Pollen evolution in cordaites and early conifers

Changes in three pollen grain characters: number of sacci, infrastructure of sacci, and number and position of apertures, are considered against a phylogenetic background of cordaites and early conifers reconstructed on the basis of female fructifications. Bisaccate pollen grains with double germination developed most probably independently in Majonicaceae and Voltziaceae, whereas the most primitive conifers produced monosaccate prepollen with a single proximal aperture. Another lineage resulting in true pollen with a single distal aperture can be observed within cordaites. The development of saccus infrastructure among early pinopsids was directed from eusaccus to protosaccus rather than vice versa. In any case, the eusaccus of modern conifers is not homologous to that of cordaites and its origin is to be elucidated. The pollen characters suggest a very early separation of conifers from cordaites, approximately simultaneous with the division of cordaites into three families.