The old generic terms Artisia and Sternbergia illustrate another source of error which can be avoided only by means of a knowledge of internal structure. The former name was proposed by Sternberg and the latter by Artis for precisely similar Carboniferous fossils, having the form of cylindrical bodies marked by numerous transverse annular ridges and grooves. These fossils are now known to be casts of the large discoid pith of the genus Cordaites, an extinct type of Palaeozoic Gymnosperms. Calamites and Tylodendron afford other instances of plants in which the supposed surface characters have been shown to be those of the pith-cast. The former genus is described at length in a later chapter, but the latter may be briefly referred to. A cast, apparently of a stem, from the Permian rocks of Russia was figured in 1870 under the name Tylodendron; the surface being characterised by spirally arranged lozenge-shaped projections, described as leaf-scars. Specimens were eventually discovered in which the supposed stem was shown to be a cast of the large pith of a plant possessing secondary wood very like that of the recent genus Araucaria. The projecting portions, instead of being leaf-cushions, were found to be the casts of depressions in the inner face of the wood where strands of vascular tissue bent outwards on their way to the leaves. If a cast is made of the comparatively large pith of Araucaria imbricata the features of Tylodendron are fairly closely reproduced[150].
A dried Bracken frond lying on the ground in the Autumn presents a very different appearance as regards the form of the ultimate segments of the frond to that of a freshly cut leaf. In the former the edges of the pinnules are strongly recurved, and their shape is considerably altered. Immersed in water for some time fern fronds or other leaves undergo maceration, and the more delicate lamina of the leaf rots away much more rapidly than the scaffolding of veins. Among fossil fern fronds differences in the form of the pinnules and in the shape and extent of the lamina, to which a specific value is assigned, are no doubt in many cases merely the expression either of differences in the state of the leaves at the time of fossilisation or of the different conditions under which they became embedded. Differential decay and disorganisation of plant tissues are factors of considerable importance with regard to the fossilisation of plants. As Lindley[151] and later writers have suggested, the absence or comparative scarcity of certain forms of plants from a particular fossil flora may in some cases be due to their rapid decay and non-preservation as fossils; it does not necessarily mean that such plants were unrepresented in the vegetation of that period. The decayed rhizomes of the Bracken fern often seen hanging from the roadside banks on a heath or moorland, and consisting of flat dark coloured bands of resistant sclerenchyma in a loose sheath of the hard shrivelled tissue, are in striking contrast to the perfect stem. A rotting Palm stem is gradually reduced to a loose stringy mass consisting of vascular strands of which the connecting parenchymatous tissue has been entirely removed. It must frequently have happened that detached vascular bundles or strands and plates of hard strengthening tissue have been preserved as fossils and mistaken for complete portions of plants.
MINERAL DEPOSITS SIMULATING PLANTS.
Apart from the necessity of keeping in view the possible differences in form due to the state of the plant fragments at the time of preservation, and the marked contrast between the same species preserved in different kinds of rock, there are numerous sources of error which belong to an entirely different category. The so-called moss-agates and the well-known dendritic markings of black oxide of manganese, are among the better known instances of purely inorganic structures simulating plant forms.
An interesting example of this striking similarity between a purely mineral deposit and the external form of a plant is afforded by some specimens originally described as impressions of the oldest known fern. The frontispiece to a well-known work on fossil plants, Le monde des plantes avant l’apparition de l’homme[152], represents a fern-like fossil on the surface of a piece of Silurian slate. The supposed plant was named Eopteris Morierei Sap., and it is occasionally referred to as the oldest land plant in books of comparatively recent date. In the Museum of the School of Mines, Berlin, there are some specimens of Angers slate on some of which the cleavage face shows a shallow longitudinal groove bearing on either side somewhat irregularly oblong and oval appendages of which the surface is traversed by fine vein-like markings. A careful examination of the slate reveals the fact that these apparent fern pinnules are merely films of iron pyrites deposited from a solution which was introduced along the rachis-like channel. Many of the extraordinary structures described as plants by Reinsch[153] in his Memoir on the minute structure of coal have been shown to be of purely mineral origin.
The innumerable casts of animal-burrows and trails as well as the casts of egg-cases and various other bodies, which have been described as fossil algae, must be included among the most fruitful sources of error.
It requires but a short experience of microscopical investigation of fossil plant structures to discover numerous pitfalls in the appearance presented by sections of calcareous and siliceous nodules. The juxtaposition of tissues apparently parts of the same plant, and the penetration by growing roots of partially decayed plant débris, serve to mislead an unpractised observer. In sections of the English ‘calcareous nodules’ one very frequently finds the tissue of Stigmarian appendages occupying every conceivable position, and preserved in places admirably calculated to lead to false interpretations. The more minute investigation of tissues is often rendered difficult by deceptive appearances simulating original structures, but which are in reality the result of mineralisation. It is no easy matter in some cases to discover whether a particular cell in a fossil tissue was originally thick-walled, or whether its sclerous appearance is due to the deposition of mineral matter on the inside of the thin cell-membrane. Examples of such sources of error as have been briefly referred to, and others, will be found in various parts of the descriptive portions of this book.
Fig. 24. A. Section of partially disorganised tissue attacked by some boring animal. c, c, coprolites; d, a tunnel made by the borer through the plant tissue.
B. Transverse section of a Lepidodendroid leaf, of which the inner tissues have been destroyed and the cavity filled with coprolites; simulating a sporangium containing spores. (A and B from specimens in the Botanical Laboratory collection, Cambridge.)