8. Eleocharis palustris L.

Pl. 76 a–d; Map 281

E. macrostachya Britton

E. smallii Britton

Plants perennial, with relatively stout rhizomes, forming loose colonies. Aerial stems 10–60(–100) cm long, 0.5–5.0 mm wide, firm, wiry, and more or less circular in cross-section to soft or flattened, finely ridged, sometimes irregularly septate with unequally spaced cross-lines. Basal sheaths green or tinged reddish purple or brown, less commonly straw-colored, sometimes darkened along the margin, the tip firm, truncate to oblique, sometimes tapered to a point on 1 side opposite a broadly V-shaped notch. Spikelets 5–40 mm long, lanceolate to narrowly ovate in outline, the tips mostly tapered and sharply pointed, with 2 or 3 sterile, basal scales. Scales 2.0–4.5 mm long, narrowly ovate to lanceolate in outline, the tips pointed, sometimes tapered, loosely ascending to appressed, light brown to dark brown, less commonly tinged reddish purple or with a green midrib, the margins sometimes darkened. Perianth bristles 4–6(–8), less commonly none, mostly about as long as to slightly longer than the fruit, retrorsely barbed. Stigmas 2. Fruits with the main body 1.4–2.0 mm long, broadly obovate in outline, biconvex to nearly circular in cross-section, the surface finely roughened, dark yellow to brown at maturity, shiny. Tubercles 0.4–0.7 mm long, narrowly conical to somewhat flattened. 2n=16, 18, 32, 36, 38. April–September.

Scattered throughout the state (U.S. and Canada north to Alaska, south discontinuously through Mexico and South America; Europe, Asia, Africa). Margins of ponds, lakes, sloughs, swampy forests, streams, and ditches, sometimes in shallow water (rarely in deeper water); also in marshes, bottomland prairies, and moist depressions of upland prairies.

The E. palustris complex (Eleocharis subseries Palustres (C.B. Clarke) Svenson) is one of the most taxonomically challenging in the genus. A sampling of treatments of this group serves to show the great variety in numbers of species accepted from the eastern and central United States: one (Gleason and Cronquist, 1991), two (Mohlenbrock, 1976; Mohlenbrock and Drapalik, 1960, 1962), three (Mohlenbrock, 1986; Steyermark, 1963); four (Harms, 1968), six (Svenson, 1957), or seven (Strandhede, 1967). Most authors have considered E. erythropoda to represent a distinct species, based on spikelet scale characters (summarized in the key above) that appear to be relatively constant, and it is treated separately above. The remaining Missouri plants often have been segregated as two species, E. macrostachya and E. smallii. Recent authors have emphasized different characters to distinguish these two taxa, as summarized in the following key:

1. Stems usually soft or flattened; spikelets usually long-tapering to narrowly pointed at the tip, 10–40 mm long; spikelet scales rather closely appressed; basal leaf sheaths truncate or only slightly oblique at the tip...E. macrostachya

1. Stems usually firm, wiry, and circular in cross-section; spikelets either pointed or blunt at the tip, 5–20 mm long; spikelet scales more loosely ascending; basal leaf sheaths noticeably oblique at the tip, with a V-shaped notch on 1 side...E. smallii

Unlike the E. ovata group, in addition to the complex morphological variation among taxa, there is also considerable cytological complexity present (Harms, 1968; Strandhede, 1967). Plants that are morphologically closest to typical E. smallii are mostly diploid (2n=16), with a few tetraploids (2n=32) recorded (Harms, 1968; Strandhede, 1967). In contrast, chromosome division in E. macrostachya is irregular, and the taxon has been described as a cytologically unstable polyploid, potentially of hybrid origin, with a suggested somatic number of 2n=38 (Harms, 1968). Thus, the cytological data support the recognition of more than a single species.

In their morphological extremes, plants corresponding to the two taxa appear strikingly distinct, especially when seen growing together. However, the majority of Missouri collections cannot conveniently be associated with either of the two taxa. There is continuous variation for all of the characters said to separate the two and no correlation between the character states listed above. In fact, American plants, particularly those ascribed to E. smallii, cannot be separated morphologically from the mostly European E. palustris with any confidence, and that name is assigned to the entire complex in the present treatment. Eleocharis palustris, as treated in this fashion, is uncomfortably variable morphologically and cytologically, and it will almost certainly be split (although perhaps along different lines) following much-needed future taxonomic research into the group.