xxx] 



STELOXYLON 



163 



cylinders in Cycas be interpreted as an indication of a closer con- 

 nexion 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. 



II. A. STELOXYLEAE. 



Steloxylon. Solms-Laubach. 



Steloxylon Ludwigii (Goeppert and Stenzel). The genus is 

 founded on a piece of stem from Siberia, possibly of Permian 



FIG. 441. Steloxylon Ludwigii. A, surface-view (longitudinal) of stem showing 

 leaf -bases. (After Solms-Laubach.) B, longitudinal section showing anasto- 

 mosing vascular strands. C, steles, the lower one showing the emission of a 

 leaf -trace. (B, C, after Schenk.) 



age though not improbably older, which was originally de- 

 scribed as Medullosa Ludwigii 1 . It is characterised by numerous 



1 Goeppert and Stenzel (81) p. 126, PL xvn. 



112 



