The comparative examination of recent Cycads naturally suggested by any attempt to compare the group as a whole with Palaeozoic types leads to some apparently contradictory results. The habit of the megasporophyll of _Cycas_ is usually quoted as a primitive attribute: the close resemblance in plan and in manner of occurrence on the stem between megasporophylls and foliage leaves recalls both Ferns and Medullosan fronds. On the other hand the production of eight body-cells in the pollen-tube of _Microcycas_[432] in place of the usual single cell may also be regarded as a primitive character. It is perhaps possible, as Miss Dorety[433] says, that the polyspermy may be a case of recurrence and not a direct inheritance. _Microcycas_ differs from _Cycas_ in having only one vascular cylinder, and if the presence of several concentric cylinders in _Cycas_ be interpreted as an indication of a closer connexion with a Medullosan ancestry, the _Microcycas_ type would represent a more advanced stage in evolution. Attempts to arrange plants according to a natural sequence are frequently frustrated by instances of unequal progress in the development of vegetative and reproductive organs; one or other set of members lags behind; some characters point to the retention of primitive traits while others indicate a marked progressive tendency. It is noteworthy that the Mesozoic Bennettitales are characterised by a greater simplicity of stem-structure than is the rule in recent Cycads, and both in their vegetative features and in the structure of the seeds they are further removed from the Medullosan type. [Illustration: Fig. 441. _Steloxylon Ludwigii._ A, surface-view (longitudinal) of stem showing leaf-bases. (After Solms-Laubach.) B, longitudinal section showing anastomosing vascular strands. C, steles, the lower one showing the emission of a leaf-trace. (B, C, after Schenk.)] II. A. =STELOXYLEAE.= =Steloxylon.= Solms-Laubach.