The ultrastructure and reproductive significance of Monoletes (Pteridospermales) pollen

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Abstract
The Carboniferous pteridosperm pollen type Monoletes is described from reproductive organs of differing preservational mode and stratigraphie occurrence. The sporoderm is constructed of two zones: an inner thin lamellated nexine and a thicker alveolate sexine. The ultrastructure of the sporoderm in the area of the proximal suture suggests that germination may have taken place from that region. Distal surface grooves are a consistent feature of the grains examined although grain ontogeny, preservation, and tapetal products may mask their presence. Features of sporoderm infrastructure suggest that Monoletes pollen grains may represent a type transitional between wind and insect pollination.