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Both the typical form and the appendageless variant of Psilotum nudum produce terminal synangia at the ultimate tips of the aerial axes. One clone in particular of the typical appendaged form produced synangia entirely at the tips of the aerial branches, as in the appendageless variant, and also developed occasional lateral transitional entities on the upper aerial axis displaying appendagelike and axislike morphological qualities. A developmental comparison of synangium development at the ultimate tips of aerial branches and of unusually elongating and normal sized fertile‐appendages showed that the morphogenetic pattern of synangium development was similar. Anatomical and morphological evidence showed the synangium to be derived from terminal subdivisions or bifurcations of the apical meristem of each structure studied. This supports the phyletic concept that the synangium of the Psilotaceae is basically terminal to an axis or an axis homologue, and that it probably evolved from terminal bifurcative branching. Occasional multiple sporangium lobes may be formed on a P. nudum synangium which may not be represented by vascular bundles. Two hypothetical phyletic models of synangium evolution are proposed that could be used to explain this phenomenon and which should be tested by further evidence. Typical and appendageless P. nudum were compared in their morphogenetic pattern developed at the upper axis vegetative apical meristem, and a reconciliation was made between the structuring of the apparently disparate forms, which involved the presence or absence of serial ordering in apical derivatives. It is suggested that this could serve as a model for appendage evolution in the family Psilotaceae.

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An otherwise normal plant of Psilotum nudum consistently produced aberrant fertile appendages over a 5-year period when the plant was grown under prolonged periods of cool-white fluorescent lighting of approximately 200–400 ft-c (0.625–1.2 MW/cm2), 16-hour photoperiod at 25–27 °C. The following variations were observed: (1) multiple synangia with or without combinations of vegetative lobing and (or) shallow dichotomies; (2) a prolonged axis growth up to 2 Cm; (3) combinations between these extremes. Vascularization of the aberrant fertile appendages showed either exaggerations of the normal condition, or multiple dichotomies. Marked asymmetrical branching and growth of the upper shoot system accompanied the development of the aberrant fertile appendages. Normal fertile appendages often followed groups of aberrant fertile appendages on the same stem axis.A temporary readjustment in the developmental pattern of the shoot systems is discussed. Variations in the timing of phases of this developmental pattern could explain the variations in aberrancies. Temporary ontogenetic reversal of an evolutionary trend is deemed possible if the evolution of Psilotum favored a proliferation of genes, rather than an increase in the number of gene controlled processes. A reconstruction of a possible ancestral fertile shoot is presented, and a Protopteridium ancestry is suggested for Psilotum.