Small, grass-like tufted herbs, annual or occasionally perennial; roots fibrous; stem solid. Leaves basal, alternate; lamina linear with an open basal sheath. Inflorescence cymose (see notes), appears spike-like or when further reduced appears head-like (Centrolepis), subtended by 1–many primary bracts. Flowers unisexual, often grouped together into bisexual or female units with 1 or 2 stamens and/or 1–several carpels variously united, often subtended by 2–many secondary bracts; perianth absent. Male flowers of 1 stamen; anther with 1 (or 2) locules. Female flowers of 1 carpel; style 1; stigma elongate; ovule 1. Fruit small, dry and usually opening by slit to release the single seed; seed with endosperm.
About 36 species in 4 genera, occurring from South-east Asia and Australia, through the Pacific to southern South America; 2 genera with 8 species in Victoria. Trithuria, which is often included in this family, is here included in the Hydatellaceae.
The inflorescence of the Centrolepidaceae is made up of greatly condensed and reduced cymose units (Cooke 1980a). These units are here referred to as 'inflorescence units'. In Aphelia, each cymose unit is subtended by a primary bract (except in A. gracilis; refer below) and the total inflorescence is arranged in a spike-like cluster which is superficially similiar to the 'spikes' of some grasses. In A. gracilis the basal primary bract usually has two male flowers (each surrounded by a hyaline secondary bract) and one bisexual unit composed of one male flower and one female flower (both are collectively surrounded by a hyaline secondary bract). In Centrolepis, the cymose units are not subtended by a primary bract, except for the two basal units. Therefore, the unisexual flowers appear to be arranged in a head-like cluster. This cluster of unisexual flowers is often incorrectly interpreted as a cluster of 1–many bisexual flowers.