4. Eleocharis equisetoides (Elliott) Torr. (horsetail spike rush)

Pl. 73 e, f; Map 277

Plants perennial, with relatively stout rhizomes connecting coarse tufts. Aerial stems 30–100 cm long, 3–5 mm wide, circular in cross-section, less commonly somewhat flattened, septate with noticeable cross-lines at regular 2–5 cm intervals. Basal sheaths loose, membranous, the lowest ones sometimes enlarged and nearly free from the stem, pale green or light brown, less commonly straw-colored, the tip oblique, tapered to a point on 1 side. Spikelets 15–40 mm long, about as wide as the aerial stems, linear in outline, the tips round to bluntly pointed, with usually 1 sterile, basal scale. Scales 4–7 mm long, papery, oblong-elliptic to oval in outline, the tips broadly rounded to bluntly pointed, the surface with fine longitudinal lines, straw-colored, often faintly purplish tinged near the lighter, irregularly scarious margins. Perianth bristles 1–4, less commonly none, slender, soft, and mostly shorter than the fruit, usually not barbed. Stigmas 2 or 3. Fruits with the main body 2.0–2.5 mm long, broadly obovate in outline, biconvex in cross-section, the surface with a fine pattern of linear, horizontal cells or nearly smooth, shiny, golden yellow to brown at maturity. Tubercles 0.7–1.2 mm long, flattened, narrowly triangular. July–October.

Known from a single, historical collection from Oregon County (eastern U.S. west to Texas). Usually a submerged aquatic in shallow to deep water; in Missouri, known only from a sinkhole pond.

A photograph from a pond in St. Charles County is purported to represent this species; however, the identity of the plants could not be verified during this study. This uncommon species may be extirpated from the state, but it should continue to be sought in a variety of aquatic habitats. It is closely related to the more common E. quadrangulata; see the treatment of that species for further discussion.