Ultrastructural studies of in situ devonian spores: Protobarinophyton pennsylvanicum Brauer

Abstract

This investigation examines sporoderm ultrastructure in Protobarinophyton pennsylvanicum Brauer (Barinophytaceae), a Devonian vascular plant that produced both large and small spores in the same sporangium. The sporoderm of the small spores is approximately 1.0 μm thick, with irregular gaps in the wall near the outer surface. In the large spores, the sporoderm is approximately 6.5 μm thick and is divided into an unornamented surface layer and a broad, alveolate inner layer. In view of the significant and consistent structural differences in the walls of the spores, it is suggested that the small spores represent microspores and not immature or aborted large spores. The general pattern of sporoderm organization in the two spore types is comparable to that reported previously for Barinophyton citrulliforme, and, more importantly, it is similar to that found in microspores and megaspores of extant heterosporous plants. This similarity suggests that Protobarinophyton pennsylvanicum, like modern heterosporous plants, produced spores that formed endosporic sexually differentiated gametophytes and provides a foundation for suggesting a more precise, biological definition of the term “heterospory” in fossil plants. In this context, sex determination in this unusual Devonian vascular plant is discussed, and it is suggested that the sex of the spores is determined by nutritional factors.