Department of Envirnomental, Population and Organismic Biology, Campus Box 334, Ramaley N122, University of Colorado, Boulder, CO 80309. The genus Platanus represents 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.