Comparative pollen morphology and ultrastructure of modern and fossil gnetophytes

A considerable disparity of pollen characters in the modern Gnetales (including the inaperturate and monosulcate germination types, the psilate, polyplicate and spinulose sculptural types) stands in sharp contrast with their ultrastructural uniformity. In all of the ultrustructurally studied living species of Ephedra, Welwitschia and Gnetum, the infratectum is granular, mostly consisting of small granules, while the endexine is uniformly thick and lamellate. The recently discovered fossil gnetophytes are a diverse group, recognized primarily on the basis of their ovulate cupules, but heterogeneous in respect to their pollen organs and pollen morphology. The pollen grains are mostly asaccate or, rarely, bisaccate (Preflosella, Dinophyton), with small ruguloid sacci (Baisianthus) or vestigial protosaccate structures (Cryptosacciferites). The germination types are inaperturate, cryptoporate, monosulcate and trisulcate. In the majority of forms the infratectum is granular, consisting of small granules, large granules, combinations of various granules, or even columella-like elements; rarely, the infratectum is alveolar. The endexine varies from distinctly lamellate to homogeneous. Our analysis of pollen grain morphology and ultrastructure of extant gnetophytes lends support to phylogenetic relatedness of extinct forms. Palaeobotanical data show a striking parallelism of morphological variations between gnetophytes and angiosperms.