Fossil plants, Vol. 1: [A text-book] for students of botany and geology

by A. C. Seward

There is a common source of danger in attempting to carry too far the venation characters as tests of affinity. The parallel venation of Monocotyledons is by no means a safe guide to follow in all cases as a distinguishing feature of this class of plants. In addition to such leaves as those of the Gymnosperm Cordaites and detached pinnæ of Cycads, there are certain species of Dicotyledons which correspond in the character of their venation to Monocotyledonous leaves. Eryngium montanum Coult., E. Lassauxi Dcne., and other species of this genus of Umbelliferæ agree closely with such a plant as Pandanus or other Monocotyledons; similarly the long linear leaves of Richea dracophylla, R. Br., one of the Ericaceæ, are identical in form with many monocotyledonous leaves. Instances might also be quoted of monocotyledonous leaves, such as species of Smilax and others which Lindley included in his family of Dictyogens which correspond closely with some types of Dicotyledons[142]. Venation characters must be used with care even in determining classes or groups, and with still greater reserve if relied on as family or generic tests.

It is too frequently the case that while we are conversant with the most detailed histological structure of a fossil plant stem, its external form is a matter of conjecture. The conditions which have favoured the petrifaction of plant tissues have as a rule not been favourable for the preservation of good casts or impressions of the external features; and, on the other hand, in the best impressions of fern fronds or other plants, in which the finest veins are clearly marked, there is no trace of internal structure. It is, however, frequently the case that a knowledge of the internal structure of a particular plant enables us to interpret certain features in a structureless cast which could not be understood without the help of histological facts. A particularly interesting example of anatomical knowledge affording a key to apparently abnormal peculiarities in a specimen preserved by incrustation, is afforded by the fructification of the genus Sphenophyllum. Some few years ago Williamson described in detail the structure of a fossil strobilus (i.e. cone) from the Coal-Measures, but owing to the isolated occurrence of the specimens he was unable to determine the plant to which the strobilus belonged. On re-examining some strobili of Sphenophyllum, preserved by incrustation, in the light of Williamson’s descriptions, Zeiller was able to explain certain features in his specimens which had hitherto been a puzzle, and he demonstrated that Williamson’s cone was that of a Sphenophyllum. Similar examples might be quoted, but enough has been said to emphasize the importance of dealing as far as possible with both petrifactions and incrustations. The facts derived from a study of a plant in one form of preservation may enable us to interpret or to amplify the data afforded by specimens preserved in another form.

DECORTICATED STEMS.

The fact that plants usually occur in detached fragments, and that they have often been sorted by water, and that portions of the same plant have been embedded in sediment considerable distances apart, is a constant source of difficulty. Deciduous leaves, cones, or angiospermous flowers, and other portions of a plant which become naturally separated from the parent tree, are met with as detached specimens, and it is comparatively seldom that we have the necessary data for reuniting the isolated members. As the result of the partial decay and separation of portions of the same stem or branch, the wood and bark may be separately preserved. Darwin[143] describes how the bark often falls from Eucalyptus trees, and hangs in long shreds, which swing about in the wind, and give to the woods a desolate and untidy appearance. In the passage already quoted from the narrative of the voyage of the Challenger, illustrations are afforded of the manner in which detached portions of plants are likely to be preserved in a fossil state. The epidermal layer of a leaf or the surface tissues of a twig may be detached from the underlying tissues and separately preserved[144]. It is exceedingly common for a stem to be partially decorticated before preservation, and the appearance presented by a cast or impression of the surface of a woody cylinder, and by the same stem with a part or the whole of its cortex intact is strikingly different. The late Prof. Balfour[145] draws attention to this source of error in his text-book of palaeobotany, and gives figures illustrating the different appearance presented by a branch of Araucaria imbricata Pav. when seen with its bark intact and more or less decorticated. Specimens that are now recognised as casts of stems from which the cortex had been more or less completely removed before preservation, were originally described under distinct generic names, such as Bergeria, Knorria and others. These are now known to be imperfect examples of Sigillarian or Lepidodendroid plants. Grand’Eury[146] quotes the bark of Lepidodendron Veltheimianum Presl. as a fossil which has been described under twenty-eight specific names, and placed in several genera.

Since the microscopical examination of fossil plant-anatomy was rendered possible, a more correct interpretation of decorticated and incomplete specimens has been considerably facilitated. The examination of tangential sections taken at different levels in the cortex of such a plant as Lepidodendron brings out the distribution of thin and thick-walled tissue. Regularly placed prominences on such a stem as the Knorria shown in fig. 23 are due to the existence in the original stem of spirally disposed areas of thin-walled and less resistant tissue; as decay proceeded, the thinner cells would be the first to disappear, and depressions would thus be formed in the surrounding thicker walled and stronger tissue. If the stem became embedded in mud or sand before the more resistant tissue had time to decay, but after the removal of the thin-walled cells, the surrounding sediment would fill up the depressions and finally, after the complete decay of the stem, the impression on the mould or on the cast, formed by the filling up of the space left by the stem, would have the form of regularly disposed projections marking the position of the more delicate tissues. The specimen represented in the figure is an exceedingly interesting and well preserved example of a Coal-Measure stem combining in itself representatives of what were formerly spoken of as distinct genera.

Fig. 23. A dichotomously branched Lepidodendroid stem (Knorria mirabilis Ren. and Zeill.). After Renault and Zeiller[147]. (¼ nat. size.) The original specimen is in the Natural History Museum, Paris.

The surface of the fossil as seen at e affords a typical example of the Knorria type of stem; the spirally disposed peg-like projections are the casts of cavities formed by the decay of the delicate cells surrounding each leaf-trace bundle on its way through the cortex of the stem. The surface g exhibits a somewhat different appearance, owing to the fact that we have the cast of the stem taken at a slightly different level. The surface of the thick layer of coal at a shows very clearly the outlines of the leaf-cushions; on the somewhat deeper surfaces b, c and d the leaf-cushions are but faintly indicated, and the long narrow lines on the coal at c represent the leaf-traces in the immediate neighbourhood of the leaf-cushions.

IMPERFECT CASTS.

It is not uncommon among the older plant-bearing rocks to find a piece of sandstone or shale of which the surface exhibits a somewhat irregular reticulate pattern, the long and oval meshes having the form of slightly raised bosses. The size of such a reticulum may vary from one in which the pattern is barely visible to the unaided eye to one with meshes more than an inch in length. The generic name Lyginodendron[148] was proposed several years ago (1843) for a specimen having such a pattern on its surface, but without any clue having been found as to the meaning of the elongated raised areas separated from one another by a narrow groove. At a later date Williamson investigated the anatomy of some petrified fragments of a Carboniferous plant which suggested a possible explanation of the surface features in the structureless specimens. The name Lyginodendron was applied to this newly discovered plant, of which one characteristic was found to be the occurrence of a hypodermal band of strong thick-walled tissue arranged in the form of a network with the meshes occupied by thin-walled parenchyma. If such a stem were undergoing gradual decay, the more delicate tissue of the meshes would be destroyed first and the harder framework left. A cast of such a partially decayed stem would take the form, therefore, of projecting areas, corresponding to the hollowed out areas of decayed tissue, and intervening depressions corresponding to the projecting framework of the more resistant fibrous tissue. A precisely similar arrangement of hypodermal strengthening tissue occurs in various Palaeozoic and other plants, and casts presenting a corresponding appearance cannot be referred with certainty to one special genus; such casts are of no real scientific value[149].