COAL PLANTS. The vegetation of the globe during the different stages of its formation has undergone very great and evident changes. There is however no reason to doubt that the fossil plants known from the different formations of our earth may all be referred to the great classes of the vegetable kingdom distinguished at the present day, viz.: the thallogens, including such plants as lichens, algae, and fungi; the acrogens, such as ferns and lycopods; the gymnosperms, such as cone-bearing plants and cycads; the endogens, such as palms, lilies, and grasses; and the exogens or dicotyledons, to which belong most of our species of trees. Though the distribution of vegetable fossil remains in these great classes is easily and truly recognized, their relations to genera and species are by no means so certain. What has been said of the formation of coal and its distribution at different geological periods (see Coal) shows that the examination of the coal floras should not be confined to the vegetation of the carboniferous period. But this limitation is admitted here, for the reason that the fossil flora of the carboniferous is more important in its study and is better known than that of any other epoch, and that its essential characters have been recognized in the contemporaneous floras over the whole world. Moreover, the essential facts concerning the floras of the periods subsequent to the carboniferous measures have been touched upon already.—The plants of the true carboniferous period are mostly, if not all, referable to the class of the acrogens or vascular cryptogams: ferns, lycopods or club mosses, and equisetaceæ or the horsetail family. A very few species have been considered as belonging to phænogamous gymnosperms: the conifers by 12 species known only by the structure of fossil trunks which represent them; the cycadeæ by so-called species of Noeggerathia, all except one, N. foliosa, of uncertain relation, and by the very common flabellaria or Cordaites borassifolia, mostly represented like Noeggerathia by long ribbon-like leaves only. One stem of it has been found in a silicified state and analyzed by Corda, who considers the plant as allied by its structure to the lycopodiaceous and to the cycadeaæ. The true place of these leaves in the vegetable scale is therefore not positively ascertained.
Fig. 1.—Flabellaria borassifolia.
—It is a remarkable fact that trunks of conifers, or those referable to this family by their structure, have been found in the upper Devonian and the subcarboniferous measures of the United States, but that as yet no remains of this kind have been discovered in the true coal measures above the millstone grit. A splendid specimen from the upper Devonian of Indiana, a stem half a foot in diameter, has its surface fluted exactly like that of a calamites, while its internal structure is related to that of araucaria.—The ferns have the most abundant species of the carboniferous period; already more than 600 have been described from this formation. Of these, about 200 species have been found in the coal fields of the United States, more than 50 of which appear to be indigenous or have not till now been observed in Europe. They represent either distinct peculiar types or some analogous ones which are distinguished by less appreciable characters, and which may perhaps be eliminated by comparison with European specimens. As now known, the North American coal flora presents differences evident enough to impress it with a distinct American character, while on the other hand the numerous identical species show it to be as positively representative of the same period as that of the European or other carboniferous floras. The determination of the species of ferns of the coal measures is rendered difficult and somewhat uncertain, at least in regard to their relation to living species, by the absence of fructifications, which essentially serve as characters for their classification. In the fossil specimens, the nervation, which is generally well preserved and distinct, together with the form of the fronds and of their divisions, the pinnae and pinnules or leaflets, are the only appreciable characters for the determination and even the general classification; as for example that of Brongniart, the oldest, simplest, and most comprehensible of those which have been attempted. It admits four general divisions:
Fig. 2.—1. Neuropteris hirsuta. 2. Adiantites Lindseæformis.
1. Neuropterideæ. Frond simple or with compound leaflets free or adhering, without middle nerve or merely with a middle nerve at the base, disappearing upward; veins dichotomous or flagellate. 2. Adiantideæ. Frond pinnate, bipinnate, or tripinnate; leaflets narrowed to the base, flabelliform, entire or scarcely lobed; veins diverging from the base, without a distinct middle nerve.
Fig. 3.—1. Sphenopteris tridactylitis. 2. Leaf enlarged.
3. Sphenopterideæ. Fronds of the same kind; veins pinnate or bipinnate near the base; secondary divisions very oblique.
Fig. 4.—Pecopteris louchitica.
4. Pecopterideæ. Fronds simple, pinnate or bipinnate, the leaflets generally adhering by the whole base, or often joined (confluent) at or near the base, then forming lobes, entire or denticulate; secondary veins pinnate, dichotomous or reticulate.—This general classification is still used as a commodious frame for establishing a large number of subdivisions, though many other systems have been proposed since. Among others, the two more important are that of Göppert, established in his Genera, where he especially considers fructifications, and that of Ettinghausen, which fixes with great detail the character of the groups and their subdivisions from similarity of nervation. These different systems are interesting to palæontologists, but present all the same insufficiency resulting from the difficulty of observing the fructification, or from the similarity of the nervation in different species or even genera. Five or six genera of living ferns, for example, exactly limited by the characters of their fructifications, all present in some of their species the same kind of nervation. It is worth remarking, however, that the fossil ferns of the carboniferous measures are in this country more generally found in fruiting specimens than in Europe.
Fig. 5.—Caulopteris Worthenii.
The same remark may be applied to the trunks of fern trees, whose scarcity has been generally remarked in Europe as a fact seemingly in contradiction with what should be expected in considering the great size of the other kinds of vegetables of the carboniferous period. In the coal measures of the United States, these trunks of ferns, named caulopteris, stemmatopteris, psaronius, &c., and generally classed by the characters of their internal structure or by the scars of leaves upon the bark, are not rare, indeed are extremely abundant in some localities. In southern Ohio and northern Virginia, trunks of ferns are imbedded in a thick bed of sandstone occupying a wide area. These trunks, mostly silicified, are either standing or prostrated, and their fragments are now strewn along the creeks, which by their course through this region have taken off and displaced these remains. They are not only in great number, but some of them are large. The average diameter is 8 in., but some measure more than a foot. A splendid specimen in the museum of comparative zoölogy at Cambridge is 2 ft. high and 14 in. in diameter, and almost exactly cylindrical. Generally, however, in connection with the roof shale of coal beds, the ferns are mostly bushy or herbaceous species, the shale being composed of soft muddy materials, forming a ground too unstable for the vegetation of large trees.
Fig. 6.—1. Lepidodendron. 2. Scars of a larger branch.
—The club moss or lycopod family is represented in the carboniferous formation mostly by large trees and floating stems, also proportionately large. They belong to several genera, of which the more remarkable are the lepidodendron, sigillaria, and stigmaria. These genera are mostly characterized by the impressions of the leaves forming at the point of attachment rhomboidal, oval, or round scars, which, small in young branches and young stems, enlarge after the fall of the leaves in proportion to the growth of the trees. In species of lepidodendron these scars are placed spirally along the trunk and contiguous to each other. In the small branches or immediately after the parting of the leaves these scars are scarcely half a line in diameter on each side; in old trees of the same species the scars are an inch and a half long and an inch broad. They form either surface or flat impressions, or are more or less deeply cut into the bark.
Fig. 7.—Sigillaria.
The sigillaria species differ essentially from the lepidodendron by the position of the scars, which are placed in vertical rows separated by convex or flat ribs, and generally at a distance from each other. The form of these scars is extremely variable, and, as in lepidodendron, their size enlarges in proportion to the growth of the trees. The identification of the species is therefore very difficult, especially when it has to be made, as is generally the case, from fragmentary specimens. It is rendered still more embarrassing by the fact that the bark of some species of sigillaria is composed of different thin layers, easily separated, on which the leaf scars, though superposed, differ upon each layer. The club mosses of our time rarely have stems half an inch in diameter; those of the coal, or the lepidodendron and sigillaria, were represented by trees sometimes 2 ft. in diameter and 40 to 60 ft. high.
Fig. 8.—1. Stigmaria with leaves. 2. Stem of Stigmaria with scars. 3. Leaf of Stigmaria.
The stigmaria are comparable to the sigillaria by their texture; but they were merely floating stems, which, suspended in water by long hollow leaves, had a mode of life independent from that of their flower- or fruit-bearing stems, the sigillaria, lepidodendron, &c., which came out only when a kind of ground had been formed and consolidated by the growth of the stigmaria. The connection of these floating stems with trunks of sigillaria and of lepidodendron has disposed many palæontologists to consider them as mere roots. But besides many other considerations, the fact that these stigmaria did live independently and for long periods without any sigillaria stems, even forming by their debris thick strata of clay, is a sufficient reason for recognizing them as true stems. In the peat bogs of our time there is a small club moss, lycopodium inundatum, which in a diminutive way shows us the mode of life of those stigmaria. It expands all around upon the soft muddy surface of the bogs, its creeping stems interlaced in every direction; and when the carpet has become solid enough, the flowering stems come up and ripen their seeds out of reach of the water. The stigmaria trunks do not vary much in size from 2 to 4 in. in diameter, but they indefinitely extended their branches by forking, and with their long leaves formed a floating network, soon solid enough to support large trees. The same result is now produced in our peat bogs by the vegetation of the floating mosses. All the scars left at the point of attachment of the leaves of stigmaria upon their stems are almost exactly round, like a swollen convex surface one and a half to two lines in diameter, surrounded by a ring and pierced in the centre by a point, the scar of a vessel. The relative position of the scars is more generally in quincuncial order, at a distance of an inch or less; they are sometimes connected by star-like wrinkles, or separated in undulate rows by narrow ridges.
Fig. 9.—1. Lepidostrobus. 2. Cross section of a small species. 3. Sporange and blade of Lepidostrobus.
The fruits of the lycopodiaceous species of the carboniferous, lepidostrobus, are in the form of seeds enclosed in receptacles or spore cases in right angle to a common axis, and forming cones covered with imbricated scales. Their form and arrangement are like those of the club mosses, but the size of the cones is proportionate to that of the trees. They vary from 1 to 2 in. in diameter, and from 2 to 18 in. or more in length.
Fig. 10.—Calamites.
—The calamites, which have left very abundant remains in the coal, are closely related by their hollow articulated stems with bark vertically and equally fluted, by their fructifications, and by their structure, to living species of equisetum (horsetail). But they have the same enormous proportions in size as remarked for lepidodendron and sigillaria compared with club mosses. Their trunks vary from 1 to 6 in. in diameter, their length from 6 to 40 ft. They appear to have grown close together and in thickets, at least in some localities; for example, near Carbondale, Pa., such forests of calamites are seen petrified, standing and imbedded in sandstone, their trunks in close proximity to one another. The branches of calamites bear whorls of linear narrow leaves, which have been described as asterophyllites. Their fruits, Wolkmannia, are cones bearing seeds like those of species of equisetum, merely differing by the presence of scales and imbricated in rows. Two other genera, represented by branches bearing leaves also in whorls, appear referable to the same family of calamites, or perhaps to one intermediate between calamites and lepidodendron.
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| Fig. 11.—Annularia longifolia. | Fig. 12.—Sphenophyllum Schlotheimii. |
One of these, annularia, bears linear, lanceolate, thickish, pointed leaflets, sometimes inflated and nearly cylindrical; it resembles asterophyllites in its facies; the other, sphenophyllum, has cuneate or flabellate leaflets, enlarging upward from the point of attachment.—It is now generally admitted that the formation of a bed of coal, and of the strata which separate it from the one above it, has demanded a long period of time, sufficient to modify somewhat the characters of the vegetation. It seems therefore that it might be easy to remark and point out the difference between the plants of two subsequent beds of coal. The miners in general appreciate this difference, and indeed it is usually an easy matter for a palæontologist to remark it when his observations are limited to the same locality. But the study is rendered very difficult by the variations in the distribution of the plants as remarked in the same bed at distant localities. The variability acting in two ways, horizontally and vertically, the result of its force in one way or the other can be appreciated only from long researches. In the coal fields of the United States, lepidodendron species predominate in the lower part of the true coal measures, and their remains have especially contributed to the formation of the big vein, B. They gradually disappear in passing up. The sigillaria species are more equally distributed from the base to the middle, while the ferns become more predominant in ascending, especially in small species. The calamites and ribbon-like flabellaria are about in the same proportion through the whole thickness of the carboniferous.—The peculiar distribution of trunks of fern trees in sandstone, which has been remarked upon already, may be taken for evidence in a question often discussed among palæontologists: Do the plant remains found in connection with the coal strata represent the whole flora of the carboniferous period? or was this flora, like that of our present peat bogs, composed of a limited number of species appropriate by their nature to the growth and composition of the coal, while a different kind of vegetation inhabited dry or high ground? All the specimens of fossil plants found in beds of sandstone, even where no trace of coal is found, represent the same species, or at least the same genera, as those found in connection with coal beds.
Fig. 13.—1. Carpolithes. 2. Trigonocarpus. 3. Cardiocarpus.
Hard fruits, resembling nuts and named trigonocarpum, carpolithes, &c., are locally abundant in sandstone, but sometimes the shale of the coal has them in plenty. Nothing is known as to the relation of these fruits, which have never been found attached to any kind of vegetable remains. They may have been floated by water from a distance, and may therefore, as has been supposed, represent species of plants not found in the true coal basins; but this is very doubtful. The composition of the coal flora, like the formation of the coal over immense surfaces, indicates a great degree of humidity in the atmosphere. The vapors covering the land, extending over the whole emerged surface, developed everywhere the same kind of vegetation, diversified only in its exuberance according to the composition, especially the solidity of the ground. Even the vegetation of the hills, if there were any in proximity to the coal basins, should have been exposed to the same influence. In the southern islands, where fern trees are most common, these plants everywhere follow or ascend the slopes of the mountains in exact concordance with the line of the fogs; therefore a general and permanent degree of humidity should have influenced the same uniform character for the vegetation of the whole land. This seems still better evidenced by the identity of the essential vegetable types in the carboniferous formation of the whole world. On this subject, however, as on other questions concerning the distribution of the old floras, there is still a great deal of uncertainty, especially caused by insufficiency of materials for observation.