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Published online by Cambridge University Press: 05 December 2011
Developmental ultrastructural studies have led to major advances in our understanding of key questions ranging from the causal basis for the alternation of generations and the role of the cytoskeleton in cellshaping processes to the phylogeny of archegoniate plants. Oogenesis is characterised by profound nuclearcytoplasmic interactions accompanied by striking changes in the egg mitochondria and plastids. At fertilization, egg penetration is a physical process and the plastids from the spermatozoids are excluded from the egg. Considerable dissimilarity between the shapes of the biflagellate spermatozoids of Lycopodium and Selaginella underlines their ancient separation. Equally striking differences in spermatozoid architecture argue against any direct phyletic link between heterosporous and homosporous ferns. Taxonomic variations between the spermatozoids of homosporous ferns suggests blepharoplast morphology to be a potentially rich source of new systematic data. Whilst there is general agreement that the multilayered structure is a cytoskeletal alignment system, a proposed shape-generating system situated near the nuclear envelope, which provides the force necessary for spermatozoid morphogensis, has not yet been identified. New fixation procedures have revealed hitherto overlooked filamentous elements associated with the nucleus. Whereas tests for actin were negative, immunoblotting suggests that these contain the intermediate filament protein antigens.