Circaeasteraceae Kuntze ex Hutch.
Habit and leaf form. Small, glabrous herbs. Plants of very peculiar vegetative form; neotenic (the cotyledons persistent, the leaves rosulate-clustered at the tip of the elongated hypocotyl). Annual. Leaves more or less opposite; simple. Lamina entire; obovate (cuneate-spathulate); dichotomously veined; without cross-venules. Leaves exstipulate. Lamina margins apically spinulose dentate.
Leaf anatomy. Stomata present; anomocytic.
Stem anatomy. Nodes unilacunar. Xylem with vessels. Vessel end-walls simple.
Reproductive type, pollination. Fertile flowers hermaphrodite. Plants hermaphrodite. Floral nectaries absent.
Inflorescence, floral, fruit and seed morphology. Flowers aggregated in ‘inflorescences’; in fascicles; regular. Free hypanthium absent.
Perianth sepaline; 2(–3); 1 whorled. Calyx 2(–3) (the third member, when present, seemingly representing a reduced stamen); polysepalous; persistent; valvate.
Androecium (1–)2. Androecial members free of the perianth; free of one another. Androecium exclusively of fertile stamens (usually), or including staminodes. Staminodes when present, 1; in the same series as the fertile stamens; sepaloid. Stamens (1–)2; reduced in number relative to the adjacent perianth, or isomerous with the perianth; alternisepalous; erect in bud. Anthers basifixed; non-versatile; dehiscing via longitudinal slits; introrse; bisporangiate. Anther epidermis persistent. Microsporogenesis simultaneous. Anther wall initially with one middle layer; of the ‘dicot’ type. Tapetum glandular. Pollen grains aperturate; 3 aperturate; colpate; 2-celled.
Gynoecium (1–)3 carpelled. Carpels reduced in number relative to the perianth to isomerous with the perianth. The pistil when monomerous, 1 celled. Gynoecium monomerous to apocarpous; of one carpel to eu-apocarpous; superior. Carpel non-stylate (with an oblique, sessile stigma); 1 ovuled. Placentation apical to marginal (subapical). Ovules pendulous; orthotropous; unitegmic; tenuinucellate. Embryo-sac development Polygonum-type. Polar nuclei fusing prior to fertilization. Antipodal cells formed; 2, or 3; not proliferating. Synergids pear-shaped. Endosperm formation cellular. Embryogeny chenopodiad.
Fruit non-fleshy; not an aggregate (when monomeric), or an aggregate. The fruiting carpel indehiscent; an achene (covered with fine, uncinate setae). Seeds copiously endospermic. Embryo well differentiated. Cotyledons 2 (persistent). Embryo straight.
Physiology, biochemistry. Not cyanogenic.
Geography, cytology. Holarctic. Temperate. Northwest Himalayas to Northwestern China. 2n = 30.
Taxonomy. Subclass Dicotyledonae; dubiously Crassinucelli (despite outstanding features of Tenuinucelli, including corolla absent, stamens free of perianth, initially one middle layer in anther wall, ovules unitegmic and tenuinucellate). Dahlgren’s Superorder Ranunculiflorae; Ranunculales. Cronquist’s Subclass Magnoliidae; Ranunculales. APG 3 core angiosperms; peripheral eudicot; Superorder Ranunculanae; Order Ranunculales.
Species 1. Genera 1; only genus, Circaeaster.