considered to be the pith of a monostelic stem, whereas in _Medullosa_ the stem is polystelic. In recent cycadean stems it is not uncommon to find patches of inversely orientated xylem and phloem internal to one or more of the cylinders of centrifugal vascular tissue. These abnormal developments are considered by Worsdell[418] to be relics of the inner portions of concentric steles possessed by the Medullosan ancestors of recent Cycads. This interpretation affords a means of bringing into closer relationship the polystelic Medulloseae and the monostelic Cycadaceae, the apparent simplicity of the latter being the result of the progressive loss of centripetal xylem and phloem, the normal cycadean cylinder being therefore regarded as a one-sided remnant of a concentric Medullosan stele. In other words, the Cycads are descended from polystelic ancestors. As further evidence in support of this view Worsdell points to the occurrence of concentric steles in the cortex of _Cycas_ and their occasional presence in the pith of other genera. Matte[419] has shown that in the seedling stem of _Encephalartos Barteri_ (fig. 396, K) there are three concentric steles each similar to a normal Medullosan stele: at a higher level in the axis the steles become ‘unrolled’ and assume the form of one-sided cylinders of centrifugal xylem and phloem. In the peduncles of some recent Cycads, _e.g._ _Stangeria_, there is a tendency towards a somewhat irregular orientation of the collateral bundles that constitute the vascular cylinder, and tracheids occasionally occur internal to the protoxylem of the individual bundles[420]. Worsdell regards these features as evidence of a Medullosan ancestry. If the sinuous plate-rings of a stem of _Medullosa Leuckarti_ (fig. 416, H) were broken up into separate portions and wholly or in part deprived of the centripetal xylem, the result would be an arrangement of bundles comparable with that in a _Stangeria_ peduncle[421]. The scattered centripetal tracheids discovered by Scott in _Stangeria_ and other cycadean peduncles are interpreted by Scott and by Worsdell as relics of some ancestral centripetal xylem, but with this important difference in the point of view; Scott believes that they represent the almost completely aborted centripetal xylem of a single stele like that of _Lyginopteris_, while Worsdell sees in them fragmentary vestiges of the central primary xylem of two or more Medullosan steles. An abnormal seedling of _Araucaria Bidwillii_ described by Shaw[422] exhibits features analogous to those in some cycadean seedlings: within the normal stele a cambium forms an inversely orientated vascular cylinder which at a lower level becomes continuous with the outer centrifugal tissue, the whole vascular system being eventually represented by two concentric steles. The polystelic stage is a development of a monostelic condition, and the inner or inverted portion of each of the two concentric steles is derived from