emission of leaf-traces. _Medullosa Leuckarti_ (fig. 416, H) resembles in its vascular plan _M. centrofilis_, but in this larger stem there are several star-rings and the enlarged peripheral steles are more or less sinuous. In _Medullosa Solmsi_ (fig. 416, E) the star-rings are still more numerous and the main vascular system consists of a double series of concentric steles, each agreeing with the larger peripheral steles of _M. Leuckarti_. Some of the forms included in _Medullosa stellata_ appear to be very different from _M. anglica_ and _M. Leuckarti_ (_cf._ fig. 416, F, A, H), but their similarity is apparent if we imagine _Medullosa anglica_ with only one main stele (with the addition of star-rings) which is stretched tangentially until it becomes a long and narrow plate-ring and is then rolled into a hollow cylinder like that in fig. 416, D. A modification of the tubular type of stele is seen in _Medullosa stellata_ var. _corticata_ (fig. 416, G) in which the vascular cylinder is broken up into two or more curved plate-rings, a change superficially similar to that by which a dictyostele is produced from a solenostele, but in _Medullosa_ it is not the overlapping of leaf-gaps that is the cause of the change. A striking feature in the stem of _Medullosa stellata_ represented in fig. 416, D and F (especially F), is the inequality in breadth of the centrifugal and centripetal xylem: this inequality is an expression of the difficulty caused by the presence of an internal as well as an external addition of secondary conducting tissue. A limit is set to the production of secondary centripetal xylem by the space available for extension, whereas there is unlimited room for increase in the case of the centrifugal tissue. This tendency to a greater development of xylem and phloem on the outer side of the primary portion of the steles is illustrated also in _M. Solmsi_ var. _lignosa_ (fig. 416, L) where the outer of the two series of peripheral steles has a much larger proportion of centrifugal xylem. In _M. anglica_ the secondary xylem on the inner side of the steles is not infrequently broader than the corresponding tissue on the abaxial side[417]; but this is exceptional in the genus. A further development of centrifugal conducting tissue without any corresponding development of centripetal tissue is effected in certain cases (fig. 416, K, L) by the addition of concentric cylinders of centrifugal xylem and phloem beyond the original concentric steles. The type illustrated by _Medullosa Solmsi_ var. _lignosa_ and _M. stellata_ var. _gigantea_ (fig. 416, L, K) at once suggests comparison with stems of _Cycas_, _Macrozamia_, and _Encephalartos_, the chief difference being the presence in _Medullosa_ of an inner series of concentric steles and a central ground-tissue containing star-rings, though the latter may be regarded as corresponding to the medullary system of bundles in _Macrozamia_. In _Macrozamia_ the central region of the stem is