Chapter 7

Fig. 40.—Vegetable tissues from coal.a,SigillariaandCordaites.Calamodendron.

Returning to the more special subject of this work, I may remark that the lepidodendroid trees and the ferns, both the arborescent and herbaceous kinds, are even more richly represented in the Carboniferous than in the preceding Erian, I must, however, content myself with merely introducing a few representatives of some of the more common kinds, in an appended note, and here give a figure of a well-known Lower Carboniferous lepidodendron, with its various forms of leaf-bases, and its foliage and fruit (Fig. 43), and a similar illustration of an allied generic form, that known asLepidophloios[CB](Fig. 44).

[CB]For full descriptions of these, see “Acadian Geology.”

Fig. 41.—Beds associated with the main coal (S. Joggins, Nova Scotia). 1, Shale and sandstone—plants withSpirorbisattached; rain-marks (?). (2, Sandstone and shale, eight feet—erectCalamites; 3, Gray sandstone, seven feet; 4, Gray shale, four feet—an erect coniferous (?) tree, rooted on the shale, passes up through fifteen feet of the sandstones and shale.) 5, Gray sandstone, four feet. 6, Gray shale, six inches—prostrate and erect trees, with rootlets, leaves,Naiadites, andSpirorbison the plants. 7, Main coal-seam, five feet of coal in two seams. 8, Underclay, with rootlets.

Fig. 42.—ErectSigillaria, standing on a coal-seam (S. Joggins, Nova Scotia).

Another group which claims our attention is that of theCalamites. These are tall, cylindrical, branchless stems, with whorls of branchlets, bearing needle-like leaves and spreading in stools from the base, so as to form dense thickets, like Southern cane-brakes (Fig. 46). They bear, in habit of growth and fructification, a closerelation to our modern equisetums, or mare’s-tails, but, as in other cases we have met with, are of gigantic size and comparatively complex structure. Their stems, in cross-section, show radiating bundles of fibres, like those of exogenous woods, yet the whole plan of structure presents some curious resemblances to the stems of their humble successors, the modern mare’s-tails. It would seem, from the manner in which dense brakes of theseCalamiteshave been preserved in the coal-formation of Nova Scotia, that they spread over low and occasionally inundated flats, and formed fringes on the seaward sides of the great Sigillaria forests. In this way they no doubt contributed to prevent the invasion of the areas of coal accumulation by the muddy waters of inundations, and thus, though they may not have furnished much of the material of coal, they no doubt contributed to its purity. Many beautiful plants of the genera Asterophyllites andAnnulariaare supposed to have been allied to theCalamites, or to have connected them with theRhizocarps. The stems and fruit of these plants have strong points of resemblance to those ofSphenophyllum, and the leaves are broad, and not narrow and angular like those of the trueCalamites(Fig. 45).

Fig. 43.—Lepidodendron corrugatum, Dawson, a tree characteristic of the Lower Carboniferous, A, Restoration. B, Leaf, natural size, C, Cone and branch, D, Branch and leaves, E. Various forms of leaf-areoles. F,Sporangium, I, L, M, Bark, with leaf-scars, N, Bark, with leaf-scars of old stem, O, Decorticated stem (Knorria).

Fig. 44.—Lepidophloios Acadianus, Dawson, a lepidodendroid tree of the coal-formation, A, Restoration. B, Portion of bark (two thirds natural size), C, Ligneous surface of the same, F, Cone (two thirds natural size). G, Leaf (natural, size), K, Portion of woody cylinder, showing outer and inner series of vessels magnified, L, Scalariform vessels (highly magnified), M, Various forms of leaf-scars and leaf-bases (natural size).

Fig. 45.—Asterophyllites,Sphenophyllum, andAnnularia. A,Asterophyllites trinerne. A1, Leaf enlarged, B,Annularia sphenophylloides. B1, Leaf enlarged, C,Sphenophyllum erosum. C1, Leaflet enlarged. C2, Scalariform vessel ofSphenophyllum. D,Pinnularia ramosissima, probably a root.

No one has done more than my friend Dr. Williamson, of Manchester, to illustrate the structure of Calamites, and he has shown that these plants, like other cryptogams of the Carboniferous, had mostly stems with regular fibrous wedges, like those of exogens. The structure of the stem is, indeed, so complex, and differs so much in different stages of growth, and different states of preservation, that we are in danger of falling into the greatest confusion in classifying these plants. Sometimes what we call a Calamite is a mere cast of its pith showing longitudinal striæ and constrictions at the nodes. Sometimes we have the form of the outer surface of the woody cylinder, showing longitudinal ribs, nodes, and marks of the emission of the branchlets. Sometimes we have the outer surface of the plant covered with a smooth bark showing flat ribs, or almost smooth, and having at the nodes regular articulations with the bases of the verticillatebranchlets, or on the lower part of the stem the marks of the attachment of the roots. The Calamites grew in dense clumps, budding off from one another, sometimes at different levels, as the mud or sand accumulated about their stems, and in some species there were creeping rhizomata or root-stocks (Figs.46to49).

Fig. 46.—Calamites. A,C. Suckovii. B,C. Cistii. (From “Acadian Geology.”)

Fig. 47.—ErectCalamites, with roots attached (Nova Scotia).

Fig. 48.—Node ofC. Cistii, with long leaves (Nova Scotia).

Fig. 49.—ErectCalamites(C. Suckovii), showing the mode of growth of new stems (b), and different forms of the ribs (a,c). (Pictou, Nova Scotia.) Half natural size.

But all Calamites were not alike in structure. In a recent paper[CC]Dr. Williamson describes three distinct structural types. What he regards as typical Calamites has in its woody zone wedges of barred vessels, with thick bands of cellular tissue separating them. A second type, whichhe refers toCalamopitus, has woody bundles composed of reticulated or multiporous fibres, with their porous sides parallel to the medullary rays, which are better developed than in the previous form. The intervening cellular masses are composed of elongated cells. This is a decided advance in structure, and is of the type of those forms having the most woody and largest stems, which Brongniart namedCalamodendron(Fig. 50). A third form, to which Dr. Williamson seems to prefer to assign this last name, has the tissue of the woody wedges barred, as in the first, but the medullary rays are better developed than in the second. In this third form the intermediate tissue, or primary medullary rays, is truly fibrous, and with secondary medullary rays traversing it. My own observations lead me to infer that there was a fourth type of calamitean stem, less endowed with woody matter, and having a larger fistulous or cellular cavity than any of those described by Dr. Williamson.

[CC]“Memoirs of the Philosophical Society,” Manchester, 1886-'87.

There is every reason to believe that all these variousand complicated stems belonged to higher and nobler types of mare’s-tails than those of the modern world, and that their fructification was equisetaceous and of the form known asCalamostachys.

We have already seen that noble tree-ferns existed in the Erian period, and these were continued, and their number and variety greatly extended, in the Carboniferous. In regard to the structure of their stems, and the method of supporting these by aërial roots, the tree-ferns of all ages have been nearly alike, and the form and structure of the leaves, except in some comparatively rare and exceptional types, has also been much the same. Any ordinary observer examining a collection of coal-formation ferns recognises at once their kinship to the familiar brackens of our own time. Their fructification is, unfortunately, rarely preserved, so that we are not able, in the case of many species, to speak confidently of their affinities with modern forms; but the knowledge of this subject has been constantly extending, and a sufficient amount of information has been obtained to enable us to say something as to their probable relationships. (Figs.51to55.)

Fig. 50.—Stems ofCalamodendronand tissues magnified (Nova Scotia),a,b, Casts of axis in sandstone, with woody envelope (reduced).c,d, Woody tissue (highly magnified).

The families into which modern ferns are divided are, it must be confessed, somewhat artificial, and in the caseof fossil ferns, in which the fructification is for the most part wanting, it is still more so, depending in great part on the form and venation of the divisions of the fronds. Of about eight families into which modern ferns are divided, seven are found in a fossil state, and of these, four at least, theCyathaceæ, theOphioglosseæ, theHymenophyllaceæ, and theMarattiaceæ, go back to the coal-formation.[CD]

[CD]Mr. R. Kidston has recently described very interesting forms of fern fructification from the coal-formation of Great Britain, and much has been done by European palæobotanists, and also by Lesquereux and Fontaine in America.

Fig. 51.—Group of coal-formation ferns, A,Odontopteris subcuneata(Bunbury), B,Neuropteris cordata(Brongniart). C,Alethopteris lonchitica(Brongniart). D,Dictyopteris obliqua(Bunbury). E,Phyllopteris antiqua(Dawson), magnified; E1, Natural size, F,Neuropteris cyclopteroides(Dawson).

Fig. 52.—Alethopteris grandis(Dawson). Middle coal-formation of Nova Scotia.

Fig. 53.—Cyclopteris(Aneimites)Acadica(Dawson), a tree-fern of the Lower Carboniferous.a, Pinnules.b, Fragment of petiole.c, Remains of fertile pinnules.

Fig. 54.—Sphenopteris latior, Dawson. Coal-formation,a, Pinnule magnified, with traces of fructification.

Fig. 55.—Fructification of Palæozoic ferns,a, Thecæ ofArchæopteris(Erian).b, Theca ofSenftenbergia(Carboniferous).c, Thecæ ofAsterotheca(Carboniferous).

Fig. 56.—Tree-ferns of the Carboniferous. A,Megaphyton magnificum, Dawson, restored. B, Leaf-scar of the same, two thirds natural size. B1, Row of leaf-scars, reduced. C,Palæopteris Harttii, scars half natural size. D,Acadica, scars half natural size.

Some of these ferns have the more complex kind of spore-case, with a jointed, elastic ring. It is to be observed, however, that those forms which have a simple spore-case, either netted or membranous, and without annulus, are most common in the Devonian and lowest Carboniferous. Some of the forms in these old rocks are somewhat difficult to place in the system. Of these, the species ofArchæopteris, of the Upper and Middle Erian, are eminent as examples. This type, however, scarcely extends as high as the coal-formation.[CE]Some of the tree-ferns of the Carboniferous present very remarkable features. One of these, of the genusMegaphyton, seems to have two rows of great leaves, one at each side of the stem, which was probably sustained by large bundles of aërial roots (Fig. 56).

[CE]The pretty little ferns of the genusBotrychium(moonwort), so common in American and European woods, seem to be their nearest modern allies.

In the Carboniferous, as in the Erian, there are leaves which have been referred to ferns, but are subject to doubt, as possibly belonging to broad-leaved taxine trees allied to the gingko-tree of China. One of these, represented inFig. 57, has been found in the coal-formation of Nova Scotia, and referred to the doubtful genusNoeggerathia. Fontaine has proposed for similar leaves found in Virginia the new generic nameSaportea.

Fig. 57.—Noeggerathia disbar(half natural size).

Fig. 58.—Cordaites(Dorycordaites), Grand d’Eury, reduced.

Ferns, as might be inferred from their great age, are at the present time dispersed over the whole world; but their headquarters, and the regions to which tree-ferns are confined, are the more moist climates of the tropics and of the southern hemisphere. The coal-swamps of the northern hemisphere seem to have excelled even these favoured regions of the present world as a paradise for ferns.

I have already stated that the Carboniferous constitutes the headquarters of theCordaites(Fig. 58), of which a large number of species have been described, both inEurope and America. We sometimes, though rarely, find their stems showing structure. In this case we have a large cellular pith, often divided by horizontal partitions into flat chambers, and constituting the objects which, when detached, are calledSternbergiæ(Fig. 62). These Sternbergia piths, however, occur in true conifers as well, as they do in the modern world in some trees, like our common butternut, of higher type; and I showed many years ago that the Sternbergia type may be detected in the young twigs of the balsam-fir (Abies balsamifera). The pith was surrounded by a ring of scalariform or barred tissue, often of considerable thickness, and in young stems so important as to have suggested lycopodiaceous affinities. But as the stem grew in size, a regular ring of woody wedges, with tissue having rounded or hexagonal pores or discs, like those of pines, was developed. Outside this was a bark, often apparently of some thickness. This structure in many important points resembles that of cycads, and also approaches to the structure of Sigillaria, while in its more highly developed forms it approximates to that of the conifers.

Fig. 59.—Fruits ofCordaitesand Taxine Conifers (coal-formation. Nova Scotia.) A,Antholithes squamosus(two thirds). B,A. rhabdocarpi. (two thirds). B1, Carpel restored. C,A. spinosus(natural size). D,Trigonocarpum intermedium. E,T. Noeggerathii. F,T. avellanum. G,Rhabdocarpus insignis, reduced. H,Antholithes pygmæus. I,Cardiocarpum fluitans. K,Cardiocarpum bisectum. L,Sporangites papillata, lycopodiaceous macrospores (natural size and magnified).

On the stems so constructed were placed long and often broad many-nerved leaves, with rows of stomata or breathing-pores, and attached by somewhat broad bases to the stem and branches. The fruit consisted of racemes, or clusters of nutlets, which seem to have been providedwith broad lateral wings for flotation in the air, or in some cases with a pulpy envelope, which flattens into a film. There seem to have been structures of both these kinds, though in the state of preservation of these curious seeds it is extremely difficult to distinguish them. In the first case they must have been intended for dissemination by the wind, like the seeds of spruces. In the latter case they may have been disseminated like the fruits of taxine trees by the agency of animals, though what these were it would be difficult to guess. These trees had very great reproductive power, since they produced numerous seeds, not singly or a few together, as in modern yews, but in long spikes or catkins bearing many seeds (Fig. 59).

It is to be observed that the Cordaites, or theCordaitinæ, as they have been called, as a family,[CF]constitute another of those intermediate groups with which we have already become familiar. On the one hand they approach closely to the broader-leaved yews like Gingko, Phyllocladus, and Podocarpus, and, on the other hand, they have affinities with Cycadaceæ, and even with Sigillariæ. They were beautiful and symmetrical trees, adding something to the variety of the rather monotonous Palæozoic forests. They contributed also somewhat to the accumulation of coal. I have found that some thin beds are almost entirely composed of their leaves, and the tissues of their wood are not infrequent in the mineral charcoal of the larger coal-seams. There is no evidence that their roots were of the stigmaroid type, though they evidently grew in the same swampy flats with the Sigillariæ and Calamites.

[CF]Engler; Cordaitées of Renault.

It may, perhaps, be well to say here that I believe there was a considerably wide range of organisation in the Cordaitinæ as well as in the Calamites and Sigillariæ, and that it will eventually be found that there were three linesof connection between the higher cryptogams and the phænogams, one leading from the lycopods by the Sigillariæ, another leading by the Cordaites, and the third leading from the Equisetums by the Calamites. Still further back the characters afterward separated in the club-mosses, mare’s-tails, and ferns, were united in the Rhizocarps, or, as some now, but I think somewhat unreasonably, prefer to call them, the “heterosporous Filicinæ.” In the more modern world, all the connecting links have become extinct and the phænogams stand widely separated from the higher cryptogams. I do not make these remarks in a Darwinian sense, but merely to state what appear to be the lines of natural affinity and the links wanting to give unity to the system of nature.

Of all the trees of the modern world, none are perhaps so widely distributed as the pines and their allies. On mountain-tops and within the Arctic zone, the last trees that can struggle against the unfavourable conditions of existence are the spruces and firs, and in the warm and moist islands of the tropics they seem equally at home with the tree-ferns and the palms. We have already seen that they are a very ancient family, and in the sandstones of the coal-formation their great trunks are frequently found, infiltrated with calcareous or silicious matter, and still retaining their structure in the greatest perfection (Fig. 60). So far as we know, the foliage of some of them which constitutes the generaWalchiaandAraucaritesof some authors (Figs.60,63) was not dissimilar from that of modern yews and spruces, though there is reason to believe that some others had broad, fern-like leaves like those of the gingko. None of them, so far as yet certainly known, were cone-bearing trees, their fruit having probably been similar to that of the yews (Fig. 61). The minute structures of their stems are nearer to those of the conifers of the islands of the southern hemisphere than to that of those in our northern climes—a correlation, no doubt, to the equable climate of the period. There is not much evidence that they grew with the Sigillariæ in the true coal-swamps, though some specimens have been found in this association. It is more likely that they were in the main inland and upland trees, and that in consequence they are mostly known to us by drifted trunks borne by river inundations into the seas and estuaries.

Fig. 60.—Coniferous wood and foliage (Carboniferous). A,Araucarites gracilis, reduced, b,Dadoxylon Acadianum(radial), 90 diams.; B1(tangential), 90 diams; B2, cell showing areolation, 250 diams. C,Dadoxylon materiarium(radial), 90 diams.; C1(tangential), 90 diams. C2, cell showing areolation, 250 diams. D,Dadoxylon antiquius(radial), 90 diams.; D1(tangential), 90 diams.; D2, cell showing areolation, 250 diams.

Fig. 61.—_Trigonocarpum Hookeri, Dawson from the coal-measures of Cape Breton. Probably the fruit of a Taxine tree. A, Broken specimen magnified twice natural size, B, Section magnified:a, the testa;b, the tegmen;c, the nucleus;d, the embryo,c, Portion of the surface of the inner coat more highly magnified.

A remarkable fact in connection with them, and showing also the manner in which the most durable vegetable structures may perish by decay, is that, like the Cordaites, they had large piths with transverse partitions, a structurewhich, as I have already mentioned, appears on a minute scale in the twigs of the fir-tree, and that sometimes casts of these piths in sandstone appear in a separate form, constituting what have been namedSternbergiæorArtisiæ. As Renault well remarks with reference to Cordaites, the existence of this chambered form of pith implies rapid elongation of the stem, so that the Cordaites and conifers of the coal-formation were probably quickly growing trees (Fig. 62).

Fig. 62.—Sternbergiapith ofDadoxylon. A, Specimen (natural size), showing remains of wood ata,a. B, Junction of wood and pith, magnified. C, Cells of the wood of do.,a,a;b, medullary ray;c, areolation.

The same general statements may be made as to the coal-vegetation as in relation to that of the Erian. In the coal period we have found none of the higher exogens, and there are only obscure and uncertain indications of the presence of endogens, which we may reserve for a future chapter; but gymnosperms abound and are highly characteristic. On the other hand, we have no mosses or lichens, and very few Algæ, but a great number of ferns and Lycopodiaceæ or club-mosses (Fig. 63). Thus, the coal-formation period is botanically a meeting-place of the lower phænogams and the higher cryptogams, and presents many forms which, when imperfectly known, have puzzled botanists in regard to their position in one or other series. In the present world, the flora most akinto that of the coal period is that of warm, temperate regions in the southern hemisphere. It is not properly a tropical flora, nor is it the flora of a cold region, but rather indicative of a moist and equable climate. Still, we must bear in mind that we may often be mistaken in reasoning as to the temperature required by extinct species of plants, differing from those now in existence. Further, we must not assume that the climatal conditions of the northern hemisphere were in the coal period at all similar to those which now prevail. As Sir Charles Lyell has shown, a less amount of land in the higher latitudes would greatly modify climates, and there is every reason to believe that in the coal period there was less land than now. Further, it has been shown by Tyndall that a very small additional amount of carbonic acid in the atmosphere would, by obstructing the radiation of heat from the earth, produce almost the effect of a glass roof or conservatory, extending over the whole world. Again, there is much in the structure of the leaves of the coal-plants, as well as in the vast amount of carbon which they accumulated in the form of coal, and the characteristics of the animal life of the period, to indicate, on independentgrounds, that the carboniferous atmosphere differed from that of the present world in this way, or in the presence of more carbonic acid—a substance now existing in the very minute proportion of one thousandth of the whole—a quantity adapted to the present requirements of vegetable and animal life, but probably not to those of the coal period.

Fig. 63.—Walchia imbricatula, S. N., Permian, Prince Edward Island.

Thus, if we inquire as to any analogous distribution of plants in the modern world, we find this only in the warmer insular climates of the southern hemisphere, where ferns, lycopods, and pines appear under forms somewhat akin to those of the Carboniferous, but mixed with other types, some of which are modern, others allied to those of the next succeeding geological ages of the Mesozoic and Tertiary; and under these periods it will be more convenient to make comparisons.

The readers of recent English popular works on geology will have observed the statement reiterated that a large proportion of the material of the great beds of bituminous coal is composed of the spore-cases of lycopodiaceous plants—a statement quite contrary to that resulting from my microscopical examinations of the coal of more than eighty coal-beds in Nova Scotia and Cape Breton, as stated in “Acadian Geology” (page 463), and more fully in my memoir of 1858 on the “Structures in Coal,”[CG]and that of 1866, on the “Conditions of Accumulation of Coal.”[CH]The reason of this mistake is, that an eminent English naturalist, happening to find in certain specimens of English coal a great quantity of remains of spores and spore-cases, though even in his specimens they constitute only a small portion of the mass, and being apparently unacquainted with what others had done in this field, wrote a popular article for the “Contemporary Review,” in which he extended an isolated andexceptional fact to all coals, and placed this supposed origin of coal in a light so brilliant and attractive that he has been followed by many recent writers. The fact is, as stated in “Acadian Geology,” that trunks ofSigillariæand similar trees constitute a great part of the denser portion of the coal, and that the cortical tissues of these rather than the wood remain as coal. But cortical or epidermal tissues in general, whether those of spore-cases or other parts of plants, are those which from their resistance to water-soakage and to decay, and from their highly carbonaceous character, are best suited to the production of coal. In point of fact, spore-cases, though often abundantly present, constitute only an infinitesimal part of the matter of the great coal-beds. In an article in “The American Journal of Science,” which appeared shortly after that above referred to, I endeavoured to correct this error, though apparently without effect in so far as the majority of British geological writers are concerned. From this article I have taken with little change the following passages, as it is of importance in theoretical geology that such mistakes, involving as they do the whole theory of coal accumulation, should not continue to pass current. The early part of the paper is occupied with facts as to the occurrence of spores and spore-cases as partial ingredients in coal. Its conclusions are as follows: It is not improbable that sporangites, or bodies resembling them, may be found in most coals; but it is most likely that their occurrence is accidental rather than essential to coal accumulation, and that they are more likely to have been abundant in shales and cannel coals, deposited in ponds or in shallow waters in the vicinity of lycopodiaceous forests, than in the swampy or peaty deposits which constitute the ordinary coals. It is to be observed, however, that the conspicuous appearance which these bodies, and also the strips and fragments of epidermal tissue, which resemble them intexture, present in slices of coal, may incline an observer, not having large experience in the examination of coals, to overrate their importance; and this I think has been done by most microscopists, especially those who have confined their attention to slices prepared by the lapidary. One must also bear in mind the danger arising from mistaking concretionary accumulations of bituminous matter for sporangia. In sections of the bituminous shales accompanying the Devonian coal above mentioned, there are many rounded yellow spots, which on examination prove to be the spaces in the epidermis ofPsilophytonthrough which the vessels passing to the leaves were emitted. To these considerations I would add the following, condensed from the paper above referred to (p. 139), in which the whole question of the origin of coal is fully discussed:[CI]

[CG]“Journal of the Geological Society,” vol. xv.

[CH]Ibid., vol. xxii.

[CI]See also “Acadian Geology,” 2d ed., pp. 138, 461, 493.

1. The mineral charcoal or ‘mother coal’ is obviously woody tissue and fibres of bark, the structure of the varieties of which, and the plants to which it probably belongs, I have discussed in the paper above mentioned.

2. The coarser layers of coal show under the microscope a confused mass of fragments of vegetable matter belonging to various descriptions of plants, and including, but not usually in large quantities, sporangites.

3. The more brilliant layers of the coal are seen, when separated by thin laminæ of clay, to have on their surfaces the markings ofSigillariæand other trees, of which they evidently represent flattened specimens, or rather the bark of such specimens. Under the microscope, when their structures are preserved, these layers show cortical tissues more abundantly than any others.

4. Some thin layers of coal consist mainly of flattened layers of leaves ofCordaitesorPychnophyllum.

5. TheStigmariaunder-clays and the stumps ofSigillariain the coal-roofs equally testify to the accumulation of coal by the growth of successive forests, more especially ofSigillariæ. There is, on the other hand, no necessary connection of sporangite-beds with Stigmarian soils. Such beds are more likely to be accumulated in water, and consequently to constitute bituminous shales and cannels.

6.Lepidodendronand its allies, to which the spore-cases in question appear to belong, are evidently much less important to coal accumulation thanSigillaria, which cannot be affirmed to have produced spore-cases similar to those in question, even though the observation of Goldenberg as to their fruit can be relied on; the accuracy of which, however, I am inclined to doubt.

On the whole, then, while giving due credit to those who have advocated the spore-theory of coal, for directing attention to this curious and no doubt important constituent of mineral fuel, and admitting that I may possibly have given too little attention to it, I must maintain that sporangite-beds are exceptional among coals, and that cortical and woody matters are the most abundant ingredients in all the ordinary kinds; and to this I cannot think that the coals of England constitute an exception.

It is to be observed, in conclusion, that the spore-cases of plants, in their indestructibility and richly carbonaceous character, only partake of qualities common to most suberous and epidermal matters, as I have explained in the publications already referred to. Such epidermal and cortical substances are extremely rich in carbon and hydrogen, in this resembling bituminous coal. They are also very little liable to decay, and they resist more than other vegetable matters aqueous infiltration—properties which have caused them to remain unchanged, and to continue free from mineral additions more than other vegetable tissues. These qualities are well seen in the bark of our American white birch. It is no wonder thatmaterials of this kind should constitute considerable portions of such vegetable accumulations as the beds of coal, and that when present in large proportion they should afford richly bituminous beds. All this agrees with the fact, apparent on examination of the common coal, that the greater number of its purest layers consist of the flattened bark ofSigillariæand similar trees, just as any single flattened trunk embedded in shale becomes a layer of pure coal. It also agrees with the fact that other layers of coal, and also the cannels and earthy bitumens, appear under the microscope to consist of finely comminuted particles, principally of epidermal tissues, not only from the fruits and spore-cases of plants, but also from their leaves and stems. These considerations impress us, just as much as the abundance of spore-cases, with the immense amount of the vegetable matter which has perished during the accumulation of coal, in comparison with that which has been preserved.

I am indebted to Dr. T. Sterry Hunt for the following very valuable information, which at once places in a clear and precise light the chemical relations of epidermal tissue and spores with coal. Dr. Hunt says: "The outer bark of the cork-tree, and the cuticle of many if not all other plants, consists of a highly carbonaceous matter, to which the name ofsuberinhas been given. The spores ofLycopodiumalso approach to this substance in composition, as will be seen by the following, one of two analyses by Duconi,[CJ]along with which I give the theoretical composition of pure cellulose or woody fibre, according to Payen and Mitscherlich, and an analysis of the suberin of cork, fromQuercus suber, from which the ash and 2·5 per cent of cellulose have been deducted.[CK]

[CJ]Liebig and Kopp, “Jahresbuch,” 1847-'48.

[CK]Gmelin, “Handbook,” xv., 145.

"This difference is not less striking when we reduce the above centesimal analyses to correspond with the formula of cellulose, C24H20O20, and represent cork andLycopodiumas containing twenty-four equivalents of carbon. For comparison I give the composition of specimens of peat, brown coal, lignite, and bituminous coal:[CL]

[CL]“Canadian Naturalist,” vi., 253.

“It will be seen from this comparison that, in ultimate composition, cork andLycopodiumare nearer to lignite than to woody fibre, and may be converted into coal with far less loss of carbon and hydrogen than the latter. They in fact approach closer in composition to resins and fats than to wood, and, moreover, like those substances repel water, with which they are not easily moistened, and thus are able to resist those atmospheric influences which effect the decay of woody tissue.”

I would add to this only one further consideration. The nitrogen present in theLycopodiumspores, no doubt, belongs to the protoplasm contained in them, a substance which would soon perish by decay; and subtracting this, the cell-walls of the spores and the walls of the spore-caseswould be most suitable material for the production of bituminous coal. But this suitableness they share with the epidermal tissue of the scales of strobiles, and of the stems and leaves of ferns and lycopods, and, above all, with the thick, corky envelope of the stems ofSigillariæand similar trees, which, as I have elsewhere shown,[CM]from its condition in the prostrate and erect trunks contained in the beds associated with coal, must have been highly carbonaceous and extremely enduring and impermeable to water. In short, if, instead of “spore-cases,” we read “epidermal tissues in general, including spore-cases,” all that has been affirmed regarding the latter will be strictly and literally true, and in accordance with the chemical composition, microscopical characters, and mode of occurrence of coal. It will also be in accordance with the following statement, from my paper on the “Structures in Coal,” published in 1859:

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