Part 4

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| Annual Production| | | Coal Formation.| of Coal in Tons.| +--------------------------------+----------------+------------------+ |Great Britain and Ireland | 5,400 | 65,887,900 | |British North America | 7,530 | 1,500,000 | |United States | 196,650 | 5,000,000 | |Belgium | 518 | 8,409,330 | |France | 1,719 | 7,740,317 | |Prussia and Austria | ---- | 4,200,000 | |Saxony | 30 | 1,000,000 | |Russia | 100 | 3,500,000 | |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:--

+-----------------------------------+------------+-------------------+ | | Average | | | COUNTRIES. | Thickness. | Tons. | | | No. Feet. | | +-----------------------------------+------------+-------------------+ |Belgium | 60 | 36,000,000,000 | |France | 60 | 59,000,000,000 | |British Islands | 35 | 190,000,000,000 | |Pennsylvania | 25 | 316,400,000,000 | |Great Appalachian Coalfield | 25 | 1,387,500,000,000 | |Indiana, Illinois, Western Kentucky| 25 | 1,277,500,000,000 | |Missouri, and Arkansas Basin | 10 | 739,000,000,000 | |North America (assumed thickness | | | | over an area of 200,000 square | | | | miles) | 20 | 4,000,000,000,000 | +-----------------------------------+------------+-------------------+

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.

[Sidenote: 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.

[Sidenote: 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._]

[Sidenote: 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."

[Sidenote: 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.]