæozoic flora of the Arctic regions also resembles that of the other quarters of the globe. Heer, in his account of the fossil flora of Bear Island,[7] enumerates the following plants:—Cardiopteris frondosa, C. polymorpha, Palæopteris Roemeriana, Sphenopteris Schimperi, Lepidodendron Veltheimianum, L. commutatum, L. Carneggiannum, L. Wilkianum, Lepidophyllum Roemeri, Knorria imbricata, K. acicularis, Calamites radiatus, Cyclostigma Kiltorkense, Stigmaria ficoides, etc., Cardiocarpum ursinum, C. punctulatum, besides various sporangia and spores.

The preponderance of Ferns over flowering plants is seen at the present day in many tropical islands, such as St. Helena and the Society group, as well as in extra-tropical islands, as New Zealand. In the latter, Hooker picked 36 kinds in an area of a few acres; they gave a luxuriant aspect to the vegetation, which presented scarcely twelve flowering plants and trees besides. An equal area in the neighbourhood of Sydney (in about the same latitude) would have yielded upwards of 100 flowering plants, and only two or three Ferns. This Acrogenous flora, then, seems to favour the idea of a humid as well as mild and equable climate at the period of the coal formation—the vegetation being that of islands in the midst of a vast ocean. Lesquereux, in Silliman's Journal, gives three sections of Ferns in the Carboniferous strata—viz. Neuropterideæ, Pecopterideæ, and Sphenopterideæ. In Neuropterideæ fructification has been seen in Odontopteris. In this genus the spores are in a peculiar bladdery sporangium. In Neuropterideæ the fructification appears to have resembled Danæa in some cases, and Osmunda in others. Professor Geikie has noticed in the lower Carboniferous shales of Slateford, near Edinburgh, a fern which has been named Adiantites Lindseæformis by Bunbury (Fig. 22, bis). It has pinnules between crescent and fan shaped. (Mem. Geol. Survey of Edinburgh, 1861, p. 151.)

Among the Ferns found in the clays, ironstones, and sandstones of the Carboniferous period, we shall give the characters of some by way of illustration.[8] Pecopteris (Fig. 23) seems to be the fossil representative, if not congener, of Pteris. Pecopteris heterophylla (Fig. 24) has a marked resemblance to Pteris esculenta of New Zealand. The frond of Pecopteris is pinnatifid, or bi-tri-pinnatifid—the leaflets adhering to the rachis by the whole length of their base, sometimes confluent; the midrib of the leaflets runs to the point, and the veins come off from it nearly perpendicularly, and the fructification when present is at the end of the veins. Neuropteris (Figs. 25, 26, 27) has a pinnate or bipinnate frond, with pinnæ somewhat cordate at the base—the midrib of the pinnæ vanishing towards the apex, and the veins coming off obliquely, and in an arched manner. Neuropteris gigantea (Fig. 26) has a thick bare rachis, according to Miller, and seems to resemble much Osmunda regalis. Odontopteris has leaves like the last, but its leaflets adhere to the stalk by their whole base, the veins spring from the base of the leaflets, and pass on towards the point. Sphenopteris (Fig. 28) has a twice or thrice pinnatifid frond, the leaflets being narrowed at the base, often wedge-shaped, and the veins generally arranged as if they radiated from the base. Sphenopteris elegans resembled Pteris aquilina in having a stout leafless rachis, which divided at a height of seven or eight inches from its club-like base into two equal parts, each of which continued to undergo two or three successive bifurcations. A little below the first forking two divided pinnæ were sent off. A very complete specimen, with the stipe, was collected in the coalfield near Edinburgh by Hugh Miller, who has described it as above. Lonchopteris has its frond multi-pinnatifid, and the leaflets more or less united together at the base; there is a distinct midrib, and the veins are reticulated. Cyclopteris (Fig. 29) has simple orbicular leaflets, undivided or lobed at the margin, the veins radiating from the base, with no midrib. Schizopteris resembles the last, but the frond is deeply divided into numerous unequal segments, which are usually lobed and taper-pointed.

The rarity of Tree-ferns in the coal-measures has often been observed, and it is the more remarkable from the durable nature of their tissues. Several species have, however, been noticed. They are referred to the genus Caulopteris. One of them, C. macrodiscus (Fig. 30) has the leaf-scars in linear series. Two other species are figured, the one a slender form with the scars widely separated, as in some Alsophilas, C. Balfouri (Fig. 31) from the Somersetshire coal-field; and the other with larger stems and more closely aggregated scars, C. Morrisi (Fig. 32), from the coal-measures at Newcastle. The latter species shows the cavities at the base of the petiole described by Mohl in many living fern-stems. The fossils named Psaronius appear to have been fern-stems with a slender axis and a large mass of adventitious roots, as in some Dicksonias and in Osmunda regalis. These stems probably belong to some of the fronds to which other names are given, but as they have not been found attached, it is impossible to determine the point. Miller has described a fern as occurring in the coal-measures, which at first sight presents more the appearance of a Cycadaceous frond than any other vegetable organism of the carboniferous age except the Cycadites Caledonicus (Salter), from Cockburnspath Cove. He thus describes it:—

"From a stipe about a line in thickness there proceed at right angles, and in alternate order, a series of sessile lanceolate leaflets, rather more than two inches in length, by about an eighth part of an inch in breadth, and about three lines apart. Each is furnished with a slender midrib; and, what seems a singular, though not entirely unique feature in a Fern, the edges of each are densely hirsute, and bristle with thick short hair. The venation is not distinctly preserved."

Sigillaria (Plate IV. Figs. 1 and 2) is perhaps the most important plant in the coal formation. The name is derived from sigillum, a seal, to indicate the seal-like markings in the stem. It is found in all coal-shales over the world. Schimper mentions 83 species. It occurs in the form of lofty stems, 40-50 feet high, and 5 feet broad (Figs. 33 and 34). Many stems of Sigillaria may be seen near Morpeth, standing erect at right angles to the planes of alternating strata of shale and sandstone (Fig. 33). They vary from 10 to 20 feet in height, and from one to three feet in diameter. Sir W. C. Trevelyan counted 20 portions of these trees within the length of half-a-mile, of which all but four or five were upright. Brongniart mentions similar erect stems as being found near St. Etienne. The stem of Sigillaria is fluted in a longitudinal manner, like a Doric column, and has a succession of single scars, which indicate the points of insertion of the leaves (Figs. 35, 36, and 37). When the outer part of the stem separates like bark, it is found that the markings presented by the inner surface differ from those seen externally. This has sometimes given rise to the erroneous multiplication of species and even of genera. Sigillaria elegans, as figured by Brongniart in Archives du Museum, i. 405, has a stem consisting of a central cellular axis or medulla, surrounded by a vascular cylinder, and this is invested by a thick cellular cortical layer, the outer portion composed of fusiform cells of less diameter than those of the inner portion. What Brongniart calls medullary rays are mere cracks or separations in the wedges traversed by vessels. In its structure it resembles its root Stigmaria, and must be referred to Lycopodiaceæ, along with Lepidodendron, Halonia, Ulodendron, etc. The small round sporangia of Sigillaria are borne in a single patch on the somewhat enlarged bases of some of the leaves. (See Carruthers on Structure and Affinities of Sigillaria, in Journ. Geol. Soc. Aug. 1869.)

It has been ascertained by Professor King and Mr. Binney of Manchester, that the plant called Stigmaria (Fig. 38) is not a separate genus, but the root of Sigillaria (Plate IV. Figs. 1 and 2). The name is derived from στίγμα, a mark, indicating the markings on the axis. It is one of the most common productions of the coal-measures, and consists of long rounded or compressed fragments, marked externally by shallow circular, oblong, or lanceolate cavities (Fig. 39) in the centre of slight tubercles, arranged more or less regularly in a quincuncial manner (Plate III. Fig. 7). The cavities occasionally present a radiating appearance. The axis of the fragments is often hollow, and different in texture from the parts around. This axis consists of a vascular cylinder or woody system, penetrated by quincuncially arranged meshes or openings, through which the vascular bundles proceed from the inner surface of the cylinder to the rootlets (Plate III. Figs. 8 and 9). From the scars and tubercles arise long ribbon-shaped processes, which were cylindrical cellular roots, now compressed (Fig. 38). The vascular cylinder of Stigmaria is composed entirely of scalariform tissue, pierced by meshes for the passage, from the inner surface of the cylinder, of the vascular bundles which supply the rootlets. (Carruthers in Geol. Proc., Aug. 1869.) Stigmaria ficoides (Fig. 38) abounds in the under-clay of a coal-seam, sending out numerous roots from its tubercles, and pushing up its aerial stem, in the form of a fluted Sigillaria. On the Bolton and Manchester Railway Mr. Binney discovered Sigillarias standing erect, and evidently connected with Stigmarias which extended 20 feet or more.[9] Stigmaria is regarded by Schimper as roots, not of Sigillaria only, but of Knorria longifolia (one of the Lepidodendreæ). The base of the stem of this species of Knorria is Ancestrophyllum, and the upper part is Didymophyllum Schottini of Goeppert. Professor King and others suppose that the Fern-like frond called Neuropteris is connected with Sigillaria, but this is a mere conjecture, set aside by the discovery of leaves attached to a species allied to Sigillaria elegans, which establishes that the long linear leaves described under the name Cyperites are the foliage of this genus. Goldenberg has figured the fructification, which consists of small sporangia like those of Flemingites, borne on the basis of but slightly modified leaves. This establishes the opinion that Sigillaria was an acrogenous plant belonging to Lycopodiaceæ. Brongniart reckons it as representing an extinct form of Gymnosperms, and King, having erroneously associated the Cyclopteris with it, places it between the Ferns and Cycadaceæ. Mr. Carruthers informs me that he has examined the stem of a true fluted Sigillaria, with the tissues preserved, and that these agree with the structure of Lepidodendron, a position in which he had already placed it from the structure of its fruit.

Lepidodendron (Figs. 40 to 44) is another genus of the coal-measures which differs from those of the present day (Plate IV. Fig. 3). Lepidodendrons, or fossil Lycopodiaceæ, had spikes of fructification comparable in size to the cones of firs and cedars, and containing very large sporangia, even larger than those of Isoetes, to which they approach in form and structure. Schimper, in 1870, enumerates 56 species of Lepidodendron, all arborescent and carboniferous. The stem of a Lepidodendron is from 20 to 45 feet high, marked outside by peculiar scale-like scars (Fig. 41), hence the name of the plant (λεπίς, a scale, and δένδρον, a tree). Although the scars on Lepidodendron are usually flattened, yet in some species they occupy the faces of diamond-shaped projections, elevated one-sixth of an inch or more above the surface of the stem, and separated from each other by deep furrows;—the surface bearing the leaf being perforated by a tubular cavity, through which the bundle of vessels that diverged from the vascular axis of the stem to the leaf passed out. The linear or lanceolate leaves are arranged in the same way as those of Lycopodiums or of Coniferæ, and the branches fork like the former. The internal structure of the stem is the same as that of Sigillaria. The fruit of Lepidodendron and allied genera is seen in Lepidostrobus and Triplosporites (Figs. 42, 43; Plate III, Fig. 10). Carruthers, in his lecture to the Royal Institution, in describing the forms of Lepidostrobus, says—"The fruit is a cone composed of imbricated scales arranged spirally on the axis like the true leaves, and bearing the sporangia on their horizontal pedicels. Three different forms of fruit belong to this genus, or it should perhaps rather be called group of plants. The first of these is the cone named by Robert Brown Triplosporites (Figs. 42, 43), and described by him from an exquisitely preserved specimen of an upper portion, in which the parts are exhibited as clearly in the petrified condition as if they belonged to a fresh and living plant. The large sporangia have a double wall, the outer composed of a compact layer of oblong cells placed endwise, or with the long diameter perpendicular to the surface; the inner is a delicate cellular membrane. The sporangium is filled with a great number of very small spores, each composed of three roundish bodies or sporules. Recently Brongniart and Schimper have described a complete specimen of this fruit, in which the minute triple spores are confined to the sporangia of the upper and middle part of the cone, but the lower portion, which was wanting in Brown's specimen, bears sporangia filled with simple spherical spores ten or twelve times larger than the others (woodcut 44, 9).

"The structure of another form of cone (Lepidostrobus) has been expounded by Dr. Hooker. The arrangement of the different parts comprising it is precisely similar to what occurs in Triplosporites; but the sporangia are filled with the minute triple spores throughout the whole cone (woodcut 44, 6 and 8).

"The third form of cone, described by me under the name Flemingites, differs from the other two in having a large number of small sporangia supported on the surface of each scale; and it agrees with Lepidostrobus in the sporangia containing only small spores (woodcut 44, 10).

"In comparing these fossils with the living club-mosses, one is struck with the singular agreement in the organisation of plants so far removed in time, and so different in size, as the recent humble club-mosses and the palæozoic tree Lepidodendrons. The fruit of Triplosporites, like that of Selaginella (woodcut 44, 1), contains large and small spores, the microspores being found in both genera on the middle and upper scales of the cone, and the macrospores on those of the lower portion (Fig. 43).

"On the other hand, the fruits of Lepidostrobus and Flemingites agree with that of Lycopodium in having only microspores. The size of the two kinds of spores also singularly agrees in the two groups. This is of some importance, for among the recent vascular Cryptogams there is a remarkable uniformity in the size of the spores in the members of the different groups, even when there is a great variety in the size of the plants. Thus the spore of our humble wall-rue is as large as that of the giant Alsophila of tropical regions. So also the spores of Equisetum and Calamites agree in size, as may be seen in woodcut 47, Figs. 3, 4, and 9, where the spores of the two genera are magnified to the same extent. And a similar comparison of the macrospore and microspore of Triplosporites with those of Selaginella, and of the microspore of Lepidostrobus with that of Lycopodium, exhibits a similar agreement. This is made apparent by the drawings in woodcut 44 of the two kinds of spores of Selaginella, 3 and 4, with those of Triplosporites, 8 and 9, which are drawn to the same scale."

The genus Sigillaria, as we have already said, has, according to the observation of Hooker, small sporangia exactly agreeing in size and form with those of Flemingites. Most probably the contents of these small sporangia were the same in both genera, so that Sigillaria would be placed with Flemingites and Lepidostrobus as arborescent Lycopodiaceæ having their affinities with Lycopodium, as they have all microspores only in their fructification.

The scales upon the Lepidodendron stems, as well as those in the cones, are arranged in a spiral manner, in the same way as plants of the present day. Professor Alexander Dickson has examined the phyllotaxis of Lepidodendrons, and gives the following results of his observations (Trans. Bot. Soc. Edin. xi. 145). The fossil remains of Lepidodendrons are often so compressed that it is difficult, or even impossible, to trace the secondary spirals round the circumference of the stem. In those cases, however, where there is comparatively little compression, i.e. where the stem is more or less cylindrical, the determination of the phyllotaxis is easy. Of such stems he has examined fifteen specimens, which may be classed according to the series of spirals to which the leaf-arrangement belongs:—

A. Ordinary series, ½, ⅓, ²/₅, ⅜, ⁵/₁₃, etc.