The piece of dried rhizome of Cyperus papyrus shown in fig. 206 is an almost exact counterpart of Stigmaria ficoides; the wrinkled and shrivelled surface and the circular root-scars containing the remains of a vascular bundle are striking features in common and, it may be added, the two plants, though very different in structure and in systematic position, illustrate anatomical adaptations to a similar environment.

Stigmaria ficoides Brongniart[565]. Figs. [204], [205], [207], [208].

The first figure of Stigmaria is said to be by Petver in 1704; Volkmann published illustrations of this common fossil in 1720 and Parkinson in 1804[566]. Binney, whose researches may be said to have inaugurated a new era in the investigation of fossil plants, wrote in 1844: “Probably no fossil plant has excited more discussion among botanists than the Stigmaria. It is the most common of the whole number of plants found in the Coal-Measures, but there has hitherto been the greatest uncertainty as to its real nature[567].” This uncertainty still exists, at least in the minds of some who know enough of the available data to realise that our knowledge is imperfect.

To pass to the questions of the affinity and nature of Stigmaria: Brongniart[568] at first compared his genus with recent Aroideae, but he afterwards[569] spoke of it as probably the root of Sigillaria. Other writers regarded Stigmaria as a dicotyledonous plant comparable with Cacti and succulent Euphorbias. For many years opinion was divided as to whether Stigmaria represents an independent and complete plant or the underground system of Sigillaria.

Artis[570], Lindley and Hutton[571], as well as Goldenberg[572], believed it to be a prostrate plant unconnected with any erect aerial stem. Goldenberg figured one of the slender rootlets terminating in an oval body described as a reproductive organ. This seed-like impression is either some extraneous body or an abnormal development at the end of a rootlet. In 1842 Logan drew attention to the almost complete monopolisation by Stigmaria of the underclays, the rock which as a general rule occurs below a seam of coal. He wrote: “The grand distinguishing feature of the underclays is the peculiar character of the vegetable organic remains; they are always of one kind (Stigmaria ficoides) and are so diffused throughout every part of the bed, that by their uniform effect alone the clay is readily recognised by the eye of the miner[573].” This fact, which has played a very conspicuous part in the perennial discussions on the origin of coal, led to the almost general recognition of the underclays as surface-soils of the Coal period forests.

The next step was the discovery of Stigmaria in the Coal-Measures of Lancashire and in the Carboniferous rocks of Cape Breton, Nova Scotia, forming the basal branches of erect stems identified by Binney[574], Bowman[575] and Richard Brown[576] as undoubted Sigillariae. In one case Brown found what he considered to be convincing evidence of the continuity between Stigmaria and Lepidodendron.

In 1842 Hawkshaw[577] described certain fossil trees, the largest of which had a circumference at the base of 15 ft., discovered, in the course of excavations for a railway in Lancashire, in soft shale at right angles to the bedding. The surface features were not sufficiently clear to enable him to decide with certainty between Sigillaria and Lepidodendron, but while inclining to the former, it is interesting to note that the occurrence of numerous Lepidostrobi near the root led him to recognise the possibility of a connexion between the Stigmarian roots and Lepidodendron stems. In 1846 Binney gave an account of similar trees found at Dukinfield near Manchester: he spoke of one stem as unquestionably a Sigillaria with vertical ribs, furrows, and scars, about 15 inches high and 4 ft. 10 inches in circumference. He expressed his conviction that “Sigillaria was a plant of an aquatic nature[578].” Similar descriptions of rooted stems in the Coal-Measures of Nova Scotia were published by Brown in 1845, 1846 and 1849; in the last paper he figured a specimen, which has become famous, showing a Syringodendron stem terminating in branching Stigmarian (or possibly Stigmariopsis) roots bearing on the lower surface a series of what he called conical tap roots[579]. A similar specimen discovered in Central France nearly fifty years later demonstrated the accuracy of Brown’s description.

Despite these discoveries the root-like nature of Stigmaria was not universally accepted. It was, however, generally agreed that Stigmaria formed the roots of Sigillaria; it was, moreover, held by some that Lepidodendron stems also possessed this type of root, an opinion based on Brown’s record and on the occurrence of Stigmaria in beds containing Lepidodendron but no Sigillaria stems, as in the volcanic beds of Arran and elsewhere, and on observations of Geinitz and others[580]. There is now general agreement that Lepidodendron and Sigillaria had the same type of “root,” though the connexion of Stigmaria with the former was not so readily admitted, and indeed the evidence in support of it is still very meagre. Goeppert and other authors were unable to believe that the numerous species of Sigillaria possessed roots of so uniform a type, but Goeppert, by his recognition of several varieties of Stigmaria, supplied a partial answer to this objection.

Messrs Mellor and Leslie[581] have described and figured some large casts of roots exposed in Permo-Carboniferous rocks in the bed of the Vaal river at Vereeniging (Transvaal) which exhibit certain features suggesting comparison with Stigmaria. Some of these reach a length of 40–50 feet and, when complete, were probably not less than 100 feet long: in some of them the centre of the cast from which forked arms spread almost horizontally shows a depression in the form of a cross indicating a regular dichotomous branching like that of Stigmaria. The authors incline to the belief that the roots belong to Noeggerathiopsis and not to a lycopodiaceous plant, though Lepidodendroid stems are abundant in the sandstone a few feet higher in the series. Despite the absence of any Stigmarian scars on the surface of the fossil it is probable that these fine specimens are the rhizomes of some lycopodiaceous plant, possibly Bothrodendron, which is not uncommon in the Vereeniging beds.