Introduction to the Study of Palæontological Botany

by John Hutton Balfour

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.