The genus Platanusrepresents an ancient, basal eudicot lineage. This taxon therefore holds a key position for understanding reproductive character diversification during the origin of the largest clade of angiosperms. We have undertaken an embryological study of Platanus racemosa in order to provide modern data on defining angiosperm reproductive characters for this important group. We report here on the development of endosperm. Development begins with numerous rounds of synchronous free-nuclear division. Rudimentary phragmoplasts are sometimes observed during these divisions, but cell walls are not formed. The nuclei are arranged in a single layer around the large vacuole except for in the chalazal region where several nuclei aggregate in a common mass of cytoplasm. The chalazal aggregation of nuclei and cytoplasm becomes cellularized first. The coenocytic portion then begins centripetal cellularization in conjunction with mitotic divisions and the formation of phragmoplasts. After compartmentalization of the coenocyte, further cell divisions occur to produce smaller cells in the mature endosperm that become filled with oil. The embryo eventually consumes most of the endosperm and grows to occupy its space in the mature seed. Our findings differ significantly from previously published reports for Platanus, in which endpsperm development was described as ab initio cellular in its development. In comparison with other descriptions of free-nuclear endosperm, Platanusis more similar to another basal eudicot than to most cereals, although it exhibits unique features. Platanusendosperm exhibits a combination of primitive and derived characters that is consistent with its position as a basal member of the eudicot clade and therefore potentially transitional between the more primitive magnoliid lineages and higher eudicots.

Key words: character evolution, embryology, endosperm, eudicot, lower hamamelid, Platanaceae