. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. 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 equiva
. The origin of a land flora, a theory based upon the facts of alternation. Plant morphology. 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 Transverse section of base of petiole of Gteichtmia a, showing a pseudo-stelar structure resulting from contraction of horse-shoe-like xylem til] 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 astclic 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. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Bower, F. O. (Frederick Orpen), 1855-1948. London, Macmillan and Co. , Ltd.
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