The presence of conspicuous medullary rays gives the secondary xylem of _Stigmaria_ the appearance of being divided into several more or less distinct groups (fig. 210, E, _St_). In tangential longitudinal section the xylem assumes the form of a broad reticulum with lenticular meshes filled with medullary-ray tissue through which strands of xylem are cut across in a transverse direction as they pass outwards from the inner edge of the wood to supply the rootlets. In addition to these broader or primary medullary rays, there were numerous secondary rays composed of narrow plates of parenchymatous cells one or several elements in depth. As Williamson pointed out, the medullary-ray tissue consists in part of radially elongated tracheal elements with spiral or scalariform thickening bands like those described in the same position in Lepidodendron stems. Our knowledge of the minute structure of the tissues abutting on the secondary xylem is far from complete. The xylem is succeeded by a zone of delicate cells which was the seat of meristematic activity. It is noteworthy that in a section figured by Williamson[600] there is the same disparity in size between the outermost elements of the xylem and the adjacent cells of the meristematic zone as in Lepidodendron stems. Beyond this region an imperfectly preserved lacunar tissue occurs like that which I have called the secretory zone in Lepidodendron stems; but information as to the structure of this part of _Stigmaria_ is much more incomplete than in the case of the aerial shoots. The middle cortex was of the same lacunar type as in the stems, and the fact that it is never well preserved in large Stigmarian axes suggests that it may have been even more richly supplied than in the aerial stems with an aerating system of spaces. The outer cortex, consisting in young examples of large-celled parenchyma, became at an early stage of growth the seat of cambial activity which resulted in the production of radially placed series of secondary elements (fig. 210, H, _p_). The outer and older elements of this secondary cortex are more tangentially stretched than the inner cells, a necessary result of the position of the phellogen on the internal edge of the tissue and of the increasing girth of the axis.