Chapter 8

[CM]“Vegetable Structures in Coal,” “Journal of Geological Society,” xv., 626. “Conditions of Accumulation of Coal,”ibid., xxii., 95. “Acadian Geology,” 197, 464.

“A single trunk ofSigillariain an erect forest presents an epitome of a coal-seam. Its roots represent theStigmariaunder-clay; its bark the compact coal; its woody axis the mineral charcoal; its fallen leaves (and fruits), with remains of herbaceous plants growing in its shade, mixed with a little earthy matter, the layers of coarse coal. The condition of the durable outer bark of erect trees concurs with the chemical theory of coal, in showing the especial suitableness of this kind of tissue for the production of the purer compact coals. It is also probable that the comparative impermeability of the bark to mineral infiltration is of importance in this respect, enabling this material to remain unaffected by causes which have filled those layers, consisting of herbaceous materials and decayed wood, with pyrites and other mineral substances.”

We need not go far in search of the uses of the coal vegetation, when .we consider the fact that the greatest civilised nations are dependent on it for their fuel. Without the coal of the Carboniferous period and the iron-ore which is one of the secondary consequences of coal accumulation, just as bog-ores of iron occur in the subsoils of modern peats, it would have been impossible either to sustain great nations in comfort in the colder climates of the northern hemisphere or to carry on our arts and manufactures. The coal-formation yields to Great Britain alone about one hundred and sixty million tons of coal annually, and the miners of the United States extract mainly from the same formation nearly a hundred million tons, while the British colonies and dependencies produce about five million tons; and it is a remarkable fact that it is to the English race that the greatest supply of this buried power and heat and light has been given.

The great forests of the coal period, while purifying the atmosphere of its excess of unwholesome carbonic acid, were storing up the light and heat of Palæozoic summers in a form in which they could be recovered in our human age, so that, independently of their uses to the animals which were their contemporaries, they are indispensable to the existence of civilised man.

Nor can we hope soon to be able to dispense with the services of this accumulated store of fuel. The forests of to-day are altogether insufficient for the supply of our wants, and though we are beginning to apply water-power to the production of electricity, and though some promising plans have been devised for the utilisation of the direct heat and light of the sun, we are still quite as dependent as any of our predecessors on what has been done for us in the Palæozoic age.

In the previous pages I have said little respecting the physical geography of the Carboniferous age; but, as maybe inferred from the vegetation, this in the northern hemisphere presented a greater expanse of swampy flats little elevated above the sea than we find in any other period. As to the southern hemisphere, less is known, but the conditions of vegetation would seem to have been essentially the same.

Taking the southern hemisphere as a whole, I have not seen any evidence of a Lower Devonian or Upper Silurian flora; but in South Africa and Australia there are remains of Upper Devonian or Lower Carboniferous plants. These were succeeded by a remarkable Upper Carboniferous or Permian group, which spread itself all over India, Australia, and South Africa,[CN]and contains some forms (Vertebraria,Phyllotheca,Glossopteris, &c. ) not found in rocks of similar age in the northern hemisphere, so that, if the age of these beds has been correctly determined, the southern hemisphere was in advance in relation to some genera of plants. This, however, is to be expected when we consider that the Triassic and Jurassic flora of the north contains or consists of intruders from more southern sites. These beds are succeeded in India by others holding cycads, &c., of Upper Jurassic or Lower Cretaceous types (Rajmahal and Jabalpur groups).

[CN]Wyley, “Journal Geol. Society,” vol. xxiii., p. 172; Daintree,ibid., vol. xxviii.; also Clarke and McCoy.

Blanford has shown that there is a very great similarity in this series all over the Australian and Indian region.[CO]Hartt and Darby have in like manner distinguished Devonian and Carboniferous forms in Brazil akin to those of the northern hemisphere. Thus the southern hemisphere would seem to have kept pace with the northern, and according to Blanford there is evidence there of cold conditions in the Permian, separating the Palæozoicflora from that of the Mesozoic, in the same manner that Ramsay has supposed a similar period of cold to have done north of the equator. This would imply a very great change of climate, since we have evidence of the extension of the Lower Carboniferous flora at least as far north as Spitzbergen. The upper coal-formation we cannot, however, trace nearly so far north; so that a gradual refrigeration may have been going on before the Permian. Thus in both hemispheres there was a general similarity in the later Palæozoic flora, and perhaps similar conditions leading to its extinction and to its replacement by that to be described in the next chapter.

[CO]“Journal Geol. Society,” vol. xxxi.

NOTES TO CHAPTER IV.

I. Characters and Classification of Palæozoic Plants.

Inthe space available in this work it would be impossible to enter fully into the classification of Palæozoic plants; but it may be well to notice some important points for the guidance of those who may desire to collect specimens; more especially as much uncertainty exists as to affinities and very contradictory statements are made. The statements below may be regarded as the results of actual observation and of the study of specimensin situin the rocks, as well as in the cabinet and under the microscope.

Gymnospermeæ.

FamilyConiferæ;GenusDadoxylon, Endlicher;Araucarites, Goeppert;Araucarioxylon, Kraus.

The trunks of this genus occur from the Middle Devonian to the Permian inclusive, as drift-logs calcified, silicified, or pyritised. The only foliage associated with them is of the type ofWalchiaandAraucarites—viz., slender branches with numerous small spiral acicular leaves. Two of the coal-formation species,D. materiarumand another, had foliage of this type. That of the others is unknown. They are all distinct from the wood ofCordaites, for which see under that genus.

The following are North American species:

[CP]“Geological Survey of Canada: Fossil Plants of Erian and Upper Silurian Formations,” by J. W. Dawson.

All of the above can be vouched for as good species based upon microscopic examination of a very large number of trunks from different parts of North America. The three Erian species ofDadoxylonandD. antiquiusfrom the Lower Carboniferous have two or more rows of cells in the medullary rays. The last named has several rows, and is a truePalæoxylonallied toD. Withamiof Great Britain.D. materiariumis specially characteristic of the upper coal-formation and Permian, and to it must belong one or both of the species of foliage indicated above.D. Clarkiihas very short, simple medullary rays of only a few cells superimposed, and has an inner cylinder of scalariform vessels, approaching in these points toCordaites.Ormoxylonhas a very peculiar articulated pith and simple medullary rays.

Witham in 1833 described several Carboniferous species of pine-wood, under the generic name Pinites, separating under the namePitusspecies which appeared to have the discs on the cell-wallsseparate and in transverse lines. Witham’s name was changed by Goeppert toAraucarites, to indicate the similarity of these woods to Araucaria,Pinitesbeing reserved for trees more closely allied to the ordinary pines. Endlicher, restricting Araucarites to foliage, etc., of Araucaria-like trees, gave the nameDadoxylonto the wood; and this, through Unger’s “Genera and Species,” has gained somewhat general acceptance. Endlicher also gave the namePissadendronto the species which Witham had calledPitus; but Brongniart proposed the namePalæoxylonto include all the species with thick and complex medullary rays, whatever the arrangement of the discs. In Schimper’s new work Kraus substitutesAraucarioxylonfor Endlicher’sDadoxylon, and includes underPissadendronall the species placed by Brongniart inPalæoxylon.

To understand all this confusion, it may be observed that the characters available in the determination of Palæozoic coniferous wood are chiefly the form and arrangement of the wood-cells, the character of the bordered pores or discs of their walls, and the form and composition of the medullary rays.

The character on which Witham separated his genusPitusfromPinitesis, as I have ascertained by examination of slices of one of his original specimens kindly presented to me by Mr. Sanderson, of Edinburgh, dependent on state of preservation, the imperfectly preserved discs or areolations of the walls of the fibre presenting the appearance of separate and distinct circles, while in other parts of the same specimens these discs are seen to be contiguous and to assume hexagonal forms, so that in this respect they do not really differ from the ordinary species ofDadoxylon. The true character for subdividing those species which are especially characteristic of the Carboniferous, is the composite structure of the medullary rays, which are thick and composed of several radial piles of cells placed side by side. This was the character employed by Brongniart in separating the genusPalæoxylon, though he might with convenience have retained Witham’s name, merely transferring to the genus the species of Witham’sPiniteswhich have complex medullary rays. The Erian rocks present the greatest variety of types, andPalæoxylonis especially characteristic of the Lower Carboniferous, while species ofDadoxylonwith two rows of bordered pores and simple medullary rays are especially plentiful in the upper coal-formation and Permo-Carboniferous.

The following table will clearly show the distinctive characters and relations of the genera in question, as held by the several authors above referred to:

Wood of Palæozoic Conifers.

[CQ]TypeO. Erianum, Dn., “Report on Canadian Plants,” 1871.[CR]TypeD. Clarkii, Dn., “Report on Canadian Plants,” 1882. This may be wood of Cordaites, to which it approaches very closely.

[CQ]TypeO. Erianum, Dn., “Report on Canadian Plants,” 1871.

[CR]TypeD. Clarkii, Dn., “Report on Canadian Plants,” 1882. This may be wood of Cordaites, to which it approaches very closely.

FamilyCordaites,GenusCordaites, Brongniart.

Trunks marked by transverse scars of attachment of bases of leaves; leaves broad, with many parallel veins, and attached by a broad base; pistillate and staminate catkins of the nature of Antholithes. Fruit winged or pulpy, of the kind known asCardiocarpum. Stem with a Sternbergia pith, usually large, surrounded by a ring of pseudo-scalariform vessels, and with a cylinder usually narrow, of woody wedges, with bordered pores in one or more series, and with simple medullary rays.

From specimens kindly presented to me by Prof. Renault, I have been able to ascertain that the stems of some at least of these plants (Eucordaites) are distinct in structure from all the species ofDadoxylon, above mentioned, exceptD. Clarkii, of the Erian. They may be regarded as intermediate between those of conifers and cycads, which is indeed the probable position of these remarkable plants.

Grand d’Eury has divided theCordaitesinto sub-genera, as follows:

1.Eucordaites.—Leaves spatulate, obovate, elliptical, or lanceolate,sessile, entire, with rounded apices and of leathery consistency. The leaves are from twenty to ninety centimetres in length. The nerves are either equally or unequally strong.

2.Dorycordaites.—Leaves lanceolate, with sharp points; nerves numerous, fine, and equal in strength. The leaves attain a length of from forty to fifty centimetres.

3.Poacordaites.—Leaves narrow, linear, entire, blunt at the point, with nerves nearly equally strong. The leaves are as much as forty centimetres in length.

To these Renault and Zeiller have added a fourth group,Scutocordaites.

GenusSternbergia.

This is merely a provisional genus intended to receive casts of the pith cylinders of various fossil trees. Their special peculiarity is that, as in the modernCecropia peltata, and some species ofFicus, the pith consists of transverse dense partitions which, on the elongation of the internodes, become separated from each other, so as to produce a chambered pith cavity, the cast of which shows transverse furrows. The young twigs of the modernAbies balsamiferapresent a similar structure on a minute scale. I have ascertained and described such pith-cylinders in large stems ofDadoxylon Ouangondianum, andD. materiarium. They occur also in the stems ofCordaitesand probably ofSigillariæ. I have discussed these curious fossils at length in “Acadian Geology” and in the “Journal of the Geological Society of London,” 1860. The following summary is from the last-mentioned paper:

a.As Prof. Williamson and the writer have shown, many of theSternbergiapiths belong to coniferous trees of the genusDadoxylon.

b.A few specimens present multiporous tissue, of the type ofDictyoxylon, a plant of unknown affinities, and which, according to Williamson, has aSternbergiapith.

c.Other examples show a true scalariform tissue, comparable with that ofLepidodendronorSigillaria, but of finer texture. Corda has shown that plants of the type of the former genus (hisLomatophloios) hadSternbergiapiths. Some plants of this group are by external characters loosely reckoned by botanists as riblessSigillariæ(Clathraria); but I believe that they are not related even ordinally to that genus.

d.Many CarboniferousSternbergiæshow structures identical with those described above as occurring inCordaites, and also in some of the trees ordinarily reckoned asSigillariæ.

GenusCardiocarpum.

I have found at least eight species of these fruits in the Erian and Carboniferous of New Brunswick and Nova Scotia, all of which are evidently fruits of gymnospermous trees. They agree in having a dense coaly nucleus of appreciable thickness, even in the flattened specimens, and surrounded by a thin and veinless wing or margin. They have thus precisely the appearance of samaras of many existing forest-trees, some of which they also resemble in the outline of the margin, except that the wings of samaras are usually veiny. The character of the nucleus, and the occasional appearance in it of marks possibly representing cotyledons or embryos, forbids the supposition that they are spore-cases. They must have been fruits of phænogams. Whether they were winged fruits or seeds, or fruits with a pulpy envelope like those of cycads and some conifers, may be considered less certain. The not infrequent distortion of the margin is an argument in favour of the latter view, though this may also be supposed to have occurred in samaras partially decayed. On the other hand, their being always apparently flattened in one plane, and the nucleus being seldom, if ever, found denuded of its margin, are arguments in favour of their having been winged nutlets or seeds. Until recently I had regarded the latter view as more probable, and so stated the matter in the second edition of “Acadian Geology.” I have, however, lately arrived at the conclusion that theCardiocarpaof the type ofC. cornutumwere gymnospermous seeds, having two cotyledons embedded in an albumen and covered with a strong membranous or woody tegmen surrounded by a fleshy outer coat, and that the notch at the apex represents the foramen or micropyle of the ovule. The structure was indeed very similar to that of the seeds ofTaxusand ofSalisburia. With respect to some of the other species, however, especially those with very broad margins, it still appears likely that they were winged.

TheCardiocarpawere borne in racemes or groups, and it seems certain that some of them at least are the seeds ofCordaites. The association of some of them and of those of the next genus withSigillariæis so constant that I cannot doubt that some of them belong to plants of that genus, or possibly to taxine conifers. The great number of distinct species of these seeds, as compared with that of known trees which could have produced them, is very remarkable.

GenusTrigonocarpum.

These are large angled nuts contained in a thick envelope, and showing internal structures resembling those of the seeds of modernTaxineæ. There are numerous species, as well as allied seeds referred to the provisional generaRhabdocarpusandCarpolithes. InTrigonocarpum HookeriI have described the internal structure of one of those seeds, and many fine examples from the coal-field of St. Etienne, in France, have been described by Brongniart, so that their internal structure is very well known.

GenusAntholithes.

This is also a provisional genus, to include spikes of floral organs, some of which are known to have belonged toCordaites, others probably toSigillariæ.

Of Uncertain Affinities.

FamilySigillariaceæ.

Under this name palæobotanists have included a great number of trees of the Carboniferous system, all of which are characterised by broad leaf-sears, with three vascular scars, and usually arranged in vertical rows, and by elongated three-nerved leaves, and roots of the stigmaria type—that is, with rounded pits, marking the attachment of rootlets spirally arranged. These trees, however, collected in the genus Sigillaria by arbitrary characters, which pass into those of the Lepidodendroid trees, have been involved in almost inextricable confusion, to disentangle which it will be necessary to consider: 1. The external characters ofSigillariæ, and trees confounded with them. 2. Subdivision ofSigillariæby external markings. 3. The microscopic character of their stems. 4. What is known of their foliage and fruit.

1.Characters of Sigillaroid and Lepidodendroid Trunks.

It may be premised that the modes of determination in fossil botany are necessarily different from those employed in recent botany. The palæobotanist must have recourse to characters derived from the leaves, the scars left by their fall, and the internal structures of the stem. These parts, held in little esteem by botanists in describing modern plants, and much neglected by them, must hold the first place in the regard of the fossil botanist, whereas the fructification, seldom preserved, and generally obscure, is of comparatively little service. It is to be remarked also that in such generalised plants as those of the Palæozoic, remarkable rather for the development of the vegetative than of the reproductive organs, the former rise in importance as compared with their value in the study of modern plants.

InSigillariæ,Lepidodendra, &c., the following surfaces of the stem may be presented to our inspection:

1. The outer surface of the epidermis without its leaves, but with the leaf-bases and leaf-scars more or less perfectly preserved. On this surface we may recognise: (1) Cellular swellings or projections of the bark to which the leaves are attached. These may be called leaf-bases, and they are sometimes very prominent. (2) The actual mark of the attachment of the leaf situated in the most prominent part of the leaf-base. This is theleaf-scar. (3) In the leaf-scar when well preserved we can see one or more minute punctures or prominences which are the points where the vascular bundles passing to the leaf found exit. These are the vascular scars.

When the leaves are attached, the leaf-scars and vascular scars cannot be seen, but the leaf-bases can be made out. Hence it is important, if possible, to secure specimens with and without the leaves. In flattened specimens the leaf-bases are often distorted by pressure and marked with furrows which must not be mistaken for true structural characters. The leaf-bases, which are in relief on the outer surface of the stem, of course appear as depressions on the mould in the containing rock, in which the markings often appear much more distinctly than on the plant itself.

2. The outer surface of the epidermis may have been removed or may be destroyed by the coarseness of the containing rock. In this case the leaf-bases are usually preserved on the surface of the outer or corky bark, but the leaf-scars and vascular scars have disappeared. This gives that condition of Lepidodendroid trees to which the nameKnorriahas been applied. When plants are in this state careful inspection may sometimes discover traces of the leaf-scars on portions of the stem, and thus enable theKnorriato be connected with the species to which it belongs.

3. The outer or corky bark may be removed, exposing the surface of the inner or fibrous and cellular bark, which in the plants in question is usually of great thickness. In this case neither the leaf-bases nor the scars are seen, but punctures or little furrows or ridges appear where the vascular bundles entered the inner bark. Specimens in this state are usually said to be decorticated, though only the outer bark is removed. It is often difficult to determine plants in this condition, unless some portion of the stem can be found still retaining the bark; but when care is taken in collecting, it will not infrequently be found that the true outer surface can be recovered from the containing rock, especially if a coaly layer representing the outer bark intervenes between this and the inner impression. Specimensof this kind, taken alone, have been referred to the generaKnorria,Bothrodendron, andHalonia.

4. In some cases, though not frequently, the outer surface of the ligneous cylinder is preserved. It almost invariably presents a regularly striated or irregularly wrinkled appearance, depending upon the vertical woody wedges, or the positions of the medullary rays or vascular bundles. Specimens of this kind constituted some of theEndogenitesof the older botanists, and the genusSchizodendronof Eichwald appears to include some of them. Many of them have also been incorrectly referred to Calamites.

5. In some cases the cast of the medullary cylinder or pith may alone be preserved. This may be nearly smooch or slightly marked by vertical striæ, but more usually presents a transverse striation, and not infrequently the transverse constrictions and septa characteristic of the genus Sternbergia. LooseSternbergiæafford little means of connecting them with the species to which they belong, except by the microscopic examination of the shreds of the ligneous cylinder which often cling to them.[CS]

[CS]See my paper, “Journal of Geological Society,” vol. xxvii.

These facts being premised, the following general statements may be made respecting some of the more common Palæozoic genera, referring, however, principally to the perfect markings as seen on the epidermis:

Sigillaria.—Leaf-bases hexagonal or elongated, or confluent on a vertical ridge. Leaf-scars hexagonal or shield-shaped. Vascular scars three, the two lateral larger than the central. This last character is constant, depending on the fact that the leaves of Sigillaria have two or more vascular bundles. All so-calledSigillariæhaving the central vascular scar largest, or only one vascular bundle, should be rejected from this genus. In young branches of branchingSigillariæthe leaf-scars sometimes appear to be spiral, but in the older stems they form vertical rows; interrupted, however, by transverse rows or bands offruit-scars, each with a single large central vascular scar, and which have borne the organs of fructification.Arthrocaulisof McCoy is founded on this peculiarity.

Syringodendron.—Differs from Sigillaria in the leaf-scars, which are circular and with a single vascular bundle. It is a matter of doubt whether these plants were of higher rank than Sigillaria tending toward the pines, or of lower rank tending toward Cyclostigma. Their leaf-bases form vertical ridges.

Lepidodendron.—Leaf-bases rhombic, oval, or lanceolate, moderatelyprominent. Leaf-scars rhombic or sometimes shield-shaped or heart-shaped, in the middle or upper part of the leaf-base. Vascular scars three—the middle one always largest and corresponding to the single nerve of the leaf; the lateral ones sometimes obsolete.

In older stems three modes of growth are observed. In some species the expansion of the bark obliterates the leaf-bases and causes the leaf-scars to appear separated by wide spaces of more or less wrinkled bark, which at length becomes longitudinally furrowed and simulates the ribbed character of Sigillaria. In others the leaf-bases grow in size as the trunk expands, so that even in large trunks they are contiguous though much larger than those on the branches. In others the outer bark, hardening at an early age, is incapable of either of the above changes, and merely becomes cleft into deep furrows in the old trunks.

Lepidophloios.—Leaf-bases transverse and prominent—often very much so. Leaf-scars transversely rhombic or oval with three vascular scars, the central largest. Leaves very long and one-nerved. Large strobiles or branchlets borne in two ranks or spirally on the sides of the stem, and leaving large, round scars (cone-scars), often with radiating impressions of the basal row of scales.

Species with long or drooping leaf-bases have been included inLepidophloiosandLomatophloios, Species with short leaf-bases and cone-scars in two rows have been calledUlodendron, and some of them have been included in Sigillaria (sub-genusClathraria). Decorticated stems are Bothrodendron andHalonia. Some of the species approach near to the last genus, especially to the Lepidodendra with rhombic leaf-bases likeL. tetragonum.

Cyclostigma.—Leaf-bases undeveloped. Leaf-scars circular or horseshoe-shaped, small, with a central vascular scar. In old trunks of Cyclostigma the leaf-scars become widely separated, and sometimes appear in vertical rows. Young branches of Lepidodendron sometimes have the leaf-scars similar to those of Cyclostigma.

Leptophleum.—Leaf-bases flat, rhombic; leaf-scars obsolete; vascular scar single, central. The last two genera are characteristically Devonian.

In contradistinction from the trees above mentioned, the following general statements may be made respecting other groups:

In conifers the leaf-bases are usually elongated vertically, often scaly in appearance, and with the leaf-scar terminal and round, oval, or rhombic, and with a single well-marked vascular scar.

In Calamites, Calamodendron, and Asterophyllites the scars of the branchlets or leaves are circular or oval, with only a single vascularscar, and situated in verticils at the top of well-marked nodes of the stem.

In tree-ferns the leaf-bases are large and usually without a distinct articulating surface. The vascular bundles are numerous. Protopteris has rounded leaf-scars with a large horseshoe-shaped bundle of vessels above and small bundles below. Caulopteris has large elliptic or oval leaf-scars with vascular scars disposed concentrically. Palæopteris,[CT]of Geinitz, has the leaf-scars transversely oval and the vascular bundles confluent in a transverse band with an appendage or outlying bundle below. Stemmatopteris has leaf-scars similar to those of Caulopteris, but the vascular bundles united into a horseshoe-shaped band.

[CT]This name, preoccupied by Geinitz, has been inadvertently misapplied to the Devonian ferns of the genusArchæopteris.

2.Subdivision of Sigillariæ in Accordance with their Markings.

The following groups may be defined in this way; but, being based on one character only, they are of course in all probability far from natural:

1.Sigillaria, Brongniart. Type,Sigillaria reniformis, Brongniart, orS. Brounii, Dawson.—Stem with broad ribs, usually much broader than the usually oval or elliptical tripunctate areoles, but disappearing at base, owing to expansion of the stem. Leaves narrow, long, three-nerved.

2.Rhytidolepis, Sternberg. Type,S. scutellata, Brongniart.—Ribs narrow, and often transversely striate. Areoles large, hexagonal or shield-shaped, tripunctate. Leaves as in last group. Kings of rounded scars on the stems and branches mark attachment of fruit. It is possible that some of the smaller stems of this group may be branches of trees of group first.

3.Syringodendron, Sternberg. Type,S. organum, L. and H.,S. oculata, Brongniart.—Stems ribbed; areoles small and round, and apparently with a single scar, or three closely approximated. These are rare, and liable to be confounded with decorticated examples of other groups; but I have some specimens which unquestionably represent the external surface.

4.Favularia, Sternberg. Type,Sigillaria elegansof Brongniart.—Leaf-bases hexagonal, or in young branches elliptical, in vertical rows, but without distinct ribs, except in old or decorticated stems. Fruit borne in verticils on the branches bearing transverse rows of rounded scars. Leaves somewhat broad and longitudinally striate.

5.Leioderma, Goldenberg. Type,S. Sydnensis, Dawson.—Ribs obsolete. Cortical and ligneous surfaces striate. Vascular scars double, elongate longitudinally, and alike on cortical and inner surfaces. Areoles in rows and distinct; stigmaria-roots striate, with small and distinct areoles.

6.Clathraria, Brongniart. Type,S. Menardi, Brongniart.—Areoles hexagonal, not in distinct rows, but having a spiral appearance. Some of the plants usually referred to this group are probably branches ofFavularia. Others are evidently fragments of plants of the genusLepidophloios.

3.Internal Structures of Sigillaria-Stems.

I long ago pointed out, on the evidence of the external markings and mode of growth, that the stems ofSigillariæmust have been exogenous, and this conclusion has now been fully confirmed by the microscopic researches of Williamson, not only in the case ofSigillariæ, but ofLepidodendraandCalamodendraas well. Confining myself to my own observations, three types ofSigillariæare known to me by their internal structures, though I cannot certainly correlate all of these with the external markings referred to above.

1.Diploxylon, in which the stem consists of a small internal axis surrounded by a very thick inner bark and a dense outer cortex. A fine example from the South Joggins is thus described:[CU]

[CU]“Journal of the Geological Society of London,” November, 1877.

"The axis of the stem is about six centimetres in its greatest diameter, and consists of a central pith-cylinder and two concentric coats of scalariform tissue. The pith-cylinder is replaced by sandstone, and is about one centimetre in diameter. The inner cylinder of scalariform tissue is perfectly continuous, not radiated, and about one millimetre in thickness. Its vessels are somewhat crushed, but have been of large diameter. Its outer surface, which readily separates from that of the outer cylinder, is striated longitudinally. The outer cylinder, which constitutes by much the largest part of the whole, is also composed of scalariform tissue; but this is radially arranged, with the individual cells quadrangular in cross-section. The cross-bars are similar on all the sides and usually simple and straight, but sometimes branching or slightly reticulated. The wall intervening between the bars has extremely delicate longitudinal waving lines of ligneous lining, in the manner first described by Williamson as occurring in the scalariform tissue of certainLepidodendra. A few small radiating spaces, partiallyoccupied with pyrites, obscurely represent the medullary rays, which must have been very feebly developed. The radiating bundles passing to the leaves run nearly horizontally; but their structure is very imperfectly preserved. The stem being old and probably long deprived of its leaves, they may have been partially disorganised before it was fossilised. The outer surface of the axis is striated longitudinally, and in some places marked with impressions of tortuous fibres, apparently those of the inner bark. In the cross-section, where weathered, it shows concentric rings; but under the microscope these appear rather as bands of compressed tissue than as proper lines of growth. They are about twenty in number. This tree has an erect, ribbed trunk, twelve feet in height and fifteen inches in diameter, swelling to about two feet at the base."

2.Favularia Type.—This has been well described by Brongniart and by Renault,[CV]and differs from the above chiefly in the fact that the outer exogenous woody zone is composed of reticulated instead of scalariform tissue, and the inner zone is of the peculiar form which I have characterised as pseudo-scalariform.

[CV]“Botanique Fossile,” Paris, 1881.

3.Sigillaria Proper.—This I have illustrated in my paper in the “Journal of the Geological Society” for May, 1871, and it appears to represent the highest and most perfect type of the larger ribbedSigillaria. This structure I have described as follows, basing my description on a very fine axis found in an erect stem, and on the fragments of the woody axis found in the bases of other erect stems:

a.A dense cellular outer bark, usually in the state of compact coal—but when its structure is preserved, showing a tissue of thickened parenchymatous cells.

b.A very thick inner bark, which has usually in great part perished, or been converted into coal, but which, in old trunks, contained a large quantity of prosenchymatous tissue, very tough and of great durability. This “bast-tissue” is comparable with that of the inner bark of modern conifers, and constitutes much of the mineral charcoal of the coal-seams.

c.An outer ligneous cylinder, composed of wood-cells, either with a single row of large bordered pores,[CW]in the manner of pinesand cycads, or with two, three, or four rows of such pores sometimes inscribed in hexagonal areoles in the manner of Dadoxylon. This woody cylinder is traversed by medullary rays, which are short, and composed of few rows of cells superimposed. It is also traversed by oblique radiating bundles of pseudo-scalariform tissue proceeding to the leaves. In someSigillariæthis outer cylinder was itself in part composed of pseudo-scalariform tissue, as in Brongniart’s specimen ofS. elegans; and in others its place may have been taken by multiporous tissue, as in a case above referred to; but I have no reason to believe that either of these variations occurred in the typical ribbed species now in question. The woody fibres of the outer cylinder may be distinguished most readily from those of conifers, as already mentioned, by the thinness of their walls, and the more irregular distribution of the pores. Additional characters are furnished by the medullary rays and the radiating bundles of scalariform tissue when these can be observed.

[CW]These are the same with the wood-cells elsewhere called discigerous tissue, and to which I have applied the terms uniporous and multiporous. The markings on the walls are caused by an unlined portion of the cell-wall placed in a disk or depression, and this often surrounded by an hexagonal rim of thickened wall; but in all cases these structures are less pronounced than inDadoxylon, and less regular in the walls of the same cell, as well as in different layers of the tissues of the axis.

d.An inner cylinder of pseudo-scalariform tissue. I have adopted the term pseudo-scalariform for this tissue, from the conviction that it is not homologous with the scalariform ducts of ferns and other acrogens, but that it is merely a modification of the discigerous wood-cells, with pores elongated transversely, and sometimes separated by thickened bars, corresponding to the hexagonal areolation of the ordinary wood-cells. A similar tissue exists in cycads, and is a substitute for the spiral vessels existing in ordinary exogens.

e.A large medulla, or pith, consisting of a hollow cylinder of cellular tissue, from which proceed numerous thin diaphragms towards the centre of the stem.

These structures of the highest type ofSigillariaare on the one hand scarcely advanced beyond those of Calamopitus, as described by Williamson, and on the other approach to those ofCordaites, as seen in specimens presented to me by Renault.

Finally, as to the fruit ofSigillariæ, I have no new facts to offer. The strobiles or spikes associated with these trees have been variously described as gymnospermous (Renault) or cryptogamous (Groldenberg and Williamson). 1 have never seen them in place. Two considerations, however, have always weighed with me in reference to this subject. One is the constant abundance of Trigonocarpaand Cardiocarpa in the soil of the Sigiliaria forests, as I have studied this at the South Joggins. The other is that the rings of fruit-scars on the branches of Sigiliaria are homologous with leaf-scars, not with branches, and therefore should have borne single carpels and not cones or spikes of inflorescence. These are merely suggestions, but I have no doubt they will be vindicated by future discoveries, which will, I have no doubt, show that in the familySigillariaceæwe have really two families, one possibly of gymnospermous rank, or at least approaching to this, the other allied to the Lepidodendra.

Cryptogamia.

(Acrogenes.)

FamilyLepidodendreæ;GenusLepidodendron, Sternberg.

These are arboreal Lycopods having linear one-nerved leaves, stems branching dichotomously, and with ovate or rhombic leaf-bases bearing rhombic leaf-scars, often very prominent. The fruit is in scaly strobiles, terminal or lateral, and there are usually, if not always, macrospores and microspores in each strobile. The young branches and stems have a central pith, a cylinder of scalariform tubes sending out ascending bundles to the leaves through a thick cellular and fibrous inner bark, and externally a dense cortex confluent with or consisting of the leaf-bases. Older stems have a second or outer layer of scalariform fibres in wedges with medullary rays, and strengthening the stem by a true exogenous growth, much as in the Diploxylon type of Sigiliaria. The development of this exogenous cylinder is different in amount and rate in different species.[CX]This different development of the exogenous axis is accompanied with appropriate external appearances in the stems, and the changes which take place in their markings. These are of three kinds. In some species the areoles, at first close together, become, in the process of the expansion of the stem, separated by intervening spaces of bark in a perfectly regular manner; so that in old stems, while widely separated, they still retain their arrangement, while in young stems they are quite close to one another. This is the case inL. corrugatum. In other species the leaf-scars or bases increase in size in the old stems, still retaining their forms and their contiguity to each other. This is the case inL. undulatum, and generally in thoseLepidodendrawhich have large leaf-bases. In these species thecontinued vitality of the bark is shown by the occasional production of lateral strobiles on large branches, in the manner of the modern red pine of America. In other species the areoles neither increase in size nor become regularly separated by growth of the intervening bark; but in old stems the bark splits into deep furrows, between which may be seen portions of bark still retaining the areoles in their original dimensions and arrangement. This is the case withL. Pictoense. This cracking of the bark no doubt occurs in very old trunks of the first two types, but not at all to the same extent.

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