Thus the roots of Sigillaria are called Stigmaria, detached leaves Sigillariophyllum, and the fructifications Sigillariostrobus; the name Sigillaria applies to the stem, which, however, when old and partly decorticated has been called Syringodendron, while its woody cylinder has often been described under the name Diploxylon.
The upright stems were attached to the soil by a number of dichotomously branched members (Stigmaria), which, whatever their morphological nature may be, appear to have performed the function of roots: they bore numerous cylindrical appendages, which penetrated the soil on all sides.
For example, the form and structure of Stigmaria have long been well known; but it is seldom possible to determine whether a given Stigmaria belonged to Sigillaria, Lepidodendron or some other genus.
To these, underground organs the name Stigmaria is applied; they are not clearly distinguishable from the corresponding parts of Sigillaria.
- On present evidence there is no satisfactory distinction to be drawn between the subterranean organs of Sigillaria and those of Lepidodendron and its immediate allies, though some progress in the identification of special forms of Stigmaria has recently been made.
These organs, to which the name Stigmaria was given by Brongniart, have been found in connexion with the upright stems both of Sigillaria and Lepidodendron.
Petrified specimens of the main Stigmaria are frequent, and those of its rootlets extraordinarily abundant.
The two parts are very different in structure: in the main axis, as shown in the common Coal Measure form Stigmaria ficoides, the centre was occupied by the pith, which was surrounded by a zone of wood, centrifugally developed throughout.
The morphology of Stigmaria has been much discussed; possibly the main axes, which do not agree perfectly either with rhizomes or roots, may best be regarded as comparable with the rhizophores of Selaginellae; they have also been compared with the embryonic stem, or protocorm, of certain species of Lycopodium; the homologies of the appendages with the roots of recent Lycopods appear manifest.
The genera Lepidodendron, Sigillaria, Stigmaria, or Calamites, which played so great a share in the vegetation of the same age in the northern hemisphere, have not been recognized among the Palaeozoic forms of India, but examples of Sigillaria, Lepidodendron and Bothrodendron are known to have existed in South Africa in the Permo-Carboniferous era.