Reproductive morphology of Nuphar (Nymphaeaceae), a member of basal angiosperms

Recent phylogenetic analyses position Nuphar as the basalmost genus in the Nymphaeaceae, a member of the ANITA grade that is constituted of the three most basal angiosperm lineages [Amborella, Nymphaeales, and Austrobaileyales (Austrobaileyaceae, Illiciaceae, Schisandraceae, and Trimeniaceae)]. In Nuphar the anther is tetrasporangiate. The endothecial cells elongate radially and develop fibrous thickenings. The glandular tapetum persists up to the two-celled pollen stage. Successive microsporogenesis results in the formation of tetrahedral and isobilateral microspore tetrads. Anasulcate pollen is two-celled when shed, with long stout echinae on the surface. A longitudinal ridge occurs in the center of the pollen furrow. The ovules are anatropous, bitegmic and crassinucellate. The inner integument arises as an annular ridge, whereas the outer one initiates as a semi-annular ridge interrupted on the concave side of the funiculus. Later the outer integument becomes a hood-shaped envelope. The micropyle structure is variable in various stages of ovule development. At anthesis the micropyle is formed by both integuments, while before and after anthesis it is formed by the inner integument. This contrasts with all previous reports, which state that the micropyle of Nuphar is formed by the inner integument alone throughout ovule development. Megasporogenesis forms a linear triad of megaspores. At the binucleate stage of female gametophyte ontogeny, the two nuclei are not separated by a large vacuole from the chalazal and micropylar ends. The wide distal region of stigma is covered with high density in unicellullar papillae whereas the narrow adaxial region has sparse unicellular papillae. The present study compares the reproductive morphological characters of Nuphar with those of other nymphaeaceous genera.