Morphological Status of the Shoot Systems of Psilotaceae

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Abstract
This paper evaluates the morphological bases for Bierhorst’s recent proposals of the systematic alliance of Psilotaceae with leptosporangiate ferns. In particular it examines the evidence which Bierhorst cites in support of the homology of the frond ofStromatopteris monilijormis with the aerial shoots ofPsilotum andTmesipteris. By a reevaluation of the data it is shown that these shoot components are not homologous but are at best analogous structures. Bierhorst’s proposals of morphological correspondence seem to have been based on the assumption that shoots of both Psilotaceae and Stromatopteridaceae are at a primitive level of evolution, intermediate between an ancestral telomic form and the more organographically differentiated higher ferns and seed plants. However, a clear histogenetic and morphogenetic distinction between leaves and buds in ferns and Psilotaceae as well as the occurrence of the same type of correlative integration of shoot components in both groups indicates that neither is morphologically very primitive. While the lack of structural correspondence between elements of one phase of the plant’s morphology does not necessarily negate Bierhorst’s general taxonomic conclusions, the fundamental nature of the differences in shoot morphology make it doubtful that the Psilotaceae should be associated with the Filicales.

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