An otherwise normal plant ofPsilotum nudumconsistently produced aberrant fertile appendages over a 5-year period when the plant was grown under prolonged periods of cool-white fluorescent lighting of approximately 200ndash;400ensp;ft-c (0.625ndash;1.2ensp;MW/cm2), 16-hour photoperiod at 25ndash;27 deg;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 2ensp;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 ofPsilotumfavored 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 aProtopteridiumancestry is suggested forPsilotum.
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