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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. Key words: Cyperaceae; embryology; Eriocaulaceae; Flagellariaceae; Juncaceae; Prionium serratum; Typhaceae.

Bioassays for allelochemical toxicity of aboveground Juncus effusus tissues were conducted with seeds and seedlings of Eleocharis obtusa and Scirpus cyperinus, two emergent sedge species (Cyperaceae) found sympatric with J. effusus, and with seeds and seedlings of J. effusus itself to evaluate potential autotoxicity. Bioassays were performed under controlled, axenic conditions with aqueous shoot extract treatments simulating in situ dissolved organic carbon concentrations. With respect to the two sedge species, neither shoot development nor seedling biomass accrual was significantly suppressed by lyophilized whole extracts from J. effusus. Although the extracts induced no significant reduction in growth of E. obtusa or S. cyperinus, biomass-specific chlorophyll a concentration was significantly reduced in E. obtusa seedlings. In contrast, seedlings of J. effusus exhibited significant reductions of biomass and chlorophyll a concentrations, and seedling shoot development was retarded in response to leachate exposure. Results of the present study suggest that J. effusus seedlings possess autotoxic sensitivity to extracts of dead, aboveground tissues of adult plants. Key words: allelochemical interactions; allelopathy; autotoxicity; Cyperaceae; Juncaceae; Juncus effusus.