Embryology of Cardiopteris (Cardiopteridaceae, Aquifoliales), with emphasis on unusual ovule and seed development.

Cardiopteris (Cardiopteridaceae), a twining herb of two or three species distributed from Southeast Asia to Northern Australia, requires an embryological study for better understanding of its reproductive features. The present study of C. quinqueloba showed that the ovule and seed development involves a number of unusual structures, most of which are unknown elsewhere in angiosperms. The ovule pendant from the apical placenta is straight (not orthotropous), ategmic, and tenuinucellate, developing a monosporic seven-celled/eight-nucleate female gametophyte with an egg apparatus on the funicular side. Fertilization occurs by a pollen tube entering from the funicular side, resulting in a zygote on the funicular side. The endosperm is formed by the cell on the funicular side in the two endosperm cell stage. While retaining a (pro)embryo/endosperm as it is, the raphe (differentiating late in pre-fertilization stages) elongates toward the antiraphal side during post-fertilization stages, resulting in an anatropous seed. The two-cell-layered nucellar epidermis (belatedly forming by periclinal divisions), along with the raphe, envelops the embryo/endosperm entirely as the seed coat. The possibility was discussed that the arrested integument development triggers a series of the subsequent unusual structures of ovule and seed development. The fertilization mode in Cardiopteris underpins the hypothesis that the Polygonum‒type female gametophyte comprises two four-celled archegonia.