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. 
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Fic. 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 aséedic 
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 
