194 ANATOMICAL EVIDENCE 



breaking up of the stele in certain axes a schizostelic state with individual 

 strands may be attained ; but comparison shows that these are not the 

 phylogenetic equivalents of the individual strands of the simple leaf, though 

 they may be continued outwards into the leaves : conversely, in the pro- 

 gressively developing leaf, a pseudostelic structure may be produced by 

 fusion of strands phylogenetically distinct ; but again comparison shows that 

 this is not the phylogenetic equivalent of the primitive stele of the axis, 

 but a condition secondarily derived. 



FIG. 98. 



Transverse section of base of petiole of Gleichenia dicarpa, showing a pseudo-stelar 

 structure resulting from contraction of horse-shoe-like xylem till the margins fuse. Photo- 

 graph by R. Kidston from section by Gwynne-Vaughan. 



It is necessary thus to differentiate characters which are primary from 

 those which are secondary. It has long been recognised that the distinction 

 cannot always be maintained between axis and leaf on the basis of strict 

 criteria of form or structure : exceptions can be found to all morphological 

 criteria proposed. Still, if on a basis of comparison the primary and 

 secondary conditions be clearly kept apart, the divergent anatomical 

 characteristics of the two parts become sufficiently obvious. Accordingly, 

 in our view the structure of the leaf is recognised as primarily astelic 

 throughout, that is, the isolated strands are not to be held as results of 

 resolution of a primitive stele ; where an apparently stelar structure appears 



