Fig. 177. Stigmaria radiculosa (Hick). (From sections in the Manchester University Collection.)
Stigmaria radiculosa (Hick).
We have no proof of the nature of the subterranean organs of Lepidodendron fuliginosum, though it is not improbable that the specimens described below may be correctly assigned by Weiss to that species. Prof. Weiss[335] has made an interesting contribution to our knowledge of a type first described by Hick[336] under the name Tylophora radiculosa, a designation which he afterwards altered to Xenophyton radiculosum[337] and for which we may now substitute Stigmaria radiculosa (Hick). Prof. Williamson expressed the opinion that Xenophyton exhibited considerable affinity with Stigmaria ficoides and Weiss’s further study of the species leads him to regard Hick’s plant as probably the Stigmarian organ of Lepidodendron fuliginosum. The diagrammatic transverse section represented in [fig. 177], A (4·5 cm. in diameter), shows an outer cortex of parenchyma, c3, consisting in part of radial rows of secondary tissue and of a band of compact parenchyma bounded by the wavy line a; at sc is a series of secretory strands exactly like those in a corresponding position in Lepidodendron fuliginosum and other species of the genus. The greater part of the organ is occupied by a lacunar and hyphal middle cortex identical in structure with that shown in [fig. 178], B, drawn from a rootlet. At d, [fig. 177], A, the middle cortex has been invaded by a narrow tongue of outer cortical tissue. The stele is characterised by a large pith filled with parenchyma; in Stigmaria ficoides[338] the general absence of pith-tissue has led to the inference that the stele was hollow. The xylem is represented by a ring of bundles separated by broad medullary rays; each bundle contains a few small, apparently primary, elements on its inner edge but is mainly composed of radial rows of secondary tracheae x2, [fig. 177], B. On the outer face of the secondary xylem occur a few smaller and thinner walled cells, c, having the appearance of meristematic tissue; from these additional tracheae were added to the xylem. This meristematic zone occurs, as in the stems of Lepidodendron, immediately internal to the secretory tissue, sc; at c1, [fig. 177], B, is seen the inner cortical tissue.
Fig. 178. Rootlet of Stigmaria. (From a section in the Manchester Collection.)
In surface-view a specimen figured by Hick[339] shows a number of circular scars agreeing in shape and arrangement with the rootlet scars of Stigmaria ficoides. At b in [fig. 177], A, the basal portion of a rootlet is shown in organic connexion with the outer cortex. The rootlet-bundles are given off from the stele as in other examples of Stigmaria; each bundle consists of a triangular strand of xylem with an endarch protoxylem at the narrow end accompanied by a portion of the secretory tissue as in the leaf-traces. As in Stigmaria ficoides the rootlets are attached to the outer cortex above a cushion of small cells. It is interesting to find that rootlet-bundles, as seen in tangential section of the main axis, are associated with a parichnos strand, but this is on the xylem side of the vascular strand, whereas in the case of leaf-traces the parichnos is on the other side of the bundle.
Fig. 178, A, represents a transverse section of a rootlet (6 mm. in diameter) associated with Stigmaria radiculosa and probably belonging to this species. The xylem strand x is composed of a group of tracheae with a single protoxylem strand, px, at the pointed end and with small metaxylem elements at the broad end next the space originally occupied by the so-called phloem. A parenchymatous sheath, c′, surrounds the bundle, and beyond this is the broad middle cortex, a small portion of which is shown on a larger scale in [fig. 178], B; as Weiss points out, some of the outermost cells of the lacunar cortex (m) are clearly in a state of meristematic activity.
The preservation of the middle cortex and the small quantity of secondary xylem are characters which this Stigmaria shares with Lepidodendron fuliginosum, and although decisive evidence is still to seek, we may express the opinion that Weiss’s surmise of a connexion between Stigmaria radiculosa and Lepidodendron fuliginosum is probably correct.