Flora of New Zealand Series

Small, tufted, grass-like annual, or moss-like perennial herbs. Stems slender, densely packed in perennial spp. Leaves linear or setaceous, all basal, or cauline and imbricate; sheaths open, distinct, membranous. Flowers minute, unisexual, without perianth, in terminal complex "spike-like" inflorescences with 2 or more distichous glume-like bracts. Each bract usually enclosing 1-several pseudanthia; each pseudanthium consisting of 1, 2 or 0 male flowers and 1-many superposed or collateral, sessile, often ± connate female flowers; pseudanthia frequently with 1-3 hyaline bracteoles. Male flower a single stamen, filament thread-like, anther 1-celled, versatile, opening lengthwise by a slit. Female flower a single unilocular ovary; style 1; ovule 1. Fruit with membranous pericarp, dehiscing longitudinally. Four genera with 30-35 spp., of Malesia, S.E. Asia, Australia, N.Z. and 1 sp. in S. America.

Two genera, Hydatella (including N.Z. H. inconspicua) and Trithuria, which were formerly included in the Centrolepidaceae were segregated into a new family Hydatellaceae by Hamann (N.Z. J. Bot. 14, 1976, 193-6). The two families are distinguished morphologically; Hydatellaceae by leaves lacking sheaths, stipitate female flowers with 2 or more septate styles, and male flowers with 2-celled anthers; Centrolepidaceae by sheathing leaves, sessile 1-styled female flowers and male flowers with 1-celled anther. The two families differ in pollen morphology, embryology and seed anatomy.