As Renault points out, roots of _Calamites_ have been figured by some writers[698] as examples of stems, but it is usually comparatively easy to distinguish between roots and stems. On examining the xylem groups more closely, one notices that the apex of each is occupied by a triangular group of centripetally-developed primary tracheids, the narrow spiral protoxylem elements occupying the outwardly directed apex. The protoxylem apex is usually followed externally by a ray of one or two radially disposed series of parenchymatous cells. This ray is not distinguished in fig. 92 _R″_ from the rows of xylem tracheids. Each xylem group is thus formed partly of centripetal xylem and in part of secondary centrifugal xylem; the latter is associated with secondary medullary rays, as in stems, and contains a broader ray (_fascicular ray_ of Williamson and Scott[699]) immediately opposite each protoxylem strand. In the roots of recent plants (_e.g._ _Cucurbita_, _Phaseolus_, &c.) a broad medullary ray is often found opposite the protoxylem, and such an arrangement is a perfectly normal structure in roots[700]. Renault has recently described several species of Calamite roots which he designates by specific names, some of them belonging to stems with the _Arthropitys_ structure, and others to _Calamodendron_. Some of the roots figured by the French author have an axial strand of xylem with 7–15 projecting angles of protoxylem[701]. These he considers true roots, but the larger specimens with a wide pith he prefers to regard as stolons. In the latter he mentions the union of the primary centripetal with the secondary centrifugal wood as a distinguishing feature. It has been shown, however, that each group of secondary xylem includes a median ray of parenchyma, and that the whole structure is essentially that of a root, and not that of a modified stem or stolon. The organs described by Renault as true roots are probably rootlets, and as Williamson and Scott have demonstrated, there is every gradation between the smaller specimens with a solid xylem axis and those with a large central pith. It is interesting to note that Renault’s figures of _Calamodendron_ roots show the closest resemblance to those of the subgenus _Arthropitys_. _d. Cones._