The embryology and systematic relationships of Prionium serratum (Juncaceae: Juncales).

Although Prionium is included in Juncaceae, rbcL sequence data indicate that Juncaceae is paraphyletic, with most genera closer to Cyperaceae than to Prionium. Cyperaceae and Juncales have embryological synapomorphies: thus embryology is used to test the monophyly of Juncaceae. The embryology of Prionium is described and its systematic position discussed. Material was prepared using standard methods of paraffin embedding. Additional embryological data were extracted from the literature. The anther in Prionium is tetrasporangiate, and the wall has an epidermis, an endothecium and middle layer, and an irregularly bilayered, glandular secretory tapetum. Microsporogenesis is probably simultaneous; pollen is ulcerate with a granular exine, in tetrahedral and cross tetrads, and trinucleate at release. The trilocular ovary contains many crassinucellate ovules probably having a Polygonum-type embryo sac. Endosperm is helobial and the embryo is of the Onagrad type, Juncus variation. The seed is testal-tegmic and germination is epigeal. The embryology of Prionium is most like that of Juncaceae, which shares several synapomorphies with Cyperaceae. Some of the characters in Cyperaceae may be interpreted as specialized forms of those found in Juncaceae. Embryology supports the monophyly of Cyperaceae and Juncales, but not Juncaceae; thus the position of Prionium remains unresolved.