(fig. 396, K) and _Cycas siamensis_ (fig. 396, L) with their polystelic type of structure and the adult stems of _Medullosa_[81]. In the stems of _Dioon_, _Microcycas_, _Stangeria_, and _Zamia_ no extrafascicular cylinders are recorded. Two main vascular bundles enter the cortex from each leaf-base and in most stems these diverge right and left and more or less completely encircle the stele before passing through the medullary rays and joining the inner portion of the xylem of the stele either as double or single bundles. These girdle-bundles (fig. 396, H) first described by Karsten and Mettenius form a very characteristic cycadean feature[82]. Adjacent girdles are joined by connecting cortical bundles and, in addition, there are cauline collateral bundles in the cortex which form an anastomosing system. In some cases, _e.g._ species of _Macrozamia_ and occasionally in _Stangeria_, the female peduncle of a _Ceratozamia_, and in seedlings of _Bowenia_ and _Cycas revoluta_[83], the leaf-traces pursue a direct course from petiole to stele as in stems of Bennettitales. It is noteworthy that in seedlings of _Microcycas_[84], a genus characterised by a large number of male gametes—presumably a primitive feature—the leaf-traces are of the girdle-type. The two bundles at the base of a petiole by repeated subdivision give rise to the numerous collateral strands of the rachis. A leaf-trace in its passage to the leaf is like that of a Conifer in having the protoxylem on its inner edge, whereas in the petiole and elsewhere in the frond it is characterised by an arrangement of the xylem that has usually been described as mesarch. A typical vascular bundle from a cycadean frond is seen in fig. 399, C; by far the greater part of the xylem is centripetal, the centrifugal xylem being confined to an arc of scattered tracheids or a small strand separated by a few parenchymatous cells from the protoxylem. [Illustration: Fig. 397. Tracheids from the stem of _Stangeria paradoxa_. (After Marsh.)] [Illustration: Fig. 398. Pith-cast of a _Macrozamia_ stem, (⅖ nat. size.)] [Illustration: Fig. 399. Sketches illustrating the changes in the structure of Cycadean vascular bundles in their course from stem to leaf: _cp_, _cf_, centripetal and centrifugal xylem; _p_, phloem; _px_, protoxylem. (After Marsh.)]