Embryology of Ceratopteris richardii (Pteridaceae, tribe Ceratopterideae), with emphasis on placental development.

This comprehensive study of early embryology in Ceratopteris richardii combines light microscopy with the first ultrastructural evaluation of any pteridophyte embryo. Emphasis is placed on ontogeny of the foot and placental transfer cells. The embryology of C. richardii shares many similarities with that of other polypodiacious ferns while exhibiting distinctive division patterns. Formative embryonic stages have been reconstructed into three-dimensional models for ease of interpretation. The zygote divides perpendicular to the gametophyte plane and anterioposterior axis. This division establishes a prone embryological habit that maximizes rapid independent establishment of a leaf-root axis in a cordate gametophyte. After the formation of a globular eight-celled stage, initials of the first leaf, and root and shoot apical meristems are defined early by discrete formative divisions. Concomitantly, the foot expands and differentiates to transport nutrients from the gametophyte for the developing embryonic organs. Transfer cell wall ingrowth deposition begins in the gametophyte placental cells before the adjacent sporophyte cells just after the eight-celled stage. These observations provide an anatomical framework for future comparative developmental genetic studies of embryogenesis in free-sporing plants.