Herbs, submersed or emergent aquatic, glabrous. Stems erect, ascending, decumbent, or prostrate, with longitudinal air spaces, rooting at nodes. Leaves: stipules membranous; petiole present or absent; blade margins entire, with hydathodes. Inflorescences: flowers usually solitary, sometimes 2[–3] per node. Pedicels present or absent. Flowers: sepals 2–4, connate basally, equal or 1 smaller, not carinate, apex obtuse; petals (0 or) 2–4, apex obtuse; stamens [0–]1–8; pistil 2–4-carpellate, ovary 2–4-locular, apex truncate; styles 2–4; stigmas 2–4. Capsules membranous. Seeds 2–33[–44] per locule, brown to yellowish brown, straight or curved (nearly circular in E. californica), surface with hexagonal, rectangular, elliptic, or ± round pits (pits oriented with longer dimension at right angles to length of seed). x = 9.

Distribution

North America, Mexico, South America, Europe, Asia, Africa, Pacific Islands, Australia.

Discussion

Species ca. 25 (10 in the flora).

Selected References

Duncan, W. H. 1964. New Elatine populations in the southeastern United States. Rhodora 66: 47–53.

Fassett, N. C. 1939. Notes from the herbarium of the University of Wisconsin. No. 17. Elatine and other aquatics. Rhodora 41: 367–377.

Fernald, M. L. 1917. The genus Elatine in eastern North America. Rhodora 19: 10–15.

Fernald, M. L. 1941b. Elatine americana and E. triandra. Rhodora 43: 208–211.

Lower Taxa

Elatine ambigua, Elatine americana, Elatine brachysperma, Elatine californica, Elatine chilensis, Elatine heterandra, Elatine minima, Elatine ojibwayensis, Elatine rubella, Elatine triandra

Key

1	Petals 2; capsules 2-locular.	Elatine minima
1	Petals usually 3–4, sometimes 0; capsules 3–4-locular.	> 2
2	Sepals 4; petals 4; capsules 4-locular.	> 3
3	Seeds curved 90–180°; pedicels 1.5–2.5(–3.5) mm, recurved in fruit; w United States.	Elatine californica
3	Seeds straight or curved to 15º; pedicels 0.1–23 mm, erect; Quebec.	Elatine ojibwayensis
2	Sepals 2–3 (rarely 4 in E. triandra); petals 3 (sometimes 0 in E. ambigua); capsules 3-locular.	> 4
4	Pedicels recurved in fruit, 0.5–2.5 mm.	Elatine ambigua
4	Pedicels erect, 0–0.5 mm.	> 5
5	Stamens 1–6, number variable within plant.	Elatine heterandra
5	Stamens 3.	> 6
6	Seed pits ± round, (9–)14–17 per row.	Elatine brachysperma
6	Seed pits angular-hexagonal, (13–)16–35 per row.	> 7
7	Seeds ellipsoid, pit length 3–5 times width; stipule margins entire.	Elatine chilensis
7	Seeds oblong or slenderly cylindric, pit length 1–3 times width; stipule margins dentate.	> 8
8	Leaves reddish green; seed pit length 1–2 times width.	Elatine rubella
8	Leaves light green to green; seed pit length 2–3 times width.	> 9
9	Leaf blades linear, lanceolate or narrowly oblong, apices acute or obtuse.	Elatine triandra
9	Leaf blades obovate or broadly spatulate, apices rounded to shallowly emarginate.	Elatine americana