On some peculiarities of sporoderm structure in members of the Cycadales and Ginkgoales

The pollen morphology and ultrastructure of Cycas micholitzii, C. simplicipinna, Cycandra profusa, Ceratozamia mexicana , and Ginkgo biloba are studied. Pollen germination is also studied in C. mexicana and G. biloba . Although dehydrated pollen grains appear monosulcate, the study of hydrated pollen shows that the aperture occupies nearly half of the pollen surface and represents a pore rather than a sulcus. In the Ginkgoales, the inaperturate ectexine is characterized by a thick solid tectum, infratectum of columella-like elements or large granules, and distinct foot layer. On the contrary, in the Cycadales, the ectexine consists of a thin tectum, alveolar infratectum, and poorly discernable foot layer. Members of the Ginkgoales have a distinct distal aperture, which is constituted by an intine, endexine, and thin ectexine. In the modern Cycadales, an ectexine is well developed throughout the pollen perimeter; in the supposed aperture region the ectexine is not reduced in thickness, although it is characterized by a thinner tectum and thinner walls of infratectal alveoli. In Cycandra profusa , no unequivocal aperture region has been found. Thickened regions were observed in the intine of both the Cycadales and Ginkgoales.