Fig. 209. Stigmariopsis and “tap-roots.” (After Grand’Eury.)
These conclusions require some modification when applied to British representatives of the arborescent Lycopodiales. The long spreading and dichotomously branched root-like organs attached to the base of Sigillarian and Lepidodendron stems are true examples of Stigmaria ficoides or other species. Stigmariopsis occurs but rarely. This marked difference between French and English specimens may be explained if we adopt the opinion of Solms-Laubach, who believes that the true Stigmaria represents both the parent rhizome and the later-formed roots of the Rhytidolepis Sigillarian species and of Lepidodendron, the Stigmariopsis form having the corresponding relation to the Leiodermarian-Clathrarian species.
The opinion expressed by Williamson[592] in 1892 that Grand’Eury’s hypothesis “appears to be identical with the vague and speculative guesses that were prevalent among us in the early years of the present [nineteenth] century” illustrates the strength of conviction based on English specimens as to the root-nature of Stigmaria.
There is undoubtedly considerable confusion, which can be cleared up only by further research, as to the precise relation between Stigmaria and Stigmariopsis on the one hand and the different types of Sigillariae on the other. The main contention, and this is the most important point, of Renault, Grand’Eury and Solms-Laubach as to the manner of formation of the aerial shoots from rhizomes and the subsequent production of forked roots and their ultimate separation from the parent rhizome is, as I believe, correct. Williamson held that Stigmaria must be regarded as a true root; he found no evidence to support the view that the large rooted stem discovered by Hawshaw, Binney, and others had been originally produced from aquatic rhizomes. It must, however, be remembered that Grand’Eury’s opinion is based on evidence afforded by the exceptionally well displayed Sigillarian forests of St Étienne, on a scale such as English strata have not as yet afforded. Moreover, the absence of any parent-rhizome in association with the rooted stumps described by Williamson and by others is not a serious argument against their rhizome origin.
The specimen represented in [fig. 209], which was examined in situ by Solms-Laubach and Grand’Eury, shows a Sigillarian stem in the Syringodendron condition bearing rows of paired parichnos scars; from the base forked and rapidly tapering arms radiate through the surrounding rock and, as shown by other specimens, these bear numerous appendages like those of the English Stigmarias. The surface-features of the arms are those of Stigmariopsis and the centre of each, as seen on the broken face, is occupied by a pith-cast characterised by parallel longitudinal ridges resembling those on the medullary casts of Calamites. It is noteworthy that the petrified rhizome originally described by Renault as Stigmaria flexuosa, and afterwards identified by him as the subterranean system of Sigillaria Brardi, possesses a vascular cylinder composed of primary xylem strands of crescentic transverse section lining the pith; a cast of the pith, after the removal by decay of its delicate parenchymatous tissue, would exhibit the surface-features of Stigmariopsis. Stigmaria flexuosa no doubt represents a true Stigmariopsis rhizome. On the other hand, as Williamson has shown, the inner surface of the wood of Stigmaria ficoides consists of a reticulum of xylem with meshes of medullary-ray tissue; a cast of such a surface presents a very different appearance from that of Stigmariopsis.
Returning to [fig. 209]: from the lower surface of the Stigmariopsis arms numerous conical outgrowths, reaching a length of several centimetres, project vertically downwards; these also possess Stigmariopsis pith-casts and are identical with the “tap-roots” of Richard Brown. The stump seen in [fig. 209] shows the characteristic hollow base of the erect stem: this is the region which, it is believed, represents the position of the Stigmarian rhizome from which the aerial shoot was developed. Although no remains of the parent rhizome were found, traces of the rootlets which probably belonged to it were found in the neighbourhood. The absence of the actual rhizome is, however, not surprising as it would not persist after its aerial Sigillarian branches had attained independence by the development of their own dichotomously branched absorbing and holdfast organs.
The Stigmarian axes of Palaeozoic Lycopods are compared by Miss Thomas[593] with the prop-roots of certain recent flowering plants which grow in tropical tidal swamps; their roots grow downwards from the stem at an angle of 50°-60° before spreading out horizontally. This author also makes some interesting suggestions in regard to the evidence afforded by anatomical structure as to the habitat of Sigillaria and Lepidodendron.
Anatomy.
The more important anatomical features of Stigmaria must be dealt with briefly. Williamson’s monograph, published in 1887[594], is considerably in advance of the work of that of any of the numerous writers who had previously dealt with the subject. The diagrammatic transverse section reproduced in fig. 210, H, illustrates the general arrangement of the tissues. The medullated stele was described by Williamson as consisting entirely of centrifugally developed secondary xylem and distinguished, therefore, from the stele of a Lepidodendron or Sigillaria by the absence of a centripetally produced primary xylem zone. The secondary xylem tracheae are characterised by scalariform pits on both radial and tangential walls and, as shown in a figure given by Solms-Laubach[595], the spaces between the transverse bars are bridged across by fine threads, as in the tracheae of Lepidodendron.