FLORA OF THE PRIMARY OR PALÆOZOIC PERIOD.

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).

Fig. 9.

Fig. 9. Tree-fern, with a slender cylindrical trunk and a crown of drooping fronds. It is a vascular acrogen.

Fig. 10.

Fig. 10. Asplenium; a species of Spleenwort. A. Rhizome, r, covered with the bases (stalks or stipes) of the fronds; f, fronds in bud, rolled up in a circinate manner (this is very rarely seen in fossil ferns); g, fronds bearing fructification on their backs. B. Portion of a frond separated to show the linear sori or clusters of sporangia (spore-cases).

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. 11, a. Fig. 11, b.Fig. 12.

Fig. 11, a. Bifurcating (forked or dichotomous) trunk (caudex) of a Tree-fern (Alsophila Perrottetiana), showing the scars (cicatrices) left by the fallen fronds. These scars exhibit the arrangement of the vascular bundles. Fig. 11, b. Rhizome of Lastrea Filix-mas (male fern), showing scars of the leaves, c, with markings of the vascular bundles.

Fig. 12. Transverse section of the stem (caudex) of a Tree-fern (Cyathea), showing the arrangement of the cellular and vascular tissue. The cellular tissue of the centre, m; that of the circumference, p; vascular cylinder, f v, consisting of dark-coloured pleurenchyma or ligneous tubes, f, and paler vessels, v, chiefly scalariform and closed spiral, and pierced by the meshes for the leaf-bundles at m; the outer cortical portion connected with the bases of the leaves, e.

Fig. 16. Fig. 15.Fig. 17.

Fig. 13. Scalariform vessels taken from a Tree-fern. They are marked with bars like the steps of a ladder, hence their name. The membrane occasionally disappears, so that the walls are made up of fibres only at some parts. 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.

Fig. 18.

Fig. 18. Fructification of Equisetum maximum, Great Water Horse-tail, showing the stalk surrounded by membranous sheaths, s s, which are fringed by numerous processes called teeth. The fructification, f, at the extremity, is in the form of a cone bearing polygonal scales, under which are spore-cases containing spores with filaments.

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. 19. Fig. 20. Fig. 21.

Fig. 19. Polygonal scale, s, of a species of Horse-tail (Equisetum), bearing membranous sacs, t, which open on their inner surface to discharge spores. 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.

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.

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]

Countries.Square Miles of Coal Formation.Annual Production of Coal in Tons.

Great Britain and Ireland5,40065,887,900

British North America7,5301,500,000

United States196,6505,000,000

Belgium5188,409,330

France1,7197,740,317

Prussia and Austria——4,200,000

Saxony301,000,000

Russia1003,500,000

Japan, China, Borneo, Australia, etc.——2,000,000

Total Produce of the World——99,237,547

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:—

Countries.Average Thickness. No. Feet.Tons.

Belgium6036,000,000,000

France6059,000,000,000

British Islands35190,000,000,000

Pennsylvania25316,400,000,000

Great Appalachian Coalfield251,387,500,000,000

Indiana, Illinois, Western Kentucky251,277,500,000,000

Missouri, and Arkansas Basin10739,000,000,000

North America (assumed thickness over an area of 200,000 square miles)204,000,000,000,000

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.

Fig. 22, bis.

Fig. 22, bis. Adiantites Lindseæformis.

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.

Fig. 23. Fig. 25. Fig. 26. Fig. 27.

Figs. 23 to 29 exhibit the fronds of some of the Ferns of the Carboniferous epoch. Fig. 23. Pecopteris (Alethopteris) aquilina. Fig. 24. Pecopteris (Alethopteris) heterophylla. Fig. 25. Neuropteris Loshii. Fig. 26. Neuropteris gigantea. Fig. 27. Neuropteris acuminata. Fig. 28. Sphenopteris affinis. Fig. 29. Cyclopteris dilatata.

Fig. 30. Fig. 31. Fig. 32.

Figs. 30 to 32. Stem of Tree-ferns, called Caulopteris. Fig. 30. Caulopteris macrodiscus. Fig. 31. Caulopteris Balfouri (Carr.), Coal-measures. Fig. 32. Caulopteris Morrisi (Carr.), Coal-measures.

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."

Fig. 33. Fig. 34.

Fig. 36. Fig. 37.

Figs. 33 to 37 exhibit forms of Sigillaria stems found in the shales of the Carboniferous epoch. Fig. 33. Stem of Sigillaria pachyderma in an erect position, covered by successive deposits of sandstone and shale; one of the stems is bifurcated. Fig. 34. Sigillaria reniformis, with its external markings, and roots which are Stigmarias, as proved by Mr. Binney. Fig. 35. Sigillaria pachyderma, after Lindley and Hutton, from the shale of Killingworth Colliery, showing the scars or places through which the vessels of the stem passed to the leaves. Fig. 36. Sigillaria (Favularia) tessellata, from the Denbigh coal-shale, showing the fluted stem with scars. Fig. 37. Sigillaria pachyderma; the stem marked with scars, and fluted longitudinally.

Fig. 35.

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.)

Fig. 38. Fig. 39.

Fig. 38. Stigmaria ficoides, root of Sigillaria, giving off rootlets, which have been compressed. Fig. 39. Stigmaria ficoides (S. Anabathra of Corda), which is the root of a Sigillaria. The markings are the points whence rootlets proceed.

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.

Fig. 40. Fig. 41.

Figs. 40 to 44 exhibit the stems and fructification of Lepidodendron. Fig. 40. Bifurcating stem of Lepidodendron obovatum (elegans), showing the scale-like scars, and the narrow-pointed leaves, resembling those of Lycopodium, but much larger. Fig. 41. Stem of Lepidodendron crenatum, with the scars of its leaves.

Fig. 42. Fig. 43.

Fig. 42. Fructification of Lepidodendron, showing its cone-like form and spiral arrangement of scales. It is called Lepidostrobus Dabadianus by Schimper, but it is probably Triplosporites. Fig. 43. Longitudinal section of the fructification, showing central axis and scales carrying sporangia. The upper sporangium contains microspores, the lower macrospores; hence it has the character of Triplosporites.

Fig. 44.

In woodcut 44 are represented the fruits of Selaginella (one of the Lycopodiums of the present day), Lepidostrobus, Triplosporites, and Flemingites. Fig. 1. Selaginella spinulosa, A. Braun (Lycopodium selaginoides, Linn.) 2. Scale and sporangium from the upper portion of the cone. 3. Antheridian microspores from the same. 4. Macrospore. 5. Scale and sporangium from the lower part of the cone, containing macrospores. 6. Lepidostrobus ornatus, Hooker. 7. Three scales and sporangia of ditto. 8. Microspores from the sporangia of the upper part of the cone of Triplosporites Brownii, Brongn. 9. Macrospore from the sporangia of the lower part (drawn from Brongniart's description and measurements). 10. Scales and sporangia of a cone of Flemingites.[10]

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.

(a.) Single spirals (D turning to the right, S to the left).

(1.) Lepidodendron (Possil Ironstone series). Stem about ¾ of an inch in diameter. Secondary spirals 8 D, 13 S, 21 D. Divergence = ¹³/₃₄ (or possibly ²¹/₅₅).

(2.) Lepidodendron (Knightswood, near Glasgow, Mr. J. Young). Stem about 1½ inch in diameter. Secondary spirals 13 D, 21 S, 34 D. Divergence = ²¹/₅₅.

(3.) Lepidodendron (Possil Sandstone series). Trunk about 2 feet long, with an average diameter of 20 inches. Steepest secondary spirals 55 S, 89 D. Divergence = ⁵⁵/₁₄₄.

(b.) Conjugate spirals.[11]

(4.) Lepidostrobus ornatus (Bathgate coal-field). About ¾ of an inch in diameter. Secondary spirals 10 D, 16 S, 26 D, 42 S. Divergence = ¹³/_(34×2) (Bijugate arrangement).

(5.) Lepidostrobus (Plean, Stirlingshire, Mr. Mackenzie). About ½ an inch in diameter. Secondary spirals 9 S, 15 D, 24 S, 39 D. Divergence = ⁸/_(21×3) (Trijugate arrangement).

(6.) Knorria taxina (from collection of Dr. Rankin, Carluke). Somewhat compressed, 2-2½ inches[12] in diameter. Secondary spirals 15 D, 24 S. Divergence = ¹⁵/_(13×3) (Trijugate arrangement).

(7.) Lepidodendron (from Dr. Rankin's collection). About 1¼ inch in diameter. Secondary spirals 10 D, 15 S, 25 D, 40 S. Divergence = ⁵/_(13×5) (Quinquejugate arrangement).

(8.) Lepidodendron (Dowanhill, Glasgow, Possil Sandstone series). Trunk about 1 foot long, and 1 foot in diameter. The upper portion exhibits secondary spirals 35 D, 56 S, 91 D; thus indicating a 7-jugate arrangement, with divergence = ⁸/_(21×7). The arrangement on the middle and lower portion is indistinct and confused; so much so as to render any determination of the arrangement doubtful.

B. Series, ⅓, ¼, ²/₇, ³/₁₁, etc.

(9.) Lepidodendron (Messrs Merry and Cunningham's Clayband Iron-Pit, Carluke). Stem 2 inches in diameter. Secondary spirals 18 S, 29 D, 47 S. Divergence = ²¹/₇₆.

C. Series, ¼, ¹/₅, ²/₉, ³/₁₄, etc.

(10.) Lepidodendron (R. B. Garden, Edinburgh, Museum). Stem somewhat flattened, 1-1½ inch in diameter. Secondary spirals 9 D, 14 S, 23 D, 37 S. Divergence = ¹³/₆₀.

(11.) Lepidodendron (Redhaugh, near Edinburgh, Mr. Peach). Stem somewhat flattened, ¾ to ½ inch in diameter. Secondary spirals 9 S, 14 D, 23 S, 37 D. Divergence = ¹³/₆₀.

D. Series, ¹/₅, ¹/₆, ²/₁₁, ³/₁₇, ⁵/₂₈, etc.

(12.) Knorria taxina (Stockbriggs, Lesmahagow,—Hunterian Museum). About 1 inch in diameter. The specimen consists of a main stem and one of the branches into which it has forked. On the main stem the secondary spirals are 6 D, 11 S, 17 D. Divergence = ⁵/₂₈ (series, ¹/₅, ¹/₆, ²/₁₁, ³/₁₇, ⁵/₂₈, etc.)—On the branch the secondary spirals are 8 S, 13 D. Divergence = ⁸/₂₁ (ordinary series, ½, ⅓, ²/₅, ⅜, etc.)

E. Series, ½, ²/₅, ³/₇, ⁵/₁₂, ⁸/₁₉, ¹³/₃₁, ²¹/₅₀, etc.

(13.) Lepidodendron (from Dr. Rankin's collection). About ⅞ inch in diameter. Secondary spirals 12 D, 19 S, 31 D. Divergence = ²¹/₅₀.

F. Series, ⅓, ³/₁₀, ⁴/₁₃, ⁷/₂₃, ¹¹/₃₆, ¹⁸/₅₉, etc.

(14.) Lepidodendron elegans (Possil Ironstone). About 1¼ inch in diameter. Secondary spirals 10 S, 13 D, 23 S, 36 D. Divergence = ¹⁸/₅₉.

(15.) Lepidodendron (Possil Ironstone). About 2¼ inches in diameter. Secondary spirals 23 S, 36 D, 59 S, 95 D. Divergence = ⁴⁷/₁₅₄.

From the above it is evident that the phyllotaxis of Lepidodendron is extremely variable, as much so perhaps as that of those most variable plants, in this respect, the Cacti. It is also clear that what has been enunciated by Professor Haughton (Manual of Geology, Lond. 1866, pp. 243, 245) as the law according to which the leaves of palæozoic plants were arranged—viz. that of alternate whorls—does not apply to these ancient Lycopods. Lepidodendron aculeatum is noted by Naumann as exhibiting an ⁸/₂₁ arrangement. (Poggendorff, Annalen, 1842, p. 5.) Professor Alexander Braun (Nov. Acta Ac. C. L. C. xv. 1, pp. 558-9), speaking of the excessive deviation from ordinary arrangements in Equisetaceæ (including Calamites), compares them in this respect with Lycopodiaceæ (including Lepidodendron), saying that in these two families "the utmost limits of the domain of all leaf-arrangement appears to be attained."

Lepidophyllum is certainly leaves of Lepidodendron, the different Lepidophylla belonging to different species of the genus. The slender terminal branches are noticed under the name of Lycopodites. In coal from Fordel Mr. Daw has detected innumerable bodies (Plate III. Figs. 1, 2, 3) which have been shown to be sporangia. (Balfour, Trans. Roy. Soc. Ed. xxi. 187.) On their under surface Mr. Carruthers has observed a triradiate ridge (Plate III. Fig. 4). (Geological Magazine, 1865, vol. ii. p. 140.) These sporangia have been found connected with the cone-like fructification called Flemingites, and resembling Lycopodium (woodcut 44, Fig. 4). Many forms of fossil plants, such as Halonia, Lepidophloios, Knorria, and Ulodendron, belong to the Lepidodendron group. Knorria is said to be the internal cast of a Lepidodendron.

Ulodendron minus and U. Taylori (Plate III. Fig. 11), found in ferruginous shale in the Water of Leith, near Colinton, exhibit beautiful sculptured scars, ranged rectilinearly along the stem. The surface is covered with small, sharply relieved obovate scales, most of them furnished with an apparent midrib, and with their edges slightly turned up. The circular or oval scars of this genus are probably impressions made by a rectilinear range of aerial roots placed on either side. When decorticated, the stem is mottled over with minute dottings arranged in a quincuncial manner, and its oval scars are devoid of the ordinary sculpturings. Bothrodendron is a decorticated condition of Ulodendron.

Fig. 45 a.

Fig. 45 b.

Fig. 45. a, Calamites Suckovii, composed of jointed striated fragments having a bark. Fig. 45. b, Septum or phragma of a Calamite.

Calamites (κάλαμος, a reed) is a reed-like fossil, having a sub-cylindrical jointed stem (Fig. 45, a and b; Fig. 46; Plate IV. Fig. 4). The stem is often crushed and flattened, and was originally hollow. Calamites is thus defined by Grand d'Eury (Ann. Nat. Hist. ser. 4, vol. iv. p. 124):—Stem articulated, fistular, and septate; outer part comparatively thin, formed of three concentric zones—1, an exterior cortical layer now converted into coal; 2, a thin subjacent zone of vascular tissue, now invariably destroyed; 3, a sort of inner lining epidermis, which is carbonified. Cortical envelope marked interiorly with regular flutings, interrupted and alternate at the articulations. Inner epidermis smooth, or scarcely striated. Vascular cylinder thin; outer surface of bark more fully fluted and articulated than the inner surface.

Fig. 46.

Fig. 46. Vertical stems of fossil trees, Calamites chiefly, found in the coal-measures of Treuil, near Saint Etienne.

Carruthers gives the following description of the structure of a species of Calamite which he examined:—The stem was composed of a central medulla, which disappeared with the growth of the plant, surrounded by a woody cylinder, composed entirely of scalariform vessels, and a thin cortical layer. The medulla penetrated the woody cylinder by a series of regular wedges, which were continued, as delicate laminæ of one or two cells in thickness, to the cortical layer. The cells of those laminæ were not muriform; their longest diameter was in the direction of the axis. The wedges were continuous, and parallel between each node. As the axial appendages were produced in whorls, the only interference with the regularity of the tissues was by the passing out through the stem at the nodes of the vascular bundles which supplied these appendages. As the leaves of each whorl were (with one or two exceptions) opposite to the interspaces of the whorls above and below, there was also at each node a re-arrangement of the wedges of vascular and cellular tissues.

Schimper considers Calamites as having an analogy with Equisetum in its fructification. He looks on them as fossil Equisetaceæ. Annularia and Sphenophyllum are considered as establishing a passage from the Equisetaceæ to the Lycopodiaceæ. Some gigantic fossil Equiseta had a diameter of nearly 5 inches, and a height of 30 or more feet. The branches, which adorned the higher part of them in the form of a crown, are simple, and have at their extremity a spike of the size of a pigeon's egg, and organised exactly like the spikes of living Equiseta. The subterranean rhizomes are well developed, and gave origin, like many Equiseta, to tubercles which had the form and size of a hen's egg.

The characters of Equisetum of the present day and Calamites, are exhibited in woodcut 47. They show a marked resemblance in the fructification. (See also page 31.)

Plants of Calamites have been seen erect by Mr. Binney, and he has determined that what were called leaves or branches by some are in reality roots. Mr. Binney gives a full description of various Calamites, under the name of Calamodendron commune, in his Memoir published by the Palæontographical Society, 1868. There are between 50 and 60 species recorded.[13]

In Spitzbergen, in rocks of the Carboniferous epoch, there have been found Calamites, Sigillaria, Lepidodendron, and ferns, apparently the same as those found in the Carboniferous epoch in Europe—Calamites radiatus, Lepidodendron Veltheimianum, Sigillaria distans, Stigmaria ficoides. Some species—Sigillaria Malmgreni, Lepidodendron Carneggiannum, and L. Wilkianum—seem to be peculiar to Bear Island.

Fig. 47.

Fig. 47. Fruits of Equisetum and Calamites. 1. Equisetum arvense, L. 2. Portion of sporangium wall. 3, 4. Spores, with the elaters free. 5. Longitudinal section of the part of one side of cone. 6. Transverse section of cone. 7. Calamites (Volkmannia) Binneyi, Carr., magnified three times. 8. Portion of the sporangium wall. 9. Two spores. 10. Longitudinal section of the part of one side of cone. 11. Transverse section of cone.

According to Carruthers the Equisetaceæ are represented in Britain by the two genera Calamites found in primary beds, and Equisetum found in secondary rocks and living at the present day. The difference in the structure of their fruits is shown in woodcut 47. The fruit of Calamites, called Volkmannia Binneyi (woodcut 47, 7), is a small slender cone composed of alternating whorls of imbricate scales, twelve in each verticil. The scales completely conceal the leaves connected with the fructification. The fruit-bearing leaves are stalked, peltate, and are arranged in whorls of 6. There are four sporangia borne on the under-surface of the peltate leaves. These spore-cases have cellular parietes, and in their interior there is a deposit of cellulose in the form of short truncate processes not unlike imperfect spirals. The spores are spherical, and appear to have thread-like processes proceeding from them, similar to elaters. The fruit-cone bears a marked resemblance to the fruit of Equisetum in its fruit-bearing leaves, sporangia, spores, and elaters (see Figs. 18, 19, 20, 21). In the modern plant all the leaves of the cone are fructiferous, while in the fossil plant some are fruit-bearing, and others are like the ordinary leaves of the plant. It is thought that the fossil may be reckoned as having a somewhat higher position than that possessed by the living genus.

Fig. 48.

Fig. 48. Foliage and fruits of Calamites. 1 and 2. Asterophyllites; 3 and 4. Annularia; 5 and 6. Sphenophyllum.

The different forms of foliage called Asterophyllites, Sphenophyllum, and Annularia, belong to the one genus Calamites, but they may form, perhaps, well-characterised sections when their fruits are better known. In woodcut 48 representations are given of the foliage and fruit of varieties of Calamites. In 1 and 2 we see the simplest form called Asterophyllites. The leaves are linear and slender, with a single rib. The form called Annularia (3 and 4) differs chiefly in having a larger amount of cellular tissue spread out on either side of the midrib. This form has a different aspect in a fossil state from the other, from its whorls of numerous broad leaves spread out on the surface of deposition, while the acicular leaves of Asterophyllites have penetrated the soft mud, and are generally preserved in the position they originally occupied in reference to the supporting branch. The third form (5 and 6) is called Sphenophyllum, and consists of whorls of wedge-shaped leaves, with one or more bifurcating veins. They occur like those of Annularia, spread out on the surface of the shale.

Fig. 49. Fig. 50.

Fig. 49. Araucarioxylon Withami, Krauss (Pinites Withami), from the Coal-measures, Craigleith, near Edinburgh, showing pleurenchyma with disks, and medullary rays. An excellent specimen of a stem of this pine may be seen in the Edinburgh Royal Botanic Garden. Fig. 50. Trigonocarpum olivæforme, an ovate, acuminate, three-ribbed, and striated fruit or seed, which some suppose to be a sporangium of a Lepidodendron, others refer it to Cycadaceæ. Hooker refers it to Coniferæ like Salisburia.

True Exogenous trees exist in the coal-fields both of England and Scotland, as at Lennel Braes and Allan Bank, in Berwickshire; High-Heworth, Fellon, Gateshead, and Wideopen, near Newcastle-upon-Tyne; and in quarries to the west of Durham; also in Craigleith quarry, near Edinburgh, and in the quarry at Granton, now under water. In the latter localities they lay diagonally athwart the sandstone strata, at an angle of about 30°, with the thicker and heavier part of their trunks below, like snags in the Mississippi. From their direction we infer that they have been drifted by a stream which has flowed from nearly north-east to south-west. At Granton, one of the specimens exhibited roots. In other places the specimens are portions of stems, one of them 6 feet in diameter by 61 feet in length, and another 4 feet in diameter by 70 feet in length. These Exogenous trees are Gymnosperms, having woody tissue like that of Coniferæ. We see under the microscope punctated woody tissue, the rows of disks being usually two, three, or more, and alternating. They seem to be allied in these respects to Araucaria and Eutassa (Fig. 61, p. 74) of the present flora. Araucarioxylon or Pinites Withami (Fig. 49) is one of the species found in Craigleith quarry; the concentric layers of the wood are obsolete; there are 2, 3, or 4 rows of disks on the wood, and 2-4 rows of small cells in the medullary rays. Along with it there have also been found Dadoxylon medullare, with inconspicuous zones, 2, 3, and 4 rows of disks, and 2-5 series of rows of cells in the rays. Pissadendron antiquum (Pitus antiqua) having 4-5 series of cells in the medullary rays, and P. primævum (Pitus primæva), with 10-15 series of cells in the medullary rays, occur at Tweedmill and Lennel Braes in Berwickshire; Peuce Withami (Fig. 1, p. 3) at Hilltop, near Durham, and at Craigleith. Sternbergia is considered by Williamson as a Dadoxylon, with a discoid pith like that seen now-a-days in the Walnut, Jasmine, and Cecropia peltata, as well as in some species of Euphorbia.[14] Sternbergia approximata is named by him Dadoxylon approximatum. Hooker believes from the structure of Trigonocarpum (Fig. 50) that it is a coniferous fruit nearly allied to Salisburia (Trans. Roy. Soc. 1854). Several species of Trigonocarpum occur in the Carboniferous rocks, such as T. olivæforme from Bolton (Plate II. Fig. 5), and T. sulcatum from Wardie, near Edinburgh (Plate II. Fig. 6). Noeggerathia and a few other plants, such as Flabellaria and Artisia, are referred by Brongniart to Cycadaceæ. Flabellaria borassifolia, according to Peach, has leaves like Yucca. Noeggerathia has pinnate leaves, cuneiform leaflets, sometimes fan-shaped; the veins arise from the base of the leaflets, are equal in size, and either remain simple or bifurcate, the nervation (venation) being similar to that of some Zamias.

The fossils of this period, referred to as Antholithes,[15] have just been shown by Mr. Carruthers to be the inflorescence of Cardiocarpum (Geol. Mag. Feb. 1872), and he proposes to set aside the former name, confining it to the tertiary fossils to which it was originally given by Brongniart, and to use the latter name. The main axis of the inflorescence is simple, stout, and marked externally with interrupted ridges. The axis bears in a distichous manner sub-opposite or alternate bracts of a linear-lanceolate form and with decurrent bases. In the axils of the bracts were developed flower-like leaf-bearing buds, and from them proceeded three or four linear pedicels, which terminated upwards in a somewhat enlarged trumpet-shaped apex. To this enlarged articulating surface was attached the fruit, to which has been given the generic name Cardiocarpum[16] (Fig. 51). The place of attachment is indicated by the short straight line which separates the cordate lobes at the base of the fruit. The fruit is flattish, broadly ovate, with a cordate base and sub-acute apex. It consists of an outer pericarp, inclosing an ovate-acute seed. That the pericarp was of some thickness, and formed probably a sub-indurated rind, is shown by a specimen preserved in the round, and figured (Fig. 53 a). The pericarp is open at the apex; and the elongated tubular apex of the spermoderm passes up to this opening. The seed forms a distinct swelling in the centre of the fruit, and a slight ridge passes up the middle to the base of the apical opening.

Fig. 51. Fig. 52.

Fig. 51. Cardiocarpum Lindleyi, Carr. Fig. 52. Do., Coal-measures, Falkirk.

These fossils are believed to be an extinct form of Gymnosperms. Two species have been described, of both of which we are able to give figures. The first figure is from the specimens collected by Mr. Peach at Falkirk. It is Cardiocarpum Lindleyi (Figs. 51, 52); it has a primary axis with sub-opposite axillary axes, bearing four to six lanceolate leaves and three or four pedicels. Primary bracts short and arcuate. Fruit ovate-cordate, with an acute bifid apex, and a ridge passing up the middle of the fruit.

Fig. 53.

Fig. 53. Cardiocarpum anomalum (Carr.), natural size: with separate fruit (a), twice natural size—Coal-measures, Coalbrookdale.

The second species is Cardiocarpum anomalum (Fig. 53) from Coalbrookdale; it has a primary axis with alternate or sub-opposite axillary axes, slender and elongated, bearing many linear leaves, and several slender pedicels; primary bracts long, slender, and straight; fruits small, margined. The somewhat magnified separate fruit (a) shows the thickness of the pericarp and the enclosed seed.

Fig. 54.

Fig. 54. Pothocites Grantoni, Paterson. a, Spike natural size; b, Portion of spike magnified; c, Perianth, 4-cleft, magnified.

In the bituminous shale at Granton, near Edinburgh, Dr. Robert Paterson discovered in 1840 a peculiar fossil plant, which he called Pothocites Grantoni (Fig. 54, a). It is figured in the Transactions of the Edinburgh Botanical Society, vol. i. March 1840. It is a spike covered by parallel rows of flowers (Fig. 54, b), each apparently with a 4-cleft calyx (Fig. 54, c). It was supposed to be allied to Potamogeton or Pothos, more probably to the latter. In that case it must be referred to the natural order Araceæ. The original specimen is deposited in the museum at the Royal Botanic Garden, Edinburgh.

Our knowledge of the real state of the vegetation of the earth when coal was formed must be very limited, when we reflect how seldom the fructification of coniferous trees has been met with in the coal-measures. A very doubtful fragment, supposed to be a cone, is given in Lindley and Hutton's work, under the title of Pinus anthracina; but it is believed by Carruthers to be a fragment of a Lepidodendroid branch. Lyell never saw a fossil fir-cone of the Carboniferous epoch, either in the rocks or museums of North America or Europe. Bunbury never heard of any other example than that noticed by Lindley and Hutton. Principal Dawson is disposed to think that the suberin of cork, of epidermis in general, and of spore-cases in particular, is a substance so rich in carbon that it is very near to coal, and so indestructible and impermeable to water, that it contributes more largely than anything else to the mineral. Sir Charles Lyell remarks—"To prevent ourselves, therefore, from hazarding false generalisations, we must ever bear in mind the extreme scantiness of our present information respecting the flora of that peculiar class of stations to which, in the Palæozoic era, the coal-measures probably belonged. I have stated elsewhere my conviction that the plants which produced coal were not drifted from a distance, but nearly all of them grew on the spot where they became fossil. They constituted the vegetation of low regions, chiefly the deltas of large rivers, slightly elevated above the level of the sea, and liable to be submerged beneath the waters of an estuary or sea by the subsidence of the ground to the amount of a few feet. That the areas where the carboniferous deposits accumulated were low, is proved not only by the occasional association of marine remains, but by the enormous thickness of strata of shale and sandstone to which the seams of coal are subordinate. The coal-measures are often thousands of feet, and sometimes two or three miles, in vertical thickness, and they imply that for an indefinite number of ages a great body of water flowed continuously in one direction, carrying down towards a given area the detritus of a large hydrographical basin, draining some large islands or continents, on the margins of which the forests of the coal period grew. If this view be correct, we can know little or nothing of the upland flora of the same era, still less of the contemporaneous plants of the mountainous or alpine regions. If so, this fact may go far to account for the apparent monotony of the vegetation, although its uniform character may doubtless be in part owing to a greater uniformity of climate then prevailing throughout the globe. Mr. Bunbury has successfully pointed out that the peculiarity of the carboniferous climate consisted more in the humidity of the atmosphere and the absence of cold, or rather the equable temperature preserved in the different seasons of the year, than in its tropical heat; but we must still presume that colder climates existed at higher elevations above the sea."

The plants of the coal-measures are evidently terrestrial plants. Brongniart agrees with Lyell in thinking that the layers of coal have in general accumulated in the situation where the plants forming them grew. The remains of these plants covered the soil in the same way as layers of peat, or the vegetable mould of great forests. In a few instances, however, the plants may have been transported from a distance, and drifted into basins. Phillips is disposed to think that this was the general mode of formation of coal-basins. He is led to this conclusion by observing the fragmentary state of the stems and branches, the general absence of roots, and the scattered condition of all the separable organs. Those who support the drift theory, look on the coal plants as having been swept from the land on which they grew by watery currents at different times, and deposited in basins and large sea-estuaries, and sometimes in lakes. The snags in the Mississippi, the St. Lawrence, and other large rivers, are given as instances of a similar drifting process.

The vegetation of the coal epoch seems to resemble most that of islands in the midst of vast oceans, and the prevalence of ferns indicates a climate similar to that of New Zealand in the present day. In speaking of the island vegetation of the coal epoch, Professor Ansted remarks (Ancient World, p. 88)—"The whole of the interior of the islands may have been clothed with thick forests, the dark verdure of which would only be interrupted by the bright green of the swamps in the hollows, or the brown tint of the ferns covering some districts near the coasts. The forests may have been formed by a mixture of several different trees. We would see then, for instance, the lofty and widely-spreading Lepidodendron, its delicate feathery fronds clothing, in rich luxuriance, branches and stems, which are built up, like the trunk of the tree-fern, by successive leafstalks that have one after another dropped away, giving by their decay additional height to the stem, which might at length be mistaken for that of a gigantic pine. There also should we find the Sigillaria, its tapering and elegant form sustained on a large and firm basis—enormous matted roots, almost as large as the trunk itself, being given off in every direction, and shooting out their fibres far into the sand and clay in search of moisture. The stem of this tree would appear like a fluted column, rising simply and gracefully without branches to a great height, and then spreading out a magnificent head of leaves like a noble palm-tree. Other trees, more or less resembling palms, and others like existing firs, also abounded, giving a richness and variety to the scene; while one gigantic species, strikingly resembling the Norfolk Island pine, might be seen towering a hundred feet or more above the rest of the forest, and exhibiting tier after tier of branches richly clothed with its peculiar pointed spear-like leaves, the branches gradually diminishing in size as they approach the apex of a lofty pyramid of vegetation. Tree-ferns also in abundance might there be recognised, occupying a prominent place in the physiognomy of vegetation, and dotted at intervals over the distant plains and valleys, the intermediate spaces being clothed with low vegetation of more humble plants of the same kind. These we may imagine exhibiting their rich crests of numerous fronds, each many feet in length, and produced in such quantity as to rival even the palm-trees in beauty. Besides all these, other lofty trees of that day, whose stems and branches are now called Calamites, existed chiefly in the midst of swamps, and bore their singular branches and leaves aloft with strange and monotonous uniformity. All these trees, and many others that might be associated with them, were, perhaps, girt round with innumerable creepers and parasitic plants, climbing to the topmost branches of the most lofty amongst them, and relieving, in some measure, the dark and gloomy character of the great masses of vegetation."

Hugh Miller remarks—"The sculpturesque character of the nobly-fluted Sigillarias was shared by not a few of its contemporaries. Ulodendrons, with their rectilinear rows of circular scars, and their stems covered with leaf-like carvings, rivalled in effect the ornately relieved torus of a Corinthian column. Favularia, Halonia, many of the Calamites, and all the Lepidodendrons, exhibited the most delicate sculpturing. In walking among the ruins of this ancient flora, the palæontologist almost feels as if he had got among the broken fragments of Italian palaces erected long years ago, when the architecture of Rome was most ornate, and every moulding was roughened with ornament; and in attempting to call up in fancy the old Carboniferous forests, he has to dwell on this peculiar feature as one of the most prominent; and to see in the multitude of trunks darkened above by clouds of foliage that rise upon him in the prospect, the slender columns of an older Alhambra, roughened with arabesque tracery and exquisite filigree work."

Flora of the Permian Epoch.

Fig. 55. Fig. 56.

Figs. 55 and 56. Walchia piniformis, Sternb., a common species in the Permian rocks of Europe. Fig. 55. Plant with leaves and fructification. Fig. 56. Fructification, natural size.

The nature of the vegetation during the Permian period, which is associated with the Carboniferous, under the reign of Acrogens, has been extensively illustrated by Goeppert. Brongniart has enumerated the fossils in three different localities, which he refers doubtfully to this period. 1. The flora of the bituminous slates of Thuringia, composed of Algæ, Ferns, and Coniferæ. 2. Flora of the Permian sandstones of Russia, comprehending Ferns, Equisetaceæ, Lycopodiaceæ, and Noeggerathiæ. 3. Flora of the slaty schists of Lodève, composed of Ferns, Asterophyllites, and Coniferæ. The genera of Ferns here met with are those found in the Carboniferous epoch; the Gymnosperms are chiefly species of Walchia and Noeggerathia (the latter is supposed by Schimper to be a Cycad); Lepidodendron elongatum, Calamites gigas, and Annularia floribunda, are also species of this period. Walchia is a conifer characteristic of the Permian epoch, of which there are eight species described (Figs. 55 and 56). It has a single seed to each scale of the cone, and two kinds of leaves, the one short and imbricated, the other long and spreading. Among the plants of the Permian formation Goeppert enumerates the following:[17]—Equisetites contractus, Calamites Suckowi, C. leioderma, Asterophyllites equisetiformis, A. elatior, Huttonia truncata, H. equisetiformis, many species of Psaronius, one of the filicoid plants, Hymenophyllites complanatus, Sphenopteris crassinervia, Sagenopteris tæniæfolia, Neuropteris imbricata, and many other species of these genera; several species of Odontopteris, Callipteris, Cyclopteris, Dioonopteris, Cyatheites, Alethopteris, Noeggerathia, Cordaites, Anthodiopsis, Dictyothalamus, Calamodendron, Arthropitys; besides species of Sigillaria, Stigmaria, and Lepidodendron. Various fruits are also mentioned, under the names of Rhabdocarpum, Cardiocarpum, Acanthocarpum, Trigonocarpum, and Lepidostrobus.