The production of secondary xylem and an increase in the girth of the whole stem led to reduction in the amount of centripetally developed conducting channels. Some of these assumed a new rôle and a shape in harmony with their functions. A later stage is represented by a further encroachment of the central parenchyma on the cylinder of centripetal xylem, as seen in _Lepidodendron Harcourtii_ and other species. The next stage is afforded by ribless species of _Sigillaria_ in which the primary xylem is broken up into separate conducting strands. As Kidston[654] reminds us, it is in the geologically more recent species of _Sigillaria_, such as _S. Brardi_, which persist into the Permian era, that this more extreme case of reduction occurs. The older genus _Lepidodendron_ seems to have retained to the last the complete cylinder of primary xylem. In the stele of _Stigmaria_, the rhizome of _Sigillaria_ and of _Lepidodendron_, reduction of the centripetal xylem has passed beyond the stage represented by the broken cylinder of the ribless Sigillarias. With the exception of the examples described by Renault[655] and by Weiss[656], _Stigmaria_ is characterised by little or no centripetal primary xylem. It is, however, noteworthy that Renault’s _Stigmaria_, in which centripetal xylem forms a prominent feature, is attributed to _Sigillaria Brardi_, a species in which the vascular cylinder of the aerial stem illustrates a later and not an earlier phase in the replacement of centripetal by centrifugal wood. It would seem, as Lady Isabel Browne[657] says, that most Stigmarian axes had reached a more advanced stage in specialisation than is shown in the stelar structure of the aerial shoots. The relatively greater and probably the more precocious development of secondary xylem in _Stigmaria_ than in _Lepidodendron_ or _Sigillaria_ may have some significance in relation to the smaller amount of “old wood[658]” (in a phylogenetic sense) in their steles. As is pointed out in a later chapter, recent researches into the anatomy of extinct members of the Osmundaceae by Kidston and Gwynne-Vaughan have brought to light a striking parallelism in evolutionary sequence between the Lepidodendreae and the ancestors of _Osmunda_ and _Todea_, the two surviving genera of one of the most ancient families of ferns.