1. Elatine triandra Schkuhr

Pl. 374 a, c; Map 1639

Plants glabrous, the vegetative parts sometimes reddish-tinged. Stems 1–6 cm long, prostrate to ascending. Leaves 3–8(–10) mm long, the blades slightly succulent, linear to spatulate, oblanceolate, or obovate, long-tapered to a sessile or short-petiolate base, rounded to bluntly pointed at the tip, sometimes with a minute, apical notch, the margins entire. Stipules 0.2–0.4 mm long, narrowly lanceolate to ovate-triangular, the margins entire. Flowers 1 per leaf axil. Sepals (2)3, 0.8–1.2 mm long, ovate, uniformly green or with narrow, white margins, often somewhat pinkish- or reddish-tinged. Petals (2)3, 0.8–1.2 mm long, greenish white, often pinkish-tinged. Stamens (2)3. Fruits globose to slightly depressed-globose, with (2)3 locules. Seeds 0.3–0.7 mm long, more or less cylindrical, straight or slightly curved, the surface usually somewhat iridescent, yellowish brown to brown, with a fine but usually noticeable network of pits, these mostly 6-angled (visible only under high magnification), becoming smaller toward the ends, in about 5–7 rows around the seed and 16–25 longitudinal rows. 2n=40. June–October.

Uncommon, known only from a few widely scattered historical collections (discontinuously nearly throughout the U.S.; Canada, Europe, Asia). Submerged aquatic or terrestrial in or around ponds, sinkhole ponds, and rivers.

Plants of Elatine growing in aquatic habitats can be quite different in appearance than those growing in moist soil. Submerged plants are a darker green color and have stems with longer internodes and leaves that are longer and narrower, whereas terrestrial individuals tend to be lighter green (but frequently reddish-tinged) and have shorter internodes and shorter, broader leaves. Because of this morphological plasticity in response to habitat differences, most botanists have depended upon subtle differences in seed morphology to distinguish entities within the complex. Given that the plants apparently engage in a lot of self-pollination (Tucker, 1986), which can facilitate the fixation of morphological variations within populations, the significance of these characters in delimiting species requires further study. There is no modern monograph of Elatine, and the taxonomy of the species is poorly understood. The E. triandra complex as it exists in the eastern United States has been treated variously as a single species without infraspecific taxa (Duncan, 1964), one species consisting of two native and one introduced varieties (Fassett, 1939; Steyermark, 1963; Gleason and Cronquist, 1991), or three species (Fernald, 1941b; Tucker, 1986; Crow and Hellquist, 2000). The present treatment follows the “one species with multiple varieties” classification. In addition to the two varieties treated below, the other eastern North American variety is var. brachysperma (A. Gray) Fassett, which occurs sporadically from Ohio to Georgia and westward to the West Coast and differs in having seeds with only 9–15 (vs. 16–25) pits per longitudinal row.