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INTRODUCTION
TO THE STUDY OF
PALÆONTOLOGICAL BOTANY

INTRODUCTION

TO THE STUDY OF

PALÆONTOLOGICAL BOTANY

BY

JOHN HUTTON BALFOUR, A.M. M.D. EDIN.

F.R.S., SEC. R.S.E., F.L.S.

PROFESSOR OF MEDICINE AND BOTANY IN THE UNIVERSITY OF EDINBURGH,
REGIUS KEEPER OF THE ROYAL BOTANIC GARDEN,
AND QUEEN'S BOTANIST FOR SCOTLAND

WITH FOUR LITHOGRAPHIC PLATES, AND UPWARDS OF
ONE HUNDRED WOODCUTS

EDINBURGH
ADAM AND CHARLES BLACK
1872

Printed by R. & R. Clark, Edinburgh.

TO

PROF. HEINRICH ROBERT GOEPPERT, M.D.,

DIRECTOR OF THE BOTANIC GARDEN, BRESLAU;

ONE OF THE MOST EMINENT PALÆONTOLOGICAL
BOTANISTS OF EUROPE,

The following Treatise

IS DEDICATED, WITH BEST RESPECTS, BY HIS
OBLIGED FRIEND

THE AUTHOR.

[PREFACE.]

The subject of Fossil Botany or Palæophytology has formed a part of the Course of Botany in the University of Edinburgh for the last twenty-five years, and the amount of time devoted to the exposition of it has increased. The recent foundation of a Chair of Geology and of a Falconer Palæontological Fellowship in the University seems to require from the Professors of Zoology and Botany special attention to the bearings of their departments of science on the structure of the animals and plants of former epochs of the Earth's history. No one can be competent to give a correct decision in regard to Fossils, unless he has studied thoroughly the present Fauna and Flora of the globe. To give a well-founded opinion in regard to extinct beings, it is essential that the observer should be conversant with the conformation and development of the living ones now on the earth; with their habits, modes of existence and reproduction, the microscopic structure of their tissues, their distribution, and their relation to soil, the atmosphere, temperature, and climate.

There can be no doubt that to become a good Fossil Geologist a student must begin with living animals and plants. The study of Geology must be shared by the Petralogist, who looks at the condition of the rocks of the globe, the minerals forming them, and their mode of formation; the Chemist, who determines the materials which enter into the composition of minerals and rocks; the Naturalist, who examines the plants and animals found in the various strata; and perhaps also the Natural Philosopher, who calculates from independent sources the phases of the Earth's history. It may be said thus to combine all these students of Science in one brotherhood. Much has been done by the efforts of such men as Hutton and Werner, who were engaged chiefly in considering the mineral department of Geology; but it is clear that the Science could not have attained its present position without the continued labours of those who have been examining fossils in their relations to time and space. Had it not been for the researches of Palæontologists, Geology could not have made its present advance.

In my Class Book of Botany I have given an introduction to Palæophytology, and it occurred to me that it might be useful to students to publish this in a separate form, with additions in both the letterpress and the illustrations. The institution of the Palæontological Fellowship, in memory of my former friend Dr. Falconer, has brought the subject specially under my notice. The Fellowship has been promoted chiefly by my friend and former pupil Dr. Charles Murchison, a gentleman fond of science and of his Alma Mater, the University of Edinburgh, where he and Falconer studied and took their degrees.

The first award of the Fellowship has been made to a distinguished student, who acquitted himself with the greatest credit during the three days of examination on Geology, Zoology, and Botany. I trust that the Fellowship will continue to stimulate our eminent students in future years.

Having been a student of Natural Science along with Dr. Falconer, I feel a peculiar interest in doing what I can to promote the study of a subject to which he so successfully devoted his energies. In my endeavour to do so I have been encouraged by my friend and former pupil, Mr. William Carruthers, at the head of the Botanical Department of the British Museum, and a former student in Edinburgh under the late Professor Fleming. He has done much to advance our knowledge of Fossil Botany, and to him I am indebted for two of the plates and some of the woodcuts which illustrate this publication. He has given me most efficient assistance, and I have to return my best thanks for his kind aid. I am also indebted to my colleague, Professor Geikie, for his valued assistance.

The neighbourhood of Edinburgh is rich in Fossils of the Carboniferous epoch, and much yet remains to be done to illustrate its Palæontology. Such labourers as Geikie and Peach may be expected to give great assistance in the furtherance of our knowledge of Scottish Geology, so as to form a school which shall revive the reputation enjoyed by Edinburgh in the days of Hutton and Jameson. If I can be useful in encouraging students to take up the study of Palæontological Botany, and to prosecute it with vigour, I shall feel that this introductory treatise has not been issued in vain. As one of the few surviving relations of Dr. James Hutton, I am glad to be able to show an interest in a science which may aid in elucidating the "Theory of the Earth."

In writing this work I have taken for granted that the reader is acquainted with the Elements of Botany, and knows the general structure of plants of the present day. I have not, therefore, hesitated to use the ordinary Botanical terms without explanation. I am satisfied that no one can study Fossil Botany properly unless he has studied Modern Botany.

Those readers who may find any difficulty as to technical terms I would refer to my Botanist's Companion, where a full Glossary is given.

27 Inverleith Row,

May 1872.

[TABLE OF CONTENTS.]

[LIST OF WOODCUTS.]

[PALÆONTOLOGICAL BOTANY].

The study of the changes which have taken place in the nature of living beings since their first appearance on the globe till the period when the surface of the earth, having assumed its present form, has been covered by the creation which now occupies it, constitutes one of the most important departments in Geology. It is, as Brongniart remarks, the history of life and its metamorphoses. The researches of geologists show clearly that the globe has undergone various alterations since that "beginning" when "God created the heavens and the earth." These alterations are exhibited in the different stratified rocks which form the outer crust of the earth, and which were chiefly sedimentary deposits produced by the weathering of the exposed rocks. Remains of the plants and animals living on the globe at the time of the formation of the different beds are preserved in them. Elevations and depressions of the surface of the earth affected the organisms on its surface, and gave to successive deposits new faunas and floras. Some of these epochs have been marked by great changes in the physical state of our planet, and they have been accompanied with equally great modifications in the nature of the living beings which inhabited it. The study of the fossil remains of animals is called Palæozoology (παλαιός, ancient, and ζῷον, animal), while the consideration of those of vegetables is denominated Palæophytology (παλαιός and φυτόν, a plant). Both are departments of the science of Palæontology, which has been the means of bringing geology to its present state of advancement. The study of these extinct forms has afforded valuable indications as to the physical state of the earth, and as to its climate at different epochs. This study requires the conjunct labours of the Zoologist, the Botanist, and the Petralogist.

The vegetation of the globe, during the different stages of its formation, has undergone very evident changes. At the same time there is no reason to doubt that the plants may all be referred to the great classes distinguished at the present day—namely, Thallogens, including such plants as Lichens, Algæ, and Fungi; Acrogens, such as Ferns and Lycopods; Gymnosperms, such as Cone-bearing plants and Cycads; Endogens, such as Palms, Lilies, and Grasses; and Exogens, such as the common trees of Britain (excluding the Fir), and the great mass of ordinary flowering plants. The relative proportion of these classes, however, has been different, and the predominance of certain forms has given a character to the vegetation of different epochs. The farther we recede in geological history from the present day, the greater is the difference between the fossil plants and those which now occupy the surface. At the time when the coal-beds were formed, the plants covering the earth belonged to genera and species not existing at the present day. As we ascend higher, the similarity between the ancient and the modern flora increases, and in the latest stratified rocks we have in certain instances an identity in species and a considerable number of existing genera. At early epochs the flora appears to have been uniform, to have presented less diversity of forms than at present, and to have been similar in the different quarters of the globe. The vegetation also indicates that the nature of the climate was different from that which characterises the countries in which these early fossil plants are now found.

[Determination of Fossil Plants.]

Fig. 1.

Fig. 1. Section of Peuce Withami, after Lindley and Hutton, a fossil Conifer of the coal epoch. Punctated woody tissue seen.

Fossil plants are by no means so easily examined as recent species. They are seldom found in a complete state. Fragments of stems, leaves, and fruits, are the data by which the plant is to be determined. It is very rare to find any traces of the flowers. The parts of fossil plants are usually separated from each other, and it is difficult to ascertain what are the portions which should be associated together so as to complete an individual plant. Specimens are sometimes preserved, so that the anatomical structure of the organs, especially of the stem, can be detected by very thin slices placed under the microscope. In the case of some stems the presence of punctated woody tissue (Fig. 1) has proved of great service as regards fossil Botany; this structure, along with the absence of large pitted ducts, serving to distinguish Conifers. The presence of scalariform vessels indicates a plant belonging to the vascular Cryptogams, of which the fern is the best known example. The cautions to be observed in determining fossil plants are noticed by Dr. Hooker in the Memoirs of the Geological Survey of Great Britain (vol. ii. p. 387). At the present day, the same fern may have different forms of fronds, which, unless they were found united, might be reckoned distinct genera; and remarkable examples are seen in Niphobolus rupestris and Lindsæa cordata. Moreover, we find the same form of frond belonging to several different genera, which can only be distinguished by the fructification; and as this is rarely seen in fossil ferns, it is often impossible to come to a decided conclusion in regard to them. A leaf of Stangeria paradoxa was considered by an eminent botanist as a barren fern frond, but it ultimately proved to be the leaf of a Cycad. The leaf of Cupania filicifolia, a Dicotyledon, might easily be mistaken for that of a fern; it resembles much the frond of a fossil fern called Coniopteris. The diverse leaves of Sterculia diversifolia, if seen separately, might easily be referred to different plants. In the same fern we meet also with different kinds of venation in the fronds. Similar remarks may be made in regard to other plants. Harvey has pointed out many difficulties in regard to sea-weeds.

As regards the materials for a fossil flora, the following remarks of Hugh Miller deserve attention:—

"The authors of Fossil Floras, however able or accomplished they may be, have often to found their genera and species, and to frame their restorations, when they attempt these, on very inadequate specimens. For, were they to pause in their labours until better ones turned up, they would find the longest life greatly too short for the completion of even a small portion of their task. Much of their work must be of necessity of a provisional character—so much so, that there are few possessors of good collections who do not find themselves in circumstances to furnish both addenda and errata to our most valuable works on Palæontology. And it is only by the free communication of these addenda and errata that geologists will be at length enabled adequately to conceive of the by-past creations, and of that gorgeous Flora of the Carboniferous age, which seems to have been by far the most luxuriant and wonderful which our emphatically ancient earth ever saw."

Fig. 2.

Fig. 2. Bark of Araucaria imbricata.

The bark of trees at the present day often exhibits different kinds of markings in its layers. This may be illustrated by a specimen of Araucaria imbricata, which was destroyed by frost in the Edinburgh Botanic Garden on 24th December 1861. The tree was 24½ feet high, with a circumference of four feet at the base of the stem, and had twenty whorls of branches. The external surface of the bark is represented in Fig. 2. There are seen scars formed in part by prolongations from the lower part of the leaves, which have been cut off close to their union with the stem. The base of each leaf remaining in the bark has the form of a narrow elongated ellipse, surrounded by cortical foliar prolongations. The markings on the bark, when viewed externally, have a somewhat oblique quadrilateral form. On removing the epiphlœum or outer bark, and examining its inner surface, we remark a difference in the appearance presented at the lower and upper part of the stem. In the lower portion the markings have an irregular elliptical form, with a deep depression, and fissures where the leaves are attached (Fig. 3). Higher up the epiphlœal markings assume rather more of a quadrilateral form, with the depressions less deep, and the fissures for the leaves giving off prolongations on either side. Farther up the markings are smaller in size, obliquely quadrilateral, and present circular clots along the boundary lines chiefly (Fig. 4). Higher still the quadrilateral form becomes more apparent, and the dots disappear (Fig. 5). The epiphlœum thus presents differences in its markings at different heights on the stem.

Fig. 3.

Fig. 4.

Fig. 5.

Figs. 3, 4, and 5. Markings on Araucaria bark.

The part of the bark immediately below the epiphlœum is well developed, and is of a spongy consistence. When examined microscopically it is seen to be composed of cells of various shapes—some elongated fusiform, others rhomboidal, others with pointed appendages. The variety of forms is very great, but it is possible that this may be partly owing to the effects of frost on the cells. On the spontaneous separation of the bark, the portion below the epiphlœum was seen to consist of distinct plates of a more or less quadrilateral form, with some of the edges concave and others convex, a part in the centre indicating the connection with the leaf, along with which it is detached. In Fig. 6 a leaf is shown with one of these plates attached.

Fig. 6.

Fig. 6. Leaf of Araucaria with a portion of bark.

The appearances presented by the outer and middle bark of Araucaria imbricata bear a marked resemblance to those exhibited by certain fossils included in the genera Sigillaria and Lepidodendron. The sculpturesque markings on the stems of these fossil plants indicate their alliance to the ferns and lycopods of the present epoch. But it is evident, from these markings, that much caution is required in making this determination. Other points of structure must be examined before a proper decision can be formed. When, for instance, the presence of scalariform tissue, or of punctated woody tissue, has been satisfactorily shown under the microscope, we are entitled to hazard an opinion as to the affinities of the fossils. In many instances, however, external appearances are the only data on which to rely for the determination of fossil genera and species; and rash conclusions have often been drawn by geologists who have not been conversant with the structure of plants. The Araucaria markings point out the need of care in drawing conclusions, and their variations at different parts of the bark indicate the danger of a rash decision as to species. There can be no doubt that in vegetable Palæontology the number of species has been needlessly multiplied—any slight variation in form having been reckoned sufficient for specific distinction. We can conceive that the Araucaria bark markings in a fossil state might easily supply several species of Lepidodendron. A naturalist, with little knowledge of the present flora of the globe, ventures sometimes to decide on an isolated fragment. Hence the crude descriptions of fossil vegetable forms, and the confusion in which Palæophytology is involved. Every geologist who examines fossil plants ought to be well acquainted with the minute structure of living plants, the forms of their roots, stems, leaves, fronds, and fructification; the markings on the outer and inner surfaces of their barks, on their stems, and on their rhizomes; the localities in which they grow, and the climates which genera and species affect in various parts of the world. (Professor Balfour in the Proceedings of the Royal Society of Edinburgh, April 1862, vol. iv. p. 577.)

[Mode of Preservation of Fossil Plants.]

The mode in which plants are preserved in a fossil state may be referred to four principal classes:—1. Casts of the plants; from which all the original substance and structure have been removed subsequently to the burial of the plants, and to the greater or less induration of the rocks in which they are entombed. Such casts are occasionally hollow, but more frequently they consist of the amorphous substance of the rock which has filled up the cavity, and which exhibits, often with remarkable minuteness, the external aspects of the original specimen. 2. Carbonisation; in which the original substance of the plant has been chemically altered and converted into lignite or coal. All trace of the form of the original plant is generally lost, as is the case with the extensive beds of coal; but frequently, when the organism has been buried in a bed of clay, the external appearance is faithfully preserved, as in the ferns and other foliage found in the shales of the coal-measures. 3. Infiltration; in which the vegetable tissues, though carbonised, retain their original form from the infiltration of some mineral in solution, chiefly lime or silex, which has filled the empty cells and vessels, and so preserved their original form. This mode of preservation occurs in the calcareous nodules in coal-beds, in the remarkable ash-beds discovered by Mr. Wünsch in Arran, and generally in the secondary rocks. 4. Petrifaction; in which the structure is preserved, but the whole of the original substance has been replaced, atom for atom, by an inorganic substance, generally lime, silex, or some ore of iron. This is the condition of the beautiful fossils from Antigua, and of many stems and fruits from rocks of all ages in Britain.

Carbonised vegetables, or those which have passed into the state of Lignites, often undergo modifications which render it difficult to understand them rightly. Sometimes a portion of the organs of vegetables which have passed into the state of lignite is transformed into pyrites, or else pyrites of a globular shape is found in the middle of the tissue, and may be taken for a character of organisation. The section of certain Dicotyledonous fossil woods, in that case, may resemble Monocotyledons. Petrifaction, as in the case of silicified woods, often preserves all the tissues equally, at other times the soft tissues are altered or destroyed; the cellular tissue being replaced by amorphous chalcedony, while the ligneous and vascular tissues alone are petrified, so as to preserve their forms. In some cases the reverse takes place as to these tissues; the fibrous portions disappear, leaving cavities, while the cells are silicified. Sometimes we find the parts regularly silicified at one place, so as to retain the structure, while at another an amorphous mass of silica is found. In such cases there appear, as it were, distinct silicified woody bundles in the midst of an amorphous mass. The appearance depends, however, merely on irregular silicification or partial petrifaction. Infiltrated fossil woods, by means of chemical tests, are shown to possess portions of vegetable tissues cemented into a mass by silica. In some cases we find the vessels and cells separately silicified, without being crushed into a compact mass. In these cases, the intercellular substance not being silicified, the mass breaks down easily; whereas, when complete silicification takes place, the mass is not friable. Coniferous wood is often friable, from silicified portions being still separated from each other by vegetable tissue more or less entire. During silicification, or subsequent to it, it frequently happens that the plant has been compressed, broken, and deformed, and that fissures have been formed which have been subsequently filled with crystallised or amorphous silica.

Fig. 7.

Fig. 7. Nicolia Owenii (Carr.), from the Tertiary Strata of Egypt.

Silicified stems of trees have been observed in various parts of the world, with their structure well preserved, so that their Endogenous and Exogenous character could be easily determined. The Rev. W. B. Clarke notices the occurrence of a fossil pine-forest at Kurrur-Kurrân, in the inlet of Awaaba, on the eastern coast of Australia. In the inlet there is a formation of conglomerate and sandstone, with subordinate beds of lignite—the lignite forming the so-called Australian coal. Throughout the alluvial flat, stumps and stools of fossilised trees are seen standing out of the ground, and one can form no better notion of their aspect than by imagining what the appearance of the existing living forest of Eucalypti and Casuarinæ would be if the trees were all cut down to a certain level. In a lake in the vicinity there are also some fossilised stumps of trees, standing vertically. In Derwent Valley, Van Diemen's Land, fossil silicified trees, in connection with trap rocks, have been found in an erect position. One was measured with a stem 6 feet high, a circumference at the base of 7 feet 3 inches, and a diameter at the top of 15 inches. The stems are Coniferous, resembling Araucaria. The outer portion of the stem is of a rich brown glossy agate, while the interior is of a snowy whiteness. One hundred concentric rings have been counted. The tissue falls into a powdery mass. Silica is found in the inside of the tubes, and their substance is also silicified. The erect silicified stems of coniferous trees exist in their natural positions in the "dirt-bed," an old surface soil in the sandstone strata of the Purbeck series in the Isle of Portland, Dorsetshire. In the petrified forests near Cairo silicified stems have been examined by Brown, Unger, and Carruthers. They belong to dicotyledonous trees (not coniferous), to which the names of Nicolia Ægyptiaca and Nicolia Owenii (Fig. 7) have been given. The wood consists of a slender prosenchyma, abundantly penetrated by large ducts. The walls of the ducts are marked by small, regularly arranged, oval, and somewhat compressed hexagonal reticulations. The ducts have transverse diaphragms. There are numerous medullary rays. The wood in their stems is converted into chalcedony. (Carruthers on Petrified Forest near Cairo. Geol. Mag., July 1870.)

[Examination of the Structure of Fossil Plants.]

When the structure of fossil plants is well preserved, it may be seen under the microscope by making thin sections after the mode recommended by Mr. William Nicol, the inventor of the prism which bears his name, and to whose memory Unger dedicated the genus Nicolia, which has just been described as constituting the petrified forest at Cairo. The following is a description of the process of preparing fossils for the microscope, by Mr. Alexander Bryson. (Edin. N. Phil. Journal, N. S. iii. 297. Balfour's Botanist's Companion, p. 30.)

"The usual mode of proceeding in making a section of fossil wood is simple, though tedious. The first process is to flatten the specimen to be operated on by grinding it on a flat lap made of lead charged with emery or corundum powder. It must now be rendered perfectly flat by hand on a plate of metal or glass, using much finer emery than in the first operation of grinding. The next operation is to cement the object to the glass plate. Both the plate of glass and the fossil to be cemented must be heated to a temperature rather inconvenient for the fingers to bear. By this means moisture and adherent air are driven off, especially from the object to be operated on. Canada balsam is now to be equally spread over both plate and object, and exposed again to heat, until the redundant turpentine in the balsam has been driven off by evaporation. The two surfaces are now to be connected while hot, and a slow circular motion, with pressure, given either to the plate or object, for the purpose of throwing out the superabundant balsam and globules of included air. The object should be below and the glass plate above, as we then can see when all the air is removed, by the pressure and motion indicated. It is proper to mention that too much balsam is more favourable for the expulsion of the air-bubbles than too little. When cold, the Canada balsam will be found hard and adhering, and the specimen fit for slitting. This process has hitherto been performed by using a disc of thin sheet-iron, so much employed by the tinsmith, technically called sheet-tin. The tin coating ought to be partially removed by heating the plate, and when hot rubbing off much of the extraneous tin by a piece of cloth. The plate has now to be planished on the polished stake of the tinsmith, until quite flat. If the plate is to be used in the lathe, and by the usual method, it ought to be planished so as to possess a slight convexity. This gives a certain amount of rigidity to the edge, which is useful in slitting by the hand; while by the method of mechanical slitting, about to be described, this convexity is inadmissible. The tin plate, when mounted on an appropriate chuck in the lathe, must be turned quite true, with its edge slightly rounded and made perfectly smooth by a fine-cut file. The edge of the disc is now to be charged with diamond powder. This is done by mingling the diamond powder with oil, and placing it on a piece of the hardest agate, and then turning the disc slowly round. Then, by holding the agate with the diamond powder with a moderate pressure against the edge of the disc, it is thoroughly charged with a host of diamond points, becoming, as it were, a saw with invisible teeth. In pounding the diamond, some care is necessary, as also a fitting mortar. The mortar should be made of an old steel die, if accessible; if not, a mass of steel, slightly conical, the base of which ought to be 2 inches in diameter, and the upper part 1½ inch. A cylindrical hole is now to be turned out in the centre, of ¾ths of an inch diameter, and about 1 inch deep. This, when hardened, is the mortar; for safety it may be annealed to a straw colour. The pestle is merely a cylinder of steel, fitting the hollow mortar but loosely, and having a ledge or edging of an eighth of an inch projecting round it, but sufficiently raised above the upper surface of the mortar, so as not to come in contact while pounding the diamond. The point of the pestle ought only to be hardened and annealed to a straw colour, and should be of course convex, fitting the opposing and equal concavity of the mortar. The purpose of the projecting ledge is to prevent the smaller particles of diamond spurting out when the pestle is struck by the hammer."

Fig. 8.

Fig. 8. Mr. Bryson's instrument for slitting fossils. A very simple slicing and polishing machine has been invented by Mr. J. B. Jordan of the Mining Record Office, and is sold by Messrs. Cotton and Johnson, Grafton Street, Soho, London. It costs about £10.

Mr. Bryson has contrived an instrument for slitting fossils. The instrument is placed on the table of a common lathe, which is, of course, the source of motion (Fig. 8). It consists of a Watt's parallel motion, with four joints, attached to a basement fixed to the table of the lathe. This base has a motion (for adjustment only) in a horizontal plane, by which we may be enabled to place the upper joint in a parallel plane with the spindle of the lathe. This may be called the azimuthal adjustment. The adjustment, which in an astronomical instrument is called the plane of right ascension, is given by a pivot in the top of the base, and clamped by a screw below. This motion in right ascension gives us the power of adjusting the perpendicular planes of motion, so that the object to be slit passes down from the circumference of the slitting-plate to nearly its centre, in a perfectly parallel plane. When this adjustment is made accurately, and the slitting-plate well primed and flat, a very thin and parallel slice is obtained. This jointed frame is counterpoised and supported by a lever, the centre of which is movable in a pillar standing perpendicularly from the lathe table. Attached to the lever is a screw of three threads, by which the counterpoise weight is adjusted readily to the varying weight of the object to be slit and the necessary pressure required on the edge of the slitting-plate.

The object is fixed to the machine by a pneumatic chuck. It consists of an iron tube, which passes through an aperture on the upper joint of the guiding-frame, into which is screwed a round piece of gun-metal, slightly hollowed in the centre, but flat towards the edge. This gun-metal disc is perforated by a small hole communicating with the interior of the iron tube. This aperture permits the air between the glass plate and the chuck to be exhausted by a small air-syringe at the other end. The face of this chuck is covered with a thin film of soft india-rubber not vulcanised, also perforated with a small central aperture. When the chuck is properly adjusted, and the india-rubber carefully stretched over the face of the gun-metal, one or two pulls of the syringe-piston is quite sufficient to maintain a very large object under the action of the slitting-plate. By this method no time is lost; the adhesion is made instantaneously, and as quickly broken by opening a small screw, to admit air between the glass plate and the chuck, when the object is immediately released. Care must be taken, in stretching the india-rubber over the face of the chuck, to make it very equal in its distribution, and as thin as is consistent with strength. When this material is obtained from the shops, it presents a series of slight grooves, and is rather hard for our purpose. It ought, therefore, to be slightly heated, which renders it soft and pliant, and in this state should now be stretched over the chuck, and a piece of soft copper wire tied round it, a slight groove being cut in the periphery of the chuck to detain the wire in its place. When by use the surface of the india-rubber becomes flat, smooth, and free from the grooves which at first mar its usefulness, a specimen may be slit of many square inches, without resort being had to another exhaustion by the syringe. But when a large, hard, siliceous object has to be slit, it is well for the sake of safety to try the syringe piston, and observe if it returns forcibly to the bottom of the cylinder, which evidences the good condition of the vacuum of the chuck.

After the operation of slitting, the plate must be removed from the spindle of the lathe, and the flat lead lap substituted. The pneumatic chuck is now to be reversed, and the specimen placed in contact with the grinder. By giving a slightly tortuous motion to the specimen, that is, using the motion of the various joints, the object is ground perfectly flat when the length of both arms of the joints is perfectly equal. Should the leg of the first joint on the right-hand side be the longer, the specimen will be ground hollow; if shorter, it will be ground convex. But if, as before stated, they are of equal length, a perfectly parallel surface will be obtained.

In operating on siliceous objects, I have found soap and water quite as speedy and efficacious as oil, which is generally used; while calcareous fossils must be slit by a solution of common soda in water. This solution of soda, if made too strong, softens the india-rubber on the face of the pneumatic chuck, and renders a new piece necessary; but if care is taken to keep the solution of moderate strength, one piece of india-rubber may last for six months. The thinner and flatter it becomes, the better hold the glass takes, until a puncture occurs in the outer portion, and a new piece is rendered necessary.

The polishing of the section is the last operation. This is performed in various ways, according to the material of which the organism is composed. If siliceous, a lap of tin is to be used, about the same size as the grinding lap. Having turned the face smooth and flat, a series of very fine notches are to be made all over the surface. This operation is accomplished by holding the edge of an old dinner-knife almost perpendicular to the surface of the lap while rotating; this produces a series of criddles, or slight asperities, which detain the polishing substance. The polishing substance used on the tin lap is technically called lapidaries' rot-stone, and is applied by slightly moistening the mass, and pressing it firmly against the polisher, care being taken to scrape off the outer surface, which often contains grit. The specimen is then to be pressed with some degree of force against the revolving tin lap or polisher, carefully changing the plane of action, by moving the specimen in various directions over the surface.

To polish calcareous objects, another method must be adopted as follows:—

A lap or disc of willow wood is to be adapted to the spindle of the lathe, three inches in thickness, and about the diameter of the other laps (10 inches), the axis of the wood being parallel to the spindle of the lathe, that is, the acting surface of the wood is the end of the fibres, the section being transverse.

This polisher must be turned quite flat and smoothed by a plane, as the willow, from its softness, is peculiarly difficult to turn. It is also of consequence to remark that both sides should be turned, so that the lap, when dry, is quite parallel. This lap is most conveniently adapted to the common face chuck of a lathe with a conical screw, so that either surface may be used. This is made evident, when we state that this polisher is always used moist, and, to keep both surfaces parallel, must be entirely plunged in water before using, as both surfaces must be equally moist, otherwise the dry surface will be concave and the moist one convex. The polishing substance used with this lap is putty powder (oxide of tin), which ought to be well washed, to free it from grit. The calcareous fossils being finely ground, are speedily polished by this method. To polish softer substances, a piece of cloth may be spread over the wooden lap, and finely-levigated chalk used as a polishing medium.

In order to study fossil plants well, there must be an acquaintance with systematic botany, a knowledge of the microscopical structure of all the organs of plants, such as their roots, stems, barks, leaves, fronds, and fruit; of the markings which they exhibit on their different surfaces, and of the scars which some of them leave when they decay. It is only thus we can expect to determine accurately the living affinities of the fossil. Brongniart says, that before comparing a fossil vegetable with living plants, it is necessary to reconstruct as completely as possible the portion of the plant under examination, to determine the relations of these portions to the other organs of the same plant, and to complete the plant if possible, by seeing whether, in the fossils of the same locality, there may not be some which belong to the same plant. The connection of the different parts of the same plant is one of the most important problems in Palæophytology, and the neglect of it has led to many mistakes. In some instances the data have been sufficient to enable botanists to refer a fossil plant to a genus of the present day, so that we have fossil species of the genera Ulmus, Alnus, Pinus, etc. Sometimes the plant is shown to be allied to a living genus, but differing in some essential point, or wanting something to complete the identity, and it is then marked by the addition of the term ites, as Pinites, Thuites, Zamites, etc.

Before drawing conclusions as to the climate or physical condition of the globe at different geological epochs, the botanist must be well informed as to the vegetation of different countries, as to the soils and localities in which certain plants grow, whether on land or in the sea, or in lakes, in dry or marshy ground, in valleys or on mountains, or in estuaries, in hot, temperate, or cold regions. Great caution must be employed also in predicating from one species the conditions of another, inasmuch as different species of the same genus frequently exist in very different habitats, and under almost opposite conditions of moisture and temperature. It is only by a careful consideration of all these particulars that any probable inferences can be drawn as to the condition of the globe. Considering the physiognomy of vegetation at the present day, we find remarkable associations of forms. The Palms, although generally characteristic of very warm countries, are by no means confined to them; Chamærops humilis extending to Europe as far as lat. 43° to 44° N., and C. palmetto in North America to lat. 34° to 36° N., while C. Fortunei, from the north of China, is perfectly hardy in the south of England. Major Madden mentions the association of Palms and Bamboos with Conifers at considerable elevations on the Himalayas. (Edin. Bot. Soc. Trans. iv., p. 185.) Epiphytic Orchids, which usually characterise warm climates, have representatives at great elevations, as Oncidium nubigenum at 14,000 feet in the Andes, and Epidendrum frigidum at from 12,000 to 13,000 feet in the Columbia mountains. These facts point out the care necessary before drawing conclusions as to the climate which fossil plants may be supposed to indicate.

[Fossiliferous Rocks.]

The rocks of which the globe is composed are divided into two great classes—the Stratified or Aqueous, and the Unstratified or Igneous. The stratified rocks frequently contain fossil remains, and are then called fossiliferous; those with no such remains are designated non-fossiliferous or azoic. The igneous unstratified rocks, included under the names of Granitic and Trappean, show no appearance of animal or vegetable remains. Those trap rocks, however, which have been formed of loose volcanic ashes have often enclosed and preserved the remains of plants and animals; while even between the successive beds of old lava-like trap rocks organic remains are sometimes found. Thus, in Antrim, near the Giant's Causeway, deposits containing vegetable remains occur inter-stratified with basaltic rocks. These remains are of Miocene age, and have been referred to coniferous plants, beeches, oaks, plane trees, etc. Similar plants have been discovered in a similar position by the Duke of Argyll in the island of Mull. In trap rocks near Edinburgh, lignite with distinct structure has also been detected. Silicified wood and coal, imbedded in trap rocks, have been seen in Kerguelen's Land. The wood is found enclosed in basalt, whilst the coal crops out in ravines, in close contact with the overlying porphyritic and amygdaloidal greenstone. Hooker has also seen silicified wood, in connection with trap, in Macquarrie's Plains, in Tasmania. Several beds of trap-tuff or ash, formed into solid compact rock by infiltrated carbonate of lime, occur in the north-east of Arran, which contain numerous stems, branches, and fruits of carboniferous plants. These represent the remains of successive forests which grew on this locality, and were one after the other destroyed by the ash-showers poured forth from a neighbouring volcano during its intermittent periods of activity.

Fossil remains are extremely rare in certain rocks, which, from the changes they have undergone, have been denominated Metamorphic. These include Gneiss and Mica-slate, which are stratified rocks subsequently altered by heat and other causes, and so completely metamorphosed that the traces of organisms have been nearly obliterated. Nevertheless, recognisable traces of plant and animal remains have been found in what were recently thought to be azoic rocks. The absence of organic remains in rocks is therefore not sufficient to enable us to state that these rocks were formed before animals or vegetables existed.

The stratified rocks which contain fossils have been divided into three great groups—the Palæozoic, the Secondary, and the Tertiary, or into Palæozoic and Neozoic groups. The formations included under these are exhibited in the following table, taken from Lyell's Manual of Geology:—

[Natural Orders to which Fossil Plants belong.]

The plants found in different strata are either terrestrial or aquatic, and the latter exhibit species allied to the salt and fresh water vegetables of the present day. Their state of preservation depends much on their structure. Cellular plants have probably in a great measure been destroyed, and hence their rarity; while those having a woody structure have been preserved. The following is the number of fossil genera and species, as compiled from Unger's work on Palæophytology—(Unger, Genera et Species Plantarum Fossilium, 1850).

These plants are arranged in the different strata as follows:—

During the twenty years that have elapsed since this enumeration was made, the number of fossil species has been very greatly increased. The proportion exhibited in this table is likewise greatly altered from the enormous additions made to the Tertiary Flora by Unger, Ettingshausen, and Heer, and from the important contributions by Principal Dawson to the Devonian Flora.

Among the fossil Thalamifloral Dicotyledons, Unger mentions species belonging to the orders—

Magnoliaceæ.
Anonaceæ.
Nymphæaceæ.
Capparidaceæ.
Malvaceæ.

Byttneriaceæ.
Tiliaceæ.
Aurantiaceæ.
Malpighiaceæ.
Aceraceæ.

Sapindaceæ.
Cedrelaceæ.
Zygophyllaceæ.
Xanthoxylaceæ.
Coriariaceæ.

Among Calycifloral Dicotyledons—

Celastraceæ.
Rhamnaceæ.
Anacardiaceæ.
Amyridaceæ.
Leguminosæ.

Rosaceæ.
Calycanthaceæ.
Combretaceæ.
Melastomaceæ.
Myrtaceæ.

Halorageaceæ.
Cucurbitaceæ.
Cornaceæ.
Loranthaceæ.
Rubiaceæ.

Among Corollifloral Dicotyledons—

Ericaceæ.
Styracaceæ.
Ebenaceæ.

Aquifoliaceæ.
Sapotaceæ.
Oleaceæ.

Apocynaceæ.
Gentianaceæ.

Among Monochlamydeous Angiosperms—

Nyctaginaceæ.
Lauraceæ.
Proteaceæ.
Aquilariaceæ.
Samydaceæ.
Santalaceæ.

Euphorbiaceæ.
Urticaceæ.
Artocarpaceæ.
Ceratophyllaceæ.
Salicaceæ.
Myricaceæ.

Betulaceæ.
Altingiaceæ.
Platanaceæ.
Corylaceæ.
Juglandaceæ.
Rafflesiaceæ.

Among Monochlamydeous Gymnosperms—

Coniferæ.

Taxaceæ.

Gnetaceæ.

Cycadaceæ.

Among Petaloid Monocotyledons—

Smilaceæ.
Orchidaceæ.
Zingiberaceæ.
Musaceæ.

Liliaceæ.
Palmæ.
Pandanaceæ.
Araceæ.

Typhaceæ.
Naiadaceæ.
Restiaceæ.

Among Glumiferous Monocotyledons—

Cyperaceæ.

Gramineæ.

Among Acrogenous Acotyledons—

Filices.
Marsileaceæ.

Lycopodiaceæ.
Equisetaceæ.

Musci.
Hepaticæ.

Among Thallogenous Acotyledons—

Lichenes.

Characeæ.

Algæ.

Fungi.

[Periods of Vegetation among Fossil Plants.]

On taking a general survey of the known fossil plants, Brongniart thought that he could trace three periods of vegetation, characterised by the predominance of certain marked forms of plants. In the ancient period there is a predominance of Acrogenous Cryptogamic plants; this is succeeded by a period in which there is a preponderance of Gymnospermous Dicotyledons; while a third period is marked by the predominance of Angiospermous Dicotyledons. There is thus—1. The reign of Acrogens, which includes the plants of the Devonian, Carboniferous, and Permian periods. During these periods there seems to be a predominance of Ferns, and a great development of arborescent Lycopodiaceæ, such as Lepidodendron and Sigillaria, and with them are associated some Gynmosperms, allied to Araucaria, and some anomalous plants, as Noeggerathia. 2. The reign of Gymnosperms, comprehending the Triassic and Jurassic periods. Here we meet with numerous Coniferæ and Cycadaceæ, while Ferns are less abundant. 3. The reign of Angiosperms, embracing the Cretaceous and the Tertiary periods. This is characterised by the predominance of Angiospermous Dicotyledons, a class of plants which constitute more than three-fourths of the present vegetable productions of the globe, and which appear to have acquired a predominance from the commencement of the Tertiary formations. These plants appear sparingly even at the beginning of the chalk formation in Europe, but are more abundant in this formation as developed in North America.

[FLORA OF THE PRIMARY OR PALÆOZOIC PERIOD.]

[Reign of Acrogens.]

In the present day, acrogenous plants are represented by cellular and vascular Cryptogams. In considering fossil plants our attention is specially directed to the latter. In the recent Floras, vascular Acrogens are represented by such plants as Ferns, Lycopods, and Equisetums. Some of them have an arborescent habit, but the greater number are shrubby and herbaceous. Many of them have creeping rhizomes, which are either subterranean, or run along the surface of the ground. One of these arborescent forms is seen in Tree-ferns (Fig. 9). Another form with a rhizome is seen in Fig. 10. The trunks of ferns are marked by scars, which indicate the parts where the bases of the fronds were attached, and where the vascular tissue passes out from the interior (Fig. 11, a and b). A transverse section of the stem (Fig. 12) shows a continuous cylinder of scalariform vessels (Fig. 13), enclosing a large mass of cellular tissue frequently penetrated by small scalariform bundles. The cylinder is pierced by meshes, from the inner sides of which rise the vascular bundles going to the leaves, while some of the free bundles of the axis pass through the mesh, carrying with them a portion of the cellular tissue into the petiole. The fructification consists of spore-cases (sporangia), often with an elastic ring round them, containing spores in their interior (Fig. 14).

Fig. 9.

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

Fig. 10.

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

Among Acrogens of the present day there are also plants belonging to the natural order Lycopodiaceæ or Club-mosses (Fig. 15), having creeping stems, which give rise to leafy branches. The leaves are small, sessile, and moss-like, and the fructification consists of two kinds of cellular bodies, small spores or microspores (Fig. 16), and large spores or macrospores (Fig. 17). They consist of cellular and vascular tissues, the latter occurring in the form of woody, annular, and scalariform vessels, which occupy the axis or central part of the stem. They differ from ferns in the distribution of their vascular bundles. The order is represented also by such plants as Selaginella, Psilotum, Phylloglossum, and Isoetes. In the plant called Isoetes (Quillwort) there is a peculiar short stem which does not increase in height. It produces additions laterally, so that the stem increases in thickness. The leaves continue to multiply, and bear fructification at their bases. They have both large and small spores.

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

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

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

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

Fig. 13. Scalariform vessels taken from a Tree-fern. They are marked with bars like the steps of a ladder, hence their name. The membrane occasionally disappears, so that the walls are made up of fibres only at some parts.
Fig. 14. Sporangia of a Fern, supported on stalks, p, each of which ends in an elastic cellular ring, s, partially surrounding the spore-case, and opening it when mature.
Fig. 15. Lycopodium clavatum, a common Club-moss. The leafy branch, l, ends in a stalk bearing two spikes of fructification, f.
Fig. 16. A kidney-shaped 2-valved case, containing small spores (microspores) of Lycopodium.
Fig. 17. Two-valved case, containing large spores (macrospores) of Selaginella.

Fig. 18.

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

Another important order of vascular Acrogens is the Equisetaceæ or Horse-tails (Fig. 18). These are Cryptogams, having rhizomes, bearing hollow, striated branches, which secrete in their epidermis a considerable amount of silex. These branches are jointed and have membranous sheaths at the articulations, which are whorls of leaves reduced to a very rudimentary condition. The fructification consists of cone-like bodies (Fig. 18, f) bearing peltate polygonal scales, under which are spore-cases (Fig. 19), enclosing spores with four hygrometric club-shaped filaments called elaters (Figs. 20 and 21). At the present day some of these plants in tropical regions have stems of 15 or 16 feet high.

Fig. 19. Fig. 20. Fig. 21.

Fig. 19. Polygonal scale, s, of a species of Horse-tail (Equisetum), bearing membranous sacs, t, which open on their inner surface to discharge spores.
Fig. 20. Spore of Equisetum, surrounded by two filaments with club-shaped extremities. The filaments are represented as coiled round the spore.
Fig. 21. Spore of Equisetum, with the filaments (elaters) expanded.

Among vascular Acrogens is included the natural order Marsileaceæ or Rhizocarpeæ, the Pepperworts (Fig. 22). The order consists of aquatic plants, with creeping stems, bearing leaves, which are either linear, or divided into three or more wedge-shaped portions not unlike clover. The fructification is at the base of the leaf-stalks, and consists of sacs (sporocarps) containing spores of two kinds, microspores and macrospores. The order contains Marsilea, Pilularia, Azolla, and Salvinia.

For a fuller account of Acrogenous plants, see Balfour's Class Book of Botany, p. 954.

These orders are represented in the Palæozoic flora. Many of the fossil species assume a large size, and show a greater degree of development than is seen in their recent congeners. The most important coal plants belong to the Ferns, Lycopods, and Horse-tails. The examination of the structure and conformation of the plants of the present flora assists much in the determination of the fossil carboniferous flora.

Fig. 22.

Fig. 22. Marsilea Fabri, a species of Pepperwort or Rhizocarp, with a creeping stem, quadrifoliate stalked leaves on one side, and roots on the other. The fructification, s, is at the base of the leaves, and consists of sporangia, called sporocarps.

In the lower Palæozoic strata the plants which have been detected are few. In the Silurian and Cambrian systems, we meet with the remains of ancient marine plants, as well as a few terrestrial species. Even in the still older Laurentian rocks, if the remarkable structure known as Eozoon canadense be considered, as it generally is, an animal, the existence of contemporary plants may be inferred, inasmuch as without vegetable life animals could not obtain food. In the Lower Silurian or Grauwacke, near Girvan, Hugh Miller found a species resembling Zostera in form and appearance. In the Lower Old Red Sandstone of Scotland he detected Fucoids, a Lepidodendron, and Lignite with a distinct Coniferous structure resembling that of Araucaria,[1] besides a remarkable pinnate frond. In the middle Old Red of Forfarshire, as seen in the Arbroath pavement, he found a fern with reniform pinnæ and a Lepidodendron. In the Upper Old Red, near Dunse, a Calamite and the well-known Irish fern Cyclopteris Hibernica occur.[2] This fern, Palæopteris Hibernica of Schimper ([Plate I. Figs. 1 to 4]), along with Sigillaria dichotoma, is very abundant in beds of the same age in the south of Ireland, from which the specimens described by Edward Forbes were obtained. The fructification has recently been discovered. This shows that the fern belongs to the Hymenophylleæ, and is consequently nearly related to the equally famous Killarney fern, Trichomanes radicans.

Mr. Carruthers states that the frond-stalk of this fern is thick, of considerable length, and clothed with large scales, which form a dense covering at the somewhat enlarged base. The well-defined separation observed in several specimens probably indicates that the frond-stalks were articulated to the stem or freely separated from it, and some root-like structures which occur on the slabs with the ferns may be their creeping rhizomes. The pinnæ are linear, obtuse, and almost sessile. The pinnules are numerous, overlapping, of an ovate or oblong-ovate form, somewhat cuneate below, and with a decurrent base. The veins are very numerous, uniform, repeatedly dichotomous, and run out to the margin, where they form a slight serration. Single pinnules rather larger than those of the pinnæ are placed over the free spaces of the rachis, as was pointed out by Brongniart. Carruthers has not met with any recent fern in which this occurs; but it has been observed in several fossil species, as in the allied American Palæopteris Halliana (Sch.), in Sphenopteris erosa (Morris), and others. The pinnules are sometimes entirely, but only partially fertile. The ovate-oblong sori are generally single and two-lipped, the slit passing one-third of the way down the sorus. The vein is continued as a free receptacle in the centre of the cup or cyst, as in existing Hymenophylleæ, in which it is included, not reaching beyond its entire portion. In some specimens the receptacle is broad or thick, indicating the presence of something besides itself in the cup, and giving the appearance that would be produced if it were covered with sporangia; there is no indication on the outer surface which might have been expected from the separate sporangia. The compression of the specimens in the rock, which has made the free receptacle appear like a vein on the wall of the cup, together with the highly altered condition of the rock in which the fossils are contained, accounts for the imperfect preservation of the minute structures. The interpretation here given of the fructification of this interesting fossil exhibits so close a resemblance to what we find in the living genus Hymenophyllum, that, were it not for the vegetative portions, it would be placed in that genus. Several ferns have been described by Bunbury from Devonian rocks at Oporto. A still more extensive and varied land flora of Devonian age (or Erian, as he calls it) has been described and illustrated by Principal Dawson from the rocks of that period occurring in Canada; and during a recent visit to Britain he has correlated many of the fragments collected by Miller, Peach, and others, with the American species he has described. The following are some of the fossil plants from beds older than the Carboniferous system:[3]—Prototaxites Logani, Dadoxylon Ouangondianum, Calamites transitionis, Asterophyllites parvulus, Sphenophyllum antiquum, Lepidodendron Gaspianum, Lepidostrobus Richardsoni, L. Matthewi, Psilophyton princeps, P. robustius, Selaginites formosus, Cordaites Robbii, C. angustifolius, Cyclopteris Jacksoni.

From the microscopic examination of the structure of specimens of fossil trunks described under the name of Prototaxites Logani, and which Principal Dawson believes to be the oldest known instance of Coniferous wood, Mr. Carruthers has come to the conclusion that they are really the stems of huge Algæ, belonging to at least more than one genus. They are very gigantic when contrasted with the ordinary Algæ of our existing seas, nevertheless some approach to them in size is made in the huge and tree-like Lessonias which Dr. Hooker found in the Antarctic Seas, and which have stems about 20 feet high, with a diameter so great that they have been collected by mariners in these regions for fuel, under the belief that they were drift-wood. They are as thick as a man's thigh. Schimper regards the Psilophyton of Dawson ([Plate IV. Fig. 5]) as allied to Pilularia, one of the Rhizocarps (Fig. 22), and Carruthers places it among the true Lycopodiaceæ.

[Flora of the Carboniferous Epoch.]

The Carboniferous period is one of the most important as regards fossil plants. The vegetable forms are numerous, and have a great similarity throughout the whole system, whether exhibited in the Old or the New World. The important substance called Coal owes its origin to the plants of this epoch. It has been subjected to great pressure and long-continued metamorphic action, and hence the appearance of the plants has been much altered. It is difficult to give a definition of Coal. The varieties of it are numerous. There is a gradual transition from Anthracite to Household and Parrot Coal; and the limit between Coal and what is called bituminous shale is by no means distinct. Coal may be said to be chemically-altered vegetable matter inter-stratified with the rocks, and capable of being used as fuel. On examining thin sections of coal under the microscope, we can detect vegetable tissues both of a cellular and vascular nature. In Wigan cannel coal, vegetable structure is seen throughout the whole mass. Such is likewise the case with other cannel, parrot, and gas coals. In common household coal, also, evident traces of organic tissue have been observed. In some kinds of coal punctated woody tissue ([Plate III. Fig. 5]) has been detected, in others scalariform tissue ([Plate III. Fig. 6]), as well as cells of different kinds. Sporangia are also frequently found in the substance of coal, as shown by Mr. Daw in that from Fordel ([Plate III. Figs. 1 to 3]); and some beds, like the Better bed of Bradford, are composed almost entirely of these sporangia imbedded in their shed microspores, as has been recently shown by Huxley. The structure of coal in different beds, and in different parts of the same bed, seems to vary according to the nature of the plants by which it has been formed, as well as to the metamorphic action which it has undergone. Hence the different varieties of coal which are worked. The occurrence of punctated tissue indicates the presence of Coniferæ in the coal-bed, while scalariform vessels point to ferns, and their allies, such as Sigillaria and Lepidodendron. The anatomical structure of the stems of these plants may have some effect on the microscopic characters of the coal produced from them. In some cannel coals structure resembling that of Acrogens has been observed. A brownish-yellow substance is occasionally present, which seems to yield abundance of carburetted hydrogen gas when exposed to heat.

It appears that in general each bed of coal is accompanied by the remains of a somewhat limited amount of species. Their number, particularly in the most ancient beds, is scarcely more than eight or ten. In other cases the number is more considerable, but rarely more than thirty or forty. In the same coal-basin each layer often contains several characteristic species which are not met with either in the beds above or below. Thus, there are sometimes small local or temporary floras, each of which has given birth to layers of coal. The quantity of carbonaceous and other matter required to form a bed of coal is immense. Maclaren has calculated that one acre of coal three feet thick is equal to the produce of 1940 acres of forest.[4] The proportion of carbon varies in different kinds of coal. Along with it there is always more or less of earthy matter which constitutes the ashes. When the earthy substances are in such quantity that the coaly deposit will not burn as fuel, then we have what is called a shale. The coal contains plants similar to those of the shales and sandstones above and below it. Underneath a coal-seam lies the Underclay, containing roots only, and representing the ancient soil; then comes the Coal, composed of plants whose roots are in the clay, with others which have grown along with and upon them, in a manner precisely similar to the growth of peat at the present day; while above the coal is the Shale, marking how mud was laid down on the plants, and bearing evidences of vigorous vegetation on neighbouring land, from which currents brought down the fine sediment, mingled with broken pieces of plants.

The total thickness of coal in the English coal-fields is about 50 or 60 feet. In the Mid-Lothian field there are 108 feet of coal. Coal-beds are worked at 1725 feet below the sea-level, and probably extend down to upwards of 20,000 feet. They rise to 12,000 feet above the sea-level, and at Huanuco, in Peru, to 14,700.[5] It is said that the first coal-works were opened at Belgium in 1198, and soon after in England and Scotland; it was not till the fifteenth century that they were opened in France and Germany.

The following calculations have been made as to the extent of the coal formation in different countries, and the amount of coal raised:—[6]

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:—

Unger enumerates 683 plants of the coal-measures, while Brongniart notices 500. Of the last number there are 6 Thallogens, 346 Acrogens, 135 Gymnosperms, and 13 doubtful plants. This appears to be a very scanty vegetation, as far as regards the number of species. It is only equal to about ¹/₂₀th of the number of species now growing on the surface of the soil of Europe. Although, however, the number of species was small, yet it is probable that the individuals of a species were numerous. The proportion of Ferns was very large. There are between 200 and 300 enumerated. Schimper thinks there are 7 species congeneric with Lycopodium found in the coal-measures. The following are some of the Cryptogamous and Phanerogamous genera belonging to the flora of the Carboniferous period:—Cyclopteris, Neuropteris, Odontopteris, Sphenopteris, Hymenophyllites, Alethopteris, Pecopteris, Coniopteris, Cladophlebis, Senftenbergia, Lonchopteris, Glossopteris, Caulopteris, Lepidodendron (Lepidostrobus, Lepidophyllum, Knorria), Flemingites, Ulodendron, Halonia, Psaronius, Sigillaria and Stigmaria, Calamites (Asterophyllites and Sphenophyllum), Noeggerathia, Walchia, Peuce, Dadoxylon, Pissadendron, Trigonocarpum.

Ferns are the carboniferous fossil group which present the most obvious and recognisable relationship to plants of the present day. While cellular plants and those with lax tissues have lost their characters by the maceration to which they were subjected before fossilisation took place, ferns are more durable, and retain their structure. It is rare, however, to find the stalk of the frond completely preserved down to its base. It is also rare to find fructification present. In this respect, fossil Ferns resemble Tree-ferns of the present day, the fronds of which rarely exhibit fructification. Hooker states that of two or three kinds of New Zealand Tree-fern, not one specimen in a thousand bears a single fertile frond, though all abound in barren ones. Only one surface of the fossil Fern-frond is exposed, and that generally the least important in a botanical point of view. Fructification is sometimes evidently seen, as figured by Corda in Senftenbergia. In this case the fructification is not unlike that of Aneimidictyon of the present day. Carruthers has recently detected the separate sporangia of Ferns full of spores in calcareous nodules in coal ([Plate I. Fig. 5]). These have the elastic ring characteristic of the Polypodiaceæ, and in their size, form, and method of attachment, they are allied to the group Hymenophylleæ. The absence of fructification presents a great obstacle to the determination of fossil Ferns. Circinate vernation, so common in modern Ferns, is rarely seen in the fossil species, and we do not in general meet with rhizomes. Characters taken from the venation and forms of the fronds are not always to be depended upon, if we are to judge from the Ferns of the present day. There is a great similarity between the carboniferous Ferns of Britain and America; and the same species, or closely allied species of the same genera as those found in Britain have been met with in South Africa, South America, and Australia. In the English coal-measures the species are about 140. The Palæozoic flora of the Arctic regions also resembles that of the other quarters of the globe. Heer, in his account of the fossil flora of Bear Island,[7] enumerates the following plants:—Cardiopteris frondosa, C. polymorpha, Palæopteris Roemeriana, Sphenopteris Schimperi, Lepidodendron Veltheimianum, L. commutatum, L. Carneggiannum, L. Wilkianum, Lepidophyllum Roemeri, Knorria imbricata, K. acicularis, Calamites radiatus, Cyclostigma Kiltorkense, Stigmaria ficoides, etc., Cardiocarpum ursinum, C. punctulatum, besides various sporangia and spores.

Fig. 22, bis.

Fig. 22, bis. Adiantites Lindseæformis.

The preponderance of Ferns over flowering plants is seen at the present day in many tropical islands, such as St. Helena and the Society group, as well as in extra-tropical islands, as New Zealand. In the latter, Hooker picked 36 kinds in an area of a few acres; they gave a luxuriant aspect to the vegetation, which presented scarcely twelve flowering plants and trees besides. An equal area in the neighbourhood of Sydney (in about the same latitude) would have yielded upwards of 100 flowering plants, and only two or three Ferns. This Acrogenous flora, then, seems to favour the idea of a humid as well as mild and equable climate at the period of the coal formation—the vegetation being that of islands in the midst of a vast ocean. Lesquereux, in Silliman's Journal, gives three sections of Ferns in the Carboniferous strata—viz. Neuropterideæ, Pecopterideæ, and Sphenopterideæ. In Neuropterideæ fructification has been seen in Odontopteris. In this genus the spores are in a peculiar bladdery sporangium. In Neuropterideæ the fructification appears to have resembled Danæa in some cases, and Osmunda in others. Professor Geikie has noticed in the lower Carboniferous shales of Slateford, near Edinburgh, a fern which has been named Adiantites Lindseæformis by Bunbury (Fig. 22, bis). It has pinnules between crescent and fan shaped. (Mem. Geol. Survey of Edinburgh, 1861, p. 151.)

Among the Ferns found in the clays, ironstones, and sandstones of the Carboniferous period, we shall give the characters of some by way of illustration.[8] Pecopteris (Fig. 23) seems to be the fossil representative, if not congener, of Pteris. Pecopteris heterophylla (Fig. 24) has a marked resemblance to Pteris esculenta of New Zealand. The frond of Pecopteris is pinnatifid, or bi-tri-pinnatifid—the leaflets adhering to the rachis by the whole length of their base, sometimes confluent; the midrib of the leaflets runs to the point, and the veins come off from it nearly perpendicularly, and the fructification when present is at the end of the veins. Neuropteris (Figs. 25, 26, 27) has a pinnate or bipinnate frond, with pinnæ somewhat cordate at the base—the midrib of the pinnæ vanishing towards the apex, and the veins coming off obliquely, and in an arched manner. Neuropteris gigantea (Fig. 26) has a thick bare rachis, according to Miller, and seems to resemble much Osmunda regalis. Odontopteris has leaves like the last, but its leaflets adhere to the stalk by their whole base, the veins spring from the base of the leaflets, and pass on towards the point. Sphenopteris (Fig. 28) has a twice or thrice pinnatifid frond, the leaflets being narrowed at the base, often wedge-shaped, and the veins generally arranged as if they radiated from the base. Sphenopteris elegans resembled Pteris aquilina in having a stout leafless rachis, which divided at a height of seven or eight inches from its club-like base into two equal parts, each of which continued to undergo two or three successive bifurcations. A little below the first forking two divided pinnæ were sent off. A very complete specimen, with the stipe, was collected in the coalfield near Edinburgh by Hugh Miller, who has described it as above. Lonchopteris has its frond multi-pinnatifid, and the leaflets more or less united together at the base; there is a distinct midrib, and the veins are reticulated. Cyclopteris (Fig. 29) has simple orbicular leaflets, undivided or lobed at the margin, the veins radiating from the base, with no midrib. Schizopteris resembles the last, but the frond is deeply divided into numerous unequal segments, which are usually lobed and taper-pointed.

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

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

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

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

The rarity of Tree-ferns in the coal-measures has often been observed, and it is the more remarkable from the durable nature of their tissues. Several species have, however, been noticed. They are referred to the genus Caulopteris. One of them, C. macrodiscus (Fig. 30) has the leaf-scars in linear series. Two other species are figured, the one a slender form with the scars widely separated, as in some Alsophilas, C. Balfouri (Fig. 31) from the Somersetshire coal-field; and the other with larger stems and more closely aggregated scars, C. Morrisi (Fig. 32), from the coal-measures at Newcastle. The latter species shows the cavities at the base of the petiole described by Mohl in many living fern-stems. The fossils named Psaronius appear to have been fern-stems with a slender axis and a large mass of adventitious roots, as in some Dicksonias and in Osmunda regalis. These stems probably belong to some of the fronds to which other names are given, but as they have not been found attached, it is impossible to determine the point. Miller has described a fern as occurring in the coal-measures, which at first sight presents more the appearance of a Cycadaceous frond than any other vegetable organism of the carboniferous age except the Cycadites Caledonicus (Salter), from Cockburnspath Cove. He thus describes it:—

"From a stipe about a line in thickness there proceed at right angles, and in alternate order, a series of sessile lanceolate leaflets, rather more than two inches in length, by about an eighth part of an inch in breadth, and about three lines apart. Each is furnished with a slender midrib; and, what seems a singular, though not entirely unique feature in a Fern, the edges of each are densely hirsute, and bristle with thick short hair. The venation is not distinctly preserved."

Fig. 33. Fig. 34.

Fig. 36. Fig. 37.

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

Fig. 35.

Sigillaria ([Plate IV. Figs. 1 and 2]) is perhaps the most important plant in the coal formation. The name is derived from sigillum, a seal, to indicate the seal-like markings in the stem. It is found in all coal-shales over the world. Schimper mentions 83 species. It occurs in the form of lofty stems, 40-50 feet high, and 5 feet broad (Figs. 33 and 34). Many stems of Sigillaria may be seen near Morpeth, standing erect at right angles to the planes of alternating strata of shale and sandstone (Fig. 33). They vary from 10 to 20 feet in height, and from one to three feet in diameter. Sir W. C. Trevelyan counted 20 portions of these trees within the length of half-a-mile, of which all but four or five were upright. Brongniart mentions similar erect stems as being found near St. Etienne. The stem of Sigillaria is fluted in a longitudinal manner, like a Doric column, and has a succession of single scars, which indicate the points of insertion of the leaves (Figs. 35, 36, and 37). When the outer part of the stem separates like bark, it is found that the markings presented by the inner surface differ from those seen externally. This has sometimes given rise to the erroneous multiplication of species and even of genera. Sigillaria elegans, as figured by Brongniart in Archives du Museum, i. 405, has a stem consisting of a central cellular axis or medulla, surrounded by a vascular cylinder, and this is invested by a thick cellular cortical layer, the outer portion composed of fusiform cells of less diameter than those of the inner portion. What Brongniart calls medullary rays are mere cracks or separations in the wedges traversed by vessels. In its structure it resembles its root Stigmaria, and must be referred to Lycopodiaceæ, along with Lepidodendron, Halonia, Ulodendron, etc. The small round sporangia of Sigillaria are borne in a single patch on the somewhat enlarged bases of some of the leaves. (See Carruthers on Structure and Affinities of Sigillaria, in Journ. Geol. Soc. Aug. 1869.)

Fig. 38. Fig. 39.

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

It has been ascertained by Professor King and Mr. Binney of Manchester, that the plant called Stigmaria (Fig. 38) is not a separate genus, but the root of Sigillaria ([Plate IV. Figs. 1 and 2]). The name is derived from στίγμα, a mark, indicating the markings on the axis. It is one of the most common productions of the coal-measures, and consists of long rounded or compressed fragments, marked externally by shallow circular, oblong, or lanceolate cavities (Fig. 39) in the centre of slight tubercles, arranged more or less regularly in a quincuncial manner ([Plate III. Fig. 7]). The cavities occasionally present a radiating appearance. The axis of the fragments is often hollow, and different in texture from the parts around. This axis consists of a vascular cylinder or woody system, penetrated by quincuncially arranged meshes or openings, through which the vascular bundles proceed from the inner surface of the cylinder to the rootlets ([Plate III. Figs. 8 and 9]). From the scars and tubercles arise long ribbon-shaped processes, which were cylindrical cellular roots, now compressed (Fig. 38). The vascular cylinder of Stigmaria is composed entirely of scalariform tissue, pierced by meshes for the passage, from the inner surface of the cylinder, of the vascular bundles which supply the rootlets. (Carruthers in Geol. Proc., Aug. 1869.) Stigmaria ficoides (Fig. 38) abounds in the under-clay of a coal-seam, sending out numerous roots from its tubercles, and pushing up its aerial stem, in the form of a fluted Sigillaria. On the Bolton and Manchester Railway Mr. Binney discovered Sigillarias standing erect, and evidently connected with Stigmarias which extended 20 feet or more.[9] Stigmaria is regarded by Schimper as roots, not of Sigillaria only, but of Knorria longifolia (one of the Lepidodendreæ). The base of the stem of this species of Knorria is Ancestrophyllum, and the upper part is Didymophyllum Schottini of Goeppert. Professor King and others suppose that the Fern-like frond called Neuropteris is connected with Sigillaria, but this is a mere conjecture, set aside by the discovery of leaves attached to a species allied to Sigillaria elegans, which establishes that the long linear leaves described under the name Cyperites are the foliage of this genus. Goldenberg has figured the fructification, which consists of small sporangia like those of Flemingites, borne on the basis of but slightly modified leaves. This establishes the opinion that Sigillaria was an acrogenous plant belonging to Lycopodiaceæ. Brongniart reckons it as representing an extinct form of Gymnosperms, and King, having erroneously associated the Cyclopteris with it, places it between the Ferns and Cycadaceæ. Mr. Carruthers informs me that he has examined the stem of a true fluted Sigillaria, with the tissues preserved, and that these agree with the structure of Lepidodendron, a position in which he had already placed it from the structure of its fruit.

Fig. 40. Fig. 41.

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

Fig. 42. Fig. 43.

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

Fig. 44.

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

Lepidodendron (Figs. 40 to 44) is another genus of the coal-measures which differs from those of the present day ([Plate IV. Fig. 3]). Lepidodendrons, or fossil Lycopodiaceæ, had spikes of fructification comparable in size to the cones of firs and cedars, and containing very large sporangia, even larger than those of Isoetes, to which they approach in form and structure. Schimper, in 1870, enumerates 56 species of Lepidodendron, all arborescent and carboniferous. The stem of a Lepidodendron is from 20 to 45 feet high, marked outside by peculiar scale-like scars (Fig. 41), hence the name of the plant (λεπίς, a scale, and δένδρον, a tree). Although the scars on Lepidodendron are usually flattened, yet in some species they occupy the faces of diamond-shaped projections, elevated one-sixth of an inch or more above the surface of the stem, and separated from each other by deep furrows;—the surface bearing the leaf being perforated by a tubular cavity, through which the bundle of vessels that diverged from the vascular axis of the stem to the leaf passed out. The linear or lanceolate leaves are arranged in the same way as those of Lycopodiums or of Coniferæ, and the branches fork like the former. The internal structure of the stem is the same as that of Sigillaria. The fruit of Lepidodendron and allied genera is seen in Lepidostrobus and Triplosporites (Figs. 42, 43; [Plate III, Fig. 10]). Carruthers, in his lecture to the Royal Institution, in describing the forms of Lepidostrobus, says—"The fruit is a cone composed of imbricated scales arranged spirally on the axis like the true leaves, and bearing the sporangia on their horizontal pedicels. Three different forms of fruit belong to this genus, or it should perhaps rather be called group of plants. The first of these is the cone named by Robert Brown Triplosporites (Figs. 42, 43), and described by him from an exquisitely preserved specimen of an upper portion, in which the parts are exhibited as clearly in the petrified condition as if they belonged to a fresh and living plant. The large sporangia have a double wall, the outer composed of a compact layer of oblong cells placed endwise, or with the long diameter perpendicular to the surface; the inner is a delicate cellular membrane. The sporangium is filled with a great number of very small spores, each composed of three roundish bodies or sporules. Recently Brongniart and Schimper have described a complete specimen of this fruit, in which the minute triple spores are confined to the sporangia of the upper and middle part of the cone, but the lower portion, which was wanting in Brown's specimen, bears sporangia filled with simple spherical spores ten or twelve times larger than the others (woodcut 44, 9).

"The structure of another form of cone (Lepidostrobus) has been expounded by Dr. Hooker. The arrangement of the different parts comprising it is precisely similar to what occurs in Triplosporites; but the sporangia are filled with the minute triple spores throughout the whole cone (woodcut 44, 6 and 8).

"The third form of cone, described by me under the name Flemingites, differs from the other two in having a large number of small sporangia supported on the surface of each scale; and it agrees with Lepidostrobus in the sporangia containing only small spores (woodcut 44, 10).

"In comparing these fossils with the living club-mosses, one is struck with the singular agreement in the organisation of plants so far removed in time, and so different in size, as the recent humble club-mosses and the palæozoic tree Lepidodendrons. The fruit of Triplosporites, like that of Selaginella (woodcut 44, 1), contains large and small spores, the microspores being found in both genera on the middle and upper scales of the cone, and the macrospores on those of the lower portion (Fig. 43).

"On the other hand, the fruits of Lepidostrobus and Flemingites agree with that of Lycopodium in having only microspores. The size of the two kinds of spores also singularly agrees in the two groups. This is of some importance, for among the recent vascular Cryptogams there is a remarkable uniformity in the size of the spores in the members of the different groups, even when there is a great variety in the size of the plants. Thus the spore of our humble wall-rue is as large as that of the giant Alsophila of tropical regions. So also the spores of Equisetum and Calamites agree in size, as may be seen in woodcut 47, Figs. 3, 4, and 9, where the spores of the two genera are magnified to the same extent. And a similar comparison of the macrospore and microspore of Triplosporites with those of Selaginella, and of the microspore of Lepidostrobus with that of Lycopodium, exhibits a similar agreement. This is made apparent by the drawings in woodcut 44 of the two kinds of spores of Selaginella, 3 and 4, with those of Triplosporites, 8 and 9, which are drawn to the same scale."

The genus Sigillaria, as we have already said, has, according to the observation of Hooker, small sporangia exactly agreeing in size and form with those of Flemingites. Most probably the contents of these small sporangia were the same in both genera, so that Sigillaria would be placed with Flemingites and Lepidostrobus as arborescent Lycopodiaceæ having their affinities with Lycopodium, as they have all microspores only in their fructification.

The scales upon the Lepidodendron stems, as well as those in the cones, are arranged in a spiral manner, in the same way as plants of the present day. Professor Alexander Dickson has examined the phyllotaxis of Lepidodendrons, and gives the following results of his observations (Trans. Bot. Soc. Edin. xi. 145). The fossil remains of Lepidodendrons are often so compressed that it is difficult, or even impossible, to trace the secondary spirals round the circumference of the stem. In those cases, however, where there is comparatively little compression, i.e. where the stem is more or less cylindrical, the determination of the phyllotaxis is easy. Of such stems he has examined fifteen specimens, which may be classed according to the series of spirals to which the leaf-arrangement belongs:—

A. Ordinary series, ½, ⅓, ²/₅, ⅜, ⁵/₁₃, etc.

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

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

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

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

(b.) Conjugate spirals.[11]

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

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

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

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

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

B. Series, ⅓, ¼, ²/₇, ³/₁₁, etc.

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

C. Series, ¼, ¹/₅, ²/₉, ³/₁₄, etc.

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

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

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

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

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

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

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

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

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

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

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

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

Fig. 45 a.

Fig. 45 b.

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

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

Fig. 46.

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

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

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

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

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

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

Fig. 47.

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

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

Fig. 48.

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

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

Fig. 49. Fig. 50.

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

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

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

Fig. 51. Fig. 52.

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

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