Admitting that Stigmaria is part of Sigillaria, the next question is, is Stigmaria a root in the ordinary sense, the underground system formed on germination of the spore and of equal age with the shoot, or did it bear a different relation to the Sigillarian stems? To this question different answers would still be given. Goeppert[582] discussed evidence in favour of the view that aerial Sigillarian shoots were produced as vegetative buds on pre-existing Stigmarian axes, like young moss plants on a protonema. At a later date Renault[583] developed a similar view as regards Sigillaria; but we may pass on to consider the more recent and complete observations of Grand’Eury[584] and Solms-Laubach[585].
The recognition of two distinct types of Stigmariae in the Coal-Measures of Central France led Grand’Eury[586] to institute a new genus, Stigmariopsis. This type, which is characterised by a difference in habit as well as by other distinguishing features, is represented by such specimens as those figured by Goldenberg as Stigmaria abbreviata, bearing lenticular scars spirally disposed on a cortical surface characterised by irregular longitudinal wrinklings. Stigmariopsis has frequently been found in direct continuity with Sigillarian stems of the Leiodermarian-Clathrarian type, spreading obliquely downwards in the form of rapidly narrowing arms clothed with slender and usually simple appendages; and from the under surface of these arms short conical outgrowths are given off. It is probable, as Solms-Laubach believes, that Stigmariopsis was represented also by long horizontally creeping rhizomes[587] of uniform breadth from which ribless Sigillarian aerial shoots arose as bud-like outgrowths. Grand’Eury, the author of the genus, confined the term to the shorter and more rapidly tapered organs spreading from the base of erect stems; the horizontal rhizomes of all Sigillarian stems he refers to Stigmaria. The pith-casts of Sigillariopsis may be recognised by their long vertical ridges and grooves, a feature readily understood by reference to the stem structure. The Stigmariopsis rhizomes though rare in England have been recognised by Dr Kidston[588] in the Middle Coal-Measures of Yorkshire; he has figured a pith-cast very like that illustrated in Solms-Laubach’s Memoir as Stigmariopsis anglica.
The surface-features of a Stigmariopsis pith-cast are clearly shown on a specimen from St Étienne in the Williamson collection[589].
STIGMARIOPSIS
The most complete account of Grand’Eury’s views in regard to the anchoring and absorbing organs of Sigillaria is given in his monograph on the Coal-field of Gard[590], St Étienne, and these are clearly stated also by Solms-Laubach[591] who confirms the conclusions of the French author as to the manner of development of the aerial shoots. Grand’Eury believes that both Stigmaria and Stigmariopsis are rhizomes and not true roots. The surface-features of Stigmaria have already been described. This type Grand’Eury speaks of as characterised by the uniform diameter and considerable horizontal elongation of the bifurcated axes; he thinks they grew both as floating rhizomes and on the ground: they may frequently be traced for a considerable distance without showing any signs of connexion with aerial shoots, but occasionally they have been seen in organic union with Sigillarian stems. He believes that these rhizomes were produced as the result of germination under water of the spores of Sigillaria or Lepidodendron and developed as long and branched aquatic rhizomes capable of independent existence. Under certain conditions, as he thinks in shallower water, the rhizomes produced bulb-like outgrowths which grew into erect stems having the surface-features of Sigillaria. This method of origin is practically the same as that described by Goeppert in 1865. The vascular medullated cylinder of these erect branches was in direct continuity with that of the Stigmarian rhizomes.
Fig. 207. An early stage in the development of Sigillaria.
A. Surface-features enlarged. (After Grand’Eury.)
Fig. 208. Later stage in the development of Sigillaria; Syringodendron with Stigmariopsis. (After Grand’Eury.)
The next stage is that in which the undifferentiated bulb becomes swollen at the base and develops four primary roots ([fig. 207] B, C) which grow obliquely downwards and produce numerous rootlets. Meanwhile the parent rhizome gradually decays, finally setting free the aerial stems which are now provided with spreading and forked roots ([fig. 208]) such as we are familiar with in English specimens as Stigmaria ficoides, but which in the French specimens show the features of Stigmariopsis. At this later stage conical outgrowths are formed from the under surface of the Stigmariopsis arranged in a more or less regular series surrounding the centre of the forked and spreading roots ([fig. 209]). These conical and positively geotropic organs were long ago described by Richard Brown as tap-roots. Grand’Eury’s conclusions are briefly as follows: Sigillaria, and we may add Lepidodendron, had no true roots and in this respect are comparable with Psilotum ([fig. 118]): the organs which are described by Grand’Eury as roots are correctly so named in a physiological sense, but morphologically they do not strictly conform, either in origin or in the arrangement of their appendages, to true roots. The question as to whether they are entitled to the designation root is one which it is needless and indeed futile to discuss in detail; it would be conceding too much to a formal academic standpoint to refrain from applying to them the term root, as that best describes their share in the life of the Sigillarian stems. The horizontal Stigmarian axes are rhizomes in the ordinary sense of the term and from these were developed Sigillarian shoots, characterised in the lower portions by large parichnos strands. From the base of the young bulbous shoots roots were formed: these roots being, in the French specimens, of the Stigmariopsis type.