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_. One of the largest specimens of a petrified _Stigmaria_ which I have seen is one lent to me by Mr Lomax from the Coal-Measures of Halifax in which the flattened transverse section measures 18 cm. × 3·5 cm., the cylinder of wood being 1·1 cm. × 7 mm. in diameter. In French examples of _Stigmaria_ or _Stigmariopsis_ it has been demonstrated by Renault[596] that primary xylem strands occur very like those in the stem of some species of _Sigillariae_ (see p. 219). If a well-preserved section of an English _Stigmaria_ is examined it will be seen that the edge of the secondary wood consists of a few narrower elements which do not exhibit the radial seriation characteristic of secondary elements.