188 



CYCADOXYLEAE 



[CH. 



by stems, our knowledge of the leaves being confined to the leaf- 

 traces in the stem which appears to have a phyllotaxis of f . The 

 stem of this Permian species has a diameter of 5 6 cm. : the 

 comparatively broad cortex contains numerous secretory canals, 

 but in place of hypodermal strands of stereome there is a super- 

 ficial periderm. The vascular tissue, consisting of secondary 



. 



FIG. 453. A. Ptychoxylon Levyi ; transverse section of stem ; a, stele of branch. 

 (After Renault.) 



B. Cycadoxylon Fremyi; transverse section of stem. (After Renault.) 



C. Cycadoxylon robustum; centre of stem. 



1 4, diagrams of sections of the stem of Ptychoxylon at different 

 levels. (Simplified from Renault ) 



xylem and phloem, assumes different patterns at different levels. 

 There is an outer vascular cylinder of centrifugally developed 

 xylem and phloem; the xylem is manoxylic and the tracheids 



