[Illustration: Fig. 416. _Medullosa_ stems. A-C, _Medullosa anglica_; A, transverse section; _a_, accessory vascular strand; _b_, accessory strand enclosed by periderm; _c_, band of periderm encircling steles; _d_, sclerenchyma between leaf and stem. B and C, longitudinal sections. (After Scott.) D, _Medullosa stellata_; _a_, star-rings; _p_, ‘partial pith.’ (After Weber and Sterzel.) E, _Medullosa Solmsi_. (After Weber and Sterzel.) F, Medullosa stellata, from a specimen in the British Museum (No. 13767). G, _Medullosa stellata_ var. _cortica_; _v_, leaf-bundles. (After Weber and Sterzel.) H, I, _Medullosa Leuckarti_. (After Weber and Sterzel and Solms-Laubach.) K, _Medullosa stellata_ var. _gigantea_; _a_, concentric stele; _b_, _b_, later cylinders of centrifugal tissue. (Adapted from Weber and Sterzel.) L, _Medullosa Solmsi_ var. _lignosa_. M, _Medullosa porosa_. (L, M, after Weber and Sterzel.)] The leaf-traces are furnished by the peripheral tracheal groups at the free surface of the primary portion of each stele: each trace is at first concentric and consists of primary xylem with one or more protoxylem strands near the outer surface and is completely or partially enclosed by secondary xylem and phloem. In the course of its passage to the leaf a leaf-trace loses its secondary tissues, which were added by the cambium during the traverse of the zone of secondary wood, and divides into small collateral bundles consisting mainly of spiral and scalariform tracheids. The collateral bundles accompanied by some narrow fibres are of the _Myeloxylon_ type (fig. 420), the xylem being wholly centripetal. In the behaviour of the leaf-traces and in the vascular system of the petioles _Medullosa_ differs from _Heterangium_ and _Lyginopteris_. Each leaf-base is supplied by sets of vascular strands which pass into it from the stem at different levels; a large leaf-base reaching 4 cm. in diameter receives as many as 70–80 bundles. The hypoderm is like that first described in the French species _Myeloxylon Landriotii_[247] and often spoken of as the _Sparganum_ type of hypoderm. The branching of the rachises points to a compound frond, and the occurrence of numerous linear pinnules with revolute margins (fig. 420, D) in association with the stem suggests that the ultimate segments were of the _Alethopteris_ form. This inference receives confirmation from the occurrence of petrified specimens of undoubted _Alethopteris_ rachises with the structure of _Myeloxylon_. It is practically certain that the leaves borne on the stems of _Medullosa anglica_ are those long known as _Alethopteris lonchitica_ (Vol. +ii.+ A, p. 553, fig. 364).