The protective qualities of humus acids, apart from the almost complete absence of Bacteria[100] from the waters of Moor- or Peat-land, is a factor of great importance in the preservation of plants against decay for many thousands of years.

From examples of fossil stems or leaves in which the organic material has been either wholly or in part replaced by coal, we may pass by a gradual transition to a mass of opaque coal in which no plant structure can be detected. It is by no means uncommon to notice on the face of a piece of coal a distinct impression of a plant stem, and in some cases the coal is obviously made up of a number of flattened and compressed branches or leaves of which the original tissues have been thoroughly carbonised. A block of French coal, represented in fig. 13, consists very largely of laminated bands composed of the long parallel veined leaves of the genus Cordaites and of the bark of Lepidodendron, Sigillaria, and other Coal-Measure genera. The long rhizomes and roots below the coal are preserved as casts in the underclay.

In examining thin sections of coal, pieces of pitted tracheids or crushed spores are frequently met with as fragments of plant structures which have withstood decay more effectually than the bulk of the vegetable débris from which the coal was formed.

The coaly layer on a fossil leaf is often found to be without any trace of the plant tissues, but not infrequently such carbonised leaves, if treated with certain reagents and examined microscopically, are seen to retain the outlines of the epidermal cells of the leaf surface. If a piece of the Carbonaceous film detached from a fossil leaf is left for some days in a small quantity of nitric acid containing a crystal of chlorate of potash, and, after washing with water, is transferred to ammonia, transparent film often shows very clearly the outlines of the epidermal cell and the form of the stomata. Such treatment has been found useful in many cases as an aid to determination[101]. Prof. Zeiller informs me that he has found it particularly satisfactory in the case of cycadean leaves.

It is sometimes possible to detach the thin lamina representing the carbonised leaf or other plant fragment from the rock on which it lies and to mount it whole on a slide. Good examples of plants treated in this way may be seen in the Edinburgh and British Museums, especially Sphenopteris fronds from the Carboniferous oil shales of Scotland. In the excellent collection of fossil plants in Stockholm there are still finer examples of such specimens, obtained by Dr Nathorst from some of the Triassic plants of Southern Sweden. In a few instances the tissues of a plant have been converted into coal in such a manner as to retain the form of the individual cells, which appear in section as a black framework in a lighter coloured matrix. Examples of such carbonised tissues were figured by some of the older writers, and Solms-Laubach has recently[102] described sections of Palaeozoic plants preserved in this manner. The section represented in fig. 70 is that of a Calamite stem (8 × 9·5 cm.) in which the wood has been converted into carbonaceous material, but the more delicate tissues have been almost completely destroyed. The thin and irregular black line a little distance outside the ring of wood, and forming the limit of the drawing, probably represents the cuticle. The whole section is embedded in a homogeneous matrix of calcareous rock, in which the more resistant tissues of the plant have been left as black patches and faint lines.

Mention should be made of a special form of preservation which has been described as fossilisation in half-relief. If a stem is imbedded in sand or mud, the matrix receives an impression of the plant surface, and if the hollow pith-cavity is filled with the surrounding sediment, the surface of the medullary cast will exhibit markings different from those seen on the surface in contact with the outside of the stem. The space separating the pith-cast from the mould bearing the impression of the stem surface may remain empty, or it may be filled with sedimentary material. In half-relief fossils, on the other hand, we have projecting from the under surface of a bed a more or less rounded and prominent ridge with certain surface markings, and fitting into a corresponding groove in the underlying rock on which the same markings have been impressed. It is conceivable that such a cast might be obtained if soft plant fragments were lying on a bed of sand, and were pressed into it by the weight of superincumbent material. The plant fragment would be squeezed into a depression, and its substance might eventually be removed and leave no other trace than the half-relief cast and hollow mould. A twig lying on sand would by its own weight gradually sink a little below the surface; if it were then blown away or in some manner removed, the depression would show the surface features of the twig. When more sand came to be spread out over the depression, it would find its way into the pattern of the mould, and so produce a cast. If at a later period when the sand had hardened, the upper portion were separated from the lower, from the former there would project a rounded cast of the hollow mould. The preservation of soft algae as half-relief casts has been doubted by Nathorst[103] and others as an unlikely occurrence in nature. They prefer to regard such ridges on a rock face as the casts of the trails or burrows of animals. This question of the preservation of the two sides of a mould showing the same impression of a plant has long been a difficult problem; it is discussed by Parkinson in his Organic Remains. In one of the letters (No. XLVI.), he quotes the objection of a sceptical friend, who refuses to believe such a manner of preservation possible, “until,” says Parkinson, “I can inform him if, by involving a guinea in plaster of Paris, I could obtain two impressions of the king’s head, without any impression of the reverse[104].”

It would occupy too much space to attempt even a brief reference to the various materials in which impressions of plants have been preserved. Carbonaceous matter is the most usual substance, and in some cases it occurs in the form of graphite which on dark grey or black rocks has the appearance of a plant drawn in lead pencil. The impressions of plants on the Jurassic (Kimeridgian) slates of Solenhofen[105] in Bavaria, like those on the Triassic sandstones of the Vosges, are usually marked out in red iron oxide.

So far we have chiefly considered examples of plants preserved in various ways by incrustation, that is, by having been enclosed in some medium which has received an impression of the surface of the plant in contact with it. By far the most valuable fossil specimens from a botanical point of view are however those in which the internal structure has been preserved; that is in which the preserving medium has not served merely as an encasing envelope or internal cast, but has penetrated into the body of the plant fragment and rendered permanent the organization of the tissues. In almost every Natural History or Geological Museum one meets with specimens of petrified trees or polished sections of fossil palm stems and other plants, in which the internal structure has been preserved in siliceous material, and admits of detailed investigation in thin sections under the microscope. Silica, calcium carbonate, with usually a certain amount of carbonate of iron and magnesium carbonate, iron pyrites, amber, and more rarely calcium fluoride or other substances have taken the place of the original cell-walls. Of silicified stems, those from Antigua, Egypt, Central France, Saxony, Brazil, Tasmania[106], and numerous other places afford good examples. Darwin records numerous silicified stems in Northern Chili, and the Uspallata Pass. In the central part of the Andes range, 7000 feet high, he describes the occurrence of “Snow-white projecting silicified columns.... They must have grown,” he adds, “in volcanic soil, and were subsequently submerged below sea-level, and covered with sedimentary beds and lava-flows[107].” A striking example of the occurrence of numerous petrified plant stems has been described by Holmes from the Tertiary forests of the Yellowstone Park. From the face of a cliff on the north side of Amethyst mountain “rows of upright trunks stand out on the ledges like the columns of a ruined temple. On the more gentle slopes farther down, but where it is still too steep to support vegetation, save a few pines, the petrified trunks fairly cover the surface, and were at first supposed by us to be the shattered remains of a recent forest[108].” Marsh[109] and Conwentz[110] have described silicified trees more than fifty feet in length from a locality in California where several large forest trees of Tertiary age have been preserved in volcanic strata. In South Africa on the Drakenberg hills there occur numerous silicified trunks, occasionally erect and often lying on the ground, probably of Triassic age[111]. In some instances the specimens measure several feet in length and diameter. Some of the coniferous stems seen in Portland, and occasionally met with reared up against a house side, illustrate the silicification of plant structure on a large scale. These are of Upper Jurassic (Purbeck) age. From Grand’Croix in France a silicified stem of Cordaites of Palaeozoic age has been recorded with a length of twenty meters. The preservation of plants by siliceous infiltrations has long been known. One of the earliest descriptions of this form of petrifaction in the British Isles is that of stems found in Lough Neagh, Ireland. In his lectures on Natural Philosophy, published at Dublin in 1751, Barton gives several figures of Irish silicified wood, and records the following occurrence in illustration of the peculiar properties erroneously attributed to the waters of Lough Neagh. Describing a certain specimen (No. XXVI), he writes:—

“This is a whetstone, which as Mr Anthony Shane, apothecary, who was born very near the lake, and is now alive, relates, he made by putting a piece of holly in the water of the lake near his father’s house, and fixing it so as to withstand the motion of the water, and marking the place so as to distinguish it, he went to Scotland to pursue his studies, and seven years after took up a stone instead of holly, the metamorphosis having been made in that time. This account he gave under his handwriting. The shore thereabouts is altogether loose sand, and two rivers discharge themselves into the lake very near that place[112].”

The well-known petrified trees from the neighbourhood of Lough Neagh are probably of Pliocene age, but their exact source has been a matter of dispute[113].

In 1836 Stokes described certain stems in which the tissues had been partially mineralised. In describing a specimen of beech from a Roman aqueduct at Eibsen in Lippe Bückeburg], he says:—

“The wood is, for the most part, in the state of very old dry wood, but there are several insulated portions, in which the place of the wood has been taken by carbonate of lime. These portions, as seen on the surface of the horizontal section, are irregularly circular, varying in size, but generally a little less or more than ⅛ inch in diameter, and they run through the whole thickness of the specimen in separate, perpendicular columns. The vessels of the wood are distinctly visible in the carbonate of lime, and are more perfect in their form and size in those portions of the specimen than in that which remains unchanged[114].”

This partial petrifaction of the structure in patches is often met with in fossil stems, and may be seriously misleading to those unfamiliar with the appearance presented by the crystallisation of silica from scattered centres in a mass of vegetable tissue. A good example of this is afforded by the gigantic stems discovered in 1829 in the Craigleith Quarry near Edinburgh[115]. Of those two large stems found in the Sandstone rock, the longest, originally 11 meters long and 3·3–3·9 meters in girth, is now set up in the grounds of the British Museum, and a large polished section (1 m. × 87 cm.) is exhibited in the Fossil-plant Gallery. The other stem is in the Botanic Garden, Edinburgh. Transverse sections of the wood of the London specimen show scattered circular patches (fig. 14 A) in the mineralised wood in which the tracheids are very clearly preserved; while in the other portion the preservation is much less perfect. The patch of tissue in fig. 14 A shows a portion of the wood of the Craigleith tree [Araucarioxylon Withami (L. and H.)] in which the mineral matter, consisting of dolomite with a little silica here and there, has crystallised in such a manner as to produce what is practically a cone-in-cone structure on a small scale, which has partially obliterated the structural features. This minute cone-in-cone structure is not uncommon in petrified tissues; it is precisely similar in appearance to that described by Cole[116] in certain minerals. The crystallisation has been set up along lines radiating from different centres, and the particles of the tissue have been pushed as it were along these lines.

A somewhat different crystallisation phenomenon is illustrated by the extremely fine section of a Lepidodendroid plant shown in fig. 15. The tissues of the primary and secondary wood (x¹ and x²) are well preserved throughout in silica, but scattered through the siliceous matrix there occur numerous circular patches, as seen in the figure. One of these is more clearly shown in fig. 14 B drawn from a longitudinal section through the secondary wood, x²; it will be noticed that where the concentric lines of the circular patch occur, the scalariform thickenings of the tracheids are sharply defined, but immediately a tracheid is free of the patch these details are lost. It would appear that in this case silicification was first completed round definite isolated centres, and the secondary crystallisation in the matrix partially obliterated some of the more delicate structural features. The same phenomenon has been observed in oolitic rocks[117], in which the oolitic grains have resisted secondary crystallisation and so retained their original structure.

Among the most important examples of silicified plants are those from a few localities in Central France. In the neighbourhood of Autun there used to be found in abundance loose nodules of siliceous rock containing numerous fragments of seeds, twigs, and leaves of different plants. The rock of which the broken portions are found on the surface of the ground was formed about the close of the Carboniferous period.

At the hands of French investigators the microscopic examination of these fragments of a Palaeozoic vegetation have thrown a flood of light on the anatomical structure of many extinct types. Sometimes the silica has penetrated the cavities of the cells and vessels, and the walls have decayed without their substance being replaced by mineral material. Sections of tissues preserved in this manner, if soaked in a coloured solution assume an appearance almost identical with that of stained sections of recent plants. The spaces left by the decayed walls act as fine capillaries and suck up the coloured solution[118].

In the Coal-Measure sandstones of England large pieces of woody stems are occasionally met with in which the mineralisation has been incomplete. A brown piece of fossil stem lying in a bed of sandstone shows on the surface a distinct woody texture, and the lines of wood elements are clearly visible. The whole is, however, very friable and falls to pieces if an attempt is made to cut thin sections of it; the tracheids of the wood easily fall apart owing to the walls being imperfectly preserved, and the absence of a connecting framework such as would have been formed had the membranes been thoroughly silicified. It is occasionally possible to obtain from petrified plant stems perfect casts in silica or other substances of the cavity of a sclerenchymatous fibre, in which the mineral has been deposited not only in the cavity but in the fine pit-canals traversing the lignified walls. Such a cast is represented in fig. 16, the fine lateral projections are the delicate casts of the pit canals. Numerous instances of minute and delicate tissues preserved in silica are recorded in later chapters. A somewhat unusual type of silicification is met with in some of the Gondwana rocks of India, in which cycadean fronds occur as white porcellaneous specimens showing a certain amount of internal structure in a siliceous matrix. Specimens of such leaves may be seen in the British Museum.

In the Coal-Measures of England, especially in the neighbourhood of Halifax in Yorkshire, and in South Lancashire, the seams of coal occasionally contain calcareous nodules varying in size from a nut to a man’s head, and consisting of about 70% of carbonate of calcium and magnesium, and 30% of oxide of iron, sulphide of iron, &c.[119] The nodules, often spoken of by English writers as ‘coal-balls,’ contain numerous fragments of plants in which the minute cellular structure is preserved with remarkable perfection. It should be noted that the term coal-ball is also applied to rounded or subangular pieces of coal which are occasionally met with in coal seams, and especially in certain French coal fields. To avoid confusion it is better to speak of the plant-containing nodules as calcareous nodules, restricting the term coal-ball to true coal pebbles. A section of a calcareous nodule, when seen under the microscope, presents the appearance of a matrix of a crystalline calcareous substance containing a heterogeneous mixture of all kinds of plant tissues, usually in the form of broken pieces and in a confused mass.

A large section of one of these nodules (12·5 cm. × 8·5 cm.) is shown in fig. 17. It illustrates the manner of occurrence of various fragments of different plants in which the structure has been more or less perfectly preserved. In this particular example we see sections of Myeloxylon (I), Calamites (II), Fern petioles (Rachiopteris) (III), Stigmarian appendages (IV), Lepidodendroid leaves (V), Myeloxylon pinnules (VI), Gymnospermous seeds (VII), Twig of a Lepidodendron, showing the central xylem cylinder and large leaf-bases on the outer cortex, (VIII), Sporangia and spores of a strobilus (IX), Tangential section of a Myeloxylon petiole (X), Rachiopteris sp. (XI), Rachiopteris sp. (XII), Band of sclerenchymatous tissue (XIII), Rachiopteris sp. (XIV).

The general appearance of a calcareous plant-nodule suggests a soft pulpy mass of decaying vegetable débris, through which roots were able to bore their way, as in a piece of peat or leafy mould. Overlying this accumulation of soft material there was spread out a bed of muddy sediment containing numerous calcareous shells, which supplied the percolating water with the material which was afterwards deposited in portions of the vegetable débris. According to this view the calcareous nodules of the coal seams represent local patches of a widespread mass of débris which were penetrated by a carbonated solution, and so preserved as samples of a decaying mass of vegetation, of which by far the greater portion became eventually converted into coal[120].

In such nodules, we find that not only has the framework of the tissues been preserved, but frequently the remains of cell contents are clearly seen. In some cases the cells of a tissue may contain in each cavity a darker coloured spot, which is probably the mineralised cell nucleus. (Fig. 42, A, 1, p. 214.) The contents of secretory sacs, such as those containing gum or resin, are frequently found as black rods filling up the cavity of the cell or canal. The contents of cells in some cases closely simulate starch grains, and such may have been actually present in the tissues of a piece of a fossil dicotyledonous stem described by Thiselton-Dyer from the Lower Eocene Thanet beds[121], and in the rhizome of a fossil Osmunda recorded by Carruthers[122]. (Fig. 42, B, p. 214.)

Schultze in 1855[123] recorded the discovery of cellulose by microchemical tests applied to macerated tissue from Tertiary lignite and coal. With reference to the possibility of recognising cell contents in fossil tissue it is interesting to find that Dr Murray of Scarborough had attempted, and apparently with success, to apply chemical tests to the tissues of Jurassic leaves. In a letter written to Hutton in 1833 Murray speaks of his experiments as follows:—

“Reverting to the Oolitic plants, I have again and with better success been experimenting upon the thin transparent films of leaves, chiefly of Taeniopteris vittata and Cyclopteris, which from their tenuity offer fine objects for the microscope.... By many delicate trials I have ascertained the existence still in these leaves of resin and of tannin.... I am seeking among the filmy leaves of the Fucoides of A. Brongniart for iodine, but hitherto without success, and indeed can hardly expect it, as probably did iodine exist in them, it must have long ago entered into new combinations[124].”

Apart from this difficulty, it is not surprising that Dr Murray’s search for iodine was unsuccessful, considering how little algal nature most of the so-called Fucoids possess.

Some of the most perfectly preserved tissues as regards the details of cell contents are those of gymnospermous seeds from Autun. In sections of one of these seeds which I recently had the opportunity of examining in Prof. Bertrand’s collection, the parenchymatous cells contained very distinct nuclei and protoplasmic contents. In one portion of the tissue in the nucellus of Sphaerospermum the cell walls had disappeared, but the nuclei remained in a remarkable state of preservation. The cells shown in fig. 42 are from the ground tissue of a petiole of Cycadeoidea gigantea Sew.[125], a magnificent Cycadean stem from Portland recently added to the British Museum collection; in the cell A, 1, the nucleus is fairly distinct and in 2 and 4 the contracted cell-contents is clearly seen. Other interesting examples of fossil nuclei are seen in a Lyginodendron leaf figured by Williamson and Scott in a recent Memoir on that genus[126]. Each mesophyll cell contains a single dark nucleus. The mineralisation of the most delicate tissues and the preservation of the various forms of cell-contents are now generally admitted by those at all conversant with the possibilities of plant petrifaction. If we consider what these facts mean—the microscopic investigation of not only the finest framework but even the very life-substance of Palaeozoic plants—we feel that the aeons since the days when these plants lived have been well-nigh obliterated.

Occasionally the plant tissues have assumed a black and somewhat ragged appearance, giving the impression of charred wood. A section of a recent burnt piece of wood resembles very closely some of the fossil twigs from the coal seam nodules. It is possible that in such cases we have portions of mineralised tissues which were first burnt in a forest fire or by lightning and then infiltrated with a petrifying solution. An example of one of these black petrified plants is shown in fig. 74 B. Chap. X. In many of the fossil plants there are distinct traces of fungus or bacterial ravages, and occasionally the section of a piece of mineralised wood shows circular spaces or canals which have the appearance of being the work of some wood-eating animal, and small oval bodies sometimes occur in such spaces which may be the coprolites of the xylophagous intruder. (Fig. 24, p. 107.)

It is well known to geologists that during the Permian and Carboniferous periods the southern portion of Scotland was the scene of widespread volcanic activity. Forests were overwhelmed by lava-streams or showers of ash, and in some districts tree stems and broken plant fragments became sealed up in a volcanic matrix. Laggan Bay in the north-east corner of the Isle of Arran, and Pettycur a short distance from Burntisland on the north shore of the Firth of Forth, are two localities where petrified plants of Carboniferous age occur in such preservation as allows of a minute investigation of their internal structure. The occurrence of plants in the former locality was first discovered by Mr Wünsch of Glasgow; the fossils occur in association with hardened shales and beds of ash, and are often exceedingly well preserved[127]. In fig. 18 is reproduced a sketch of a hollow tree trunk from Arran, probably a Lepidodendron stem, in which only the outer portion of the bark has been preserved, while the inner cortical tissues have been removed and the space occupied by volcanic detritus.

The smaller cylindrical structures in the interior of the hollow trunk are the central woody cylinders of Lepidodendroid trees; each consists of an axial pith surrounded by a band of primary wood and a broader zone of secondary wood. One of the axes probably belonged to the stem of which only the shell has been preserved, the others must have come from other trees and may have been floated in by water[128]. The microscopic details of the wood and outer cortex have in this instance been preserved in a calcareous material, which was no doubt derived by water percolating through the volcanic ash. It is frequently found that in fossil trees or twigs a separation of the tissues has taken place along such natural lines of weakness as the cambium or the phellogen, before the petrifying medium had time to permeate the entire structure. Tree stems recently killed by lava streams during volcanic eruptions at the present day supply a parallel with the Palaeozoic forest trees of Carboniferous times.

Guillemard in describing a volcanic crater in Celebes, speaks of burnt trees still standing in the lava stream, “so charred at the base of the trunk that we could easily push them down[129].” An interesting case is quoted by Hooker in his Himalayan Journals, illustrating the occurrence of a hollow shell of a tree, in which the outer portions of a stem had been left while the inner portions had disappeared, the wood being hollow and so favourable to the production of a current of air which accelerated the destruction of the internal tissues.

On the coast near Burntisland on the Firth of Forth blocks of rock are met with in which numerous plant fragments of Carboniferous age are scattered in a confused mass through a calcareous volcanic matrix. The twigs, leaves, spores, and other portions are in small fragments, and their delicate cells are often preserved in wonderful perfection.

The manner of occurrence of plants in sandstones, shales or other rocks is often of considerable importance to the botanist and geologist, as an aid to the correct interpretation of the actual conditions which obtained at the time when the plant remains were accumulating in beds of sediment. To attempt to restore the conditions under which any set of plants became preserved, we have to carefully consider each special case. A nest of seeds preserved as internal casts in a mass of sandstone, such as is represented by the block of Carboniferous sandstone in fig. 19, suggests a quiet spot in an eddy where seeds were deposited in the sandy sediment. Delicate leaf structures with sporangia still intact, point to quietly flowing water and a transport of no great distance. Occasionally the large number of delicate and light plant fragments, associated it may be with insect wings, may favour the idea of a wind storm which swept along the lighter pieces from a forest-clad slope and deposited them in the water of a lake. In some Tertiary plant-beds the manner of occurrence of leaves and flowers is such as to suggest a seasonal alternation, and the different layers of plant débris may be correlated with definite seasons of growth[130].

The predominance of certain classes of plants in a particular bed may be due to purely mechanical causes and to differential sorting by water, or it may be that the district traversed by the stream which carried down the fragments was occupied almost exclusively by one set of plants. The trees from higher ground may be deposited in a different part of a river’s course to those growing in the plains or lowland marshes. It is obviously impossible to lay down any definite rules as to the reading of plant records, as aids to the elucidation of past physical and botanical conditions. Each case must be separately considered, and the various probabilities taken into account, judging by reference to the analogy of present day conditions.

Various attempts, more or less successful, have been made to imitate the natural processes of plant mineralisation[131]. By soaking sections of wood for some time in different solutions, and then exposing them to heat, the organic substance of the cell walls has been replaced by a deposit of oxide of iron and other substances. Fern leaves heated to redness between pieces of shale have been reduced to a condition very similar to that of fossil fronds. Pieces of wood left for centuries in disused mines have been found in a state closely resembling lignite[132]. Attempts have also been made to reproduce the conditions under which vegetable tissues were converted into coal, but as yet these have not yielded results of much scientific value. The Geysers of Yellowstone Park have thrown some light on the manner in which wood may be petrified by the percolation of siliceous solutions; and it has been suggested that the silicification of plants may have been effected by the waters of hot springs holding silica in solution. Examples of wood in process of petrifaction in the Geyser district of North America have been recorded by Kuntze[133], and discussed by Schweinfurth[134], Solms-Laubach[135] and others[136]. The latter expresses the opinion that by a long continuance of such action as may now be observed in the neighbourhood of hot springs, the organic substance of wood might be replaced by siliceous material. The exact manner of replacement needs more thorough investigation. Kuntze describes the appearance of forest trees which have been reached by the waters of neighbouring Geysers. The siliceous solution rises in the wood by capillarity; the leaves, branches and bark are gradually lost, and the outer tissues of the wood become hardened and petrified as the result of evaporation from the exposed surface of the stem. The products of decay going on in the plant tissues must be taken into account, and the double decomposition which might result. There is no apparent reason why experiments undertaken with pieces of recent wood exposed to permeation by various calcareous and siliceous solutions under different conditions should not furnish useful results.