Flora of North America species comparison

Eleocharis palustris is the most widespread and common species of the extremely difficult circumboreal “E. palustris complex,” which in North America comprises E. palustris, E. mamillata, E. macrostachya, E. erythropoda, E. uniglumis, E. kamtschatica, and E. ambigens. Two or more of these species have been combined by recent authors. The complex has been studied extensively only in northern Europe (S.-O. Strandhede 1965, 1966), where E. palustris, E. mamillata, and E. uniglumis are recognized (S.-O. Strandhede 1966). European studies and preliminary studies in North America by S.-O. Strandhede (1967) and L. J. Harms (1968) indicate that unstable chromosome structure and number as well as interspecific hybridization contribute to the taxonomic complexity of the E. palustris complex.

Eleocharis palustris is extremely variable worldwide. Recognition of infraspecific taxa outside northwestern Europe is premature. For northern Europe, S.-O. Strandhede (1966) recognized E. palustris subsp. palustris, with two varieties, for which the chromosome numbers 2n = (14–)16(–17) have been reported, and E. palustris subsp. vulgaris, without varieties, for which the chromosome numbers 2n = (33–)38–39(–40) have been reported. Eleocharis palustris subsp. vulgaris is morphologically intermediate between E. palustris and E. uniglumis and may be of hybrid origin. Its North American counterpart appears to be the polyploid populations of E. macrostachya (variants b and c, at least in part), as defined herein. For North America, S.-O. Strandhede (1967) and L. J. Harms (1968) recognized two “cytotypes” among the plants with the morphology of E. smallii, one with 2n = 16 (variant a below) and one with 2n = 36 (variant c below). Much of the variation in habit is undoubtedly because of modification of the phenotype by environmental conditions as described for Europe by S.-O. Strandhede (1966). The more robust plants are often emergent in open water and may be called “crassa” phenotypes; the more slender plants often grow in densely vegetated marshes and meadows and may be called “meadow” or “grassland” phenotypes. Intermediates between E. palustris variant b (below) and E. erythropoda are common in zones of sympatry.

At least 4 variants are notable in North America.

Variant a (Eleocharis smallii in the strict sense) has culms mostly 1–3 mm wide; distal leaf sheaths sometimes disintegrating, often splitting adaxially, summits often with red margins, apices obtuse to broadly acute; floral scales 3–4 mm; achenes to 1.5(–1.6) mm; culm stomates 39–48 µm (based on very few measurements). Reported chromosome numbers for which I have seen vouchers, all from Kansas, are 2n = 16, 17 (L. J. Harms 1968). The range of variant a is mostly northeastern, where it is known from elevations to 1700 m in Newfoundland, west to Manitoba and south to North Carolina, Kentucky, Missouri, and Kansas, with one collection from east-central Alaska.

Variant b is similar to variant a and intergrades with it. It has culms only 0.5–1.2 mm wide; distal leaf sheaths persistent and not splitting, summits usually with markedly red margins, markedly oblique when viewed from the side, apices acute to narrowly obtuse; and spikelets with proximal scale often clasping 3/4 of the culm. At least some of these slender plants may simply be meadow or grassland forms produced by the direct effects of unfavorable enviromental factors such as competition. Variant b is mostly sympatric with variant a; it is more common in the Southeast, where it is known south to Louisiana and Arkansas. Plants from the more southern part of the range are especially striking because of their extremely oblique, brightly red-margined sheath summits and proximal floral scales usually clasping to 3/4 of the culm.

Variant c may be called Eleocharis palustris var. vigens L. H. Bailey. The lectotype is from the shores of Lake Champlain in Vermont (S. G. Smith 2001). It is similar to unusually robust forms of variant a, from which it differs in that its achenes are 1.6–2 mm, culm stomates 52–65 µm, and floral scales mostly 3.5–4.5 mm. Because of its large achenes and stomates, variant c is assumed to be tetraploid with 2n = 36 (S.-O. Strandhede 1967; L. J. Harms 1968). Variant c apparently grows mostly as an emergent in open water to about 1 m deep. Its known range is northeastern, from Newfoundland and Labrador to Manitoba, south to New York, Michigan, Wisconsin, and Nebraska.

Variant d comprises most of the plants that cannot be placed in the preceding variants. Most of these plants closely resemble most specimens that I have seen from northern Eurasia and as described for Eleocharis palustris subsp. palustris by S.-O. Strandhede (1966). Variant d has distal leaf sheaths often splitting or disintegrating, the summit margins not reddish, and apices usually broadly obtuse. In North America variant d is mostly subarctic and boreal; it is known from Newfoundland and Labrador to Alaska, south to New York, Wisconsin, Minnesota, Iowa, New Mexico, and California. Some plants of variant d that have markedly narrow tubercles mostly much (to 2 times) higher than wide and narrow achenes only 0.9–1.1 mm wide may deserve taxonomic recognition; they are known from Manitoba west to British Columbia and Alaska, south to Colorado, Utah, and California. Specimens of variant d from scattered western localities from Alaska and Yukon south to California have floral scales 4–5 mm and achenes 1.6–1.9 mm and are very similar to variant c.

(Discussion copyrighted by Flora of North America; reprinted with permission.)

Species ca. 200 (67 in the flora).

The name of the genus has sometimes been given as Heleocharis Lestibudois; this is now regarded as an orthographic variant of Eleocharis.

Eleocharis dulcis (Burman f.) Trinius ex Henschel is sometimes cultivated for its edible tubers. Some species are weeds in rice fields, mostly extraterritorially. Almost all species are restricted to wetlands, often emergent, and sometimes submerged aquatic.

No recent comprehensive worldwide taxonomic treatment of Eleocharis is available. The treatment herein is based mostly on the extensive studies by H. K. Svenson (1929, 1932, 1934, 1937, 1939, 1947, 1957), which were mostly restricted to species of North America. Classification of Eleocharis is unusually difficult because relatively few macroscopic characters are provided by the simple structure characteristic of the genus (only two leaves, basal, without blades or with only rudimentary blades, and unbranched aerial stems, each with a single terminal spikelet and without an involucral bract). Undoubtedly much evolutionary convergence has occurred in most vegetative and reproductive structures (M. S. González-E. and J. A. Tena-F. 2000; E. H. Roalson and E. A. Friar 2000).

North American Eleocharis includes some extremely difficult species complexes that need taxonomic revision: (1) The E. palustris complex (species 1–7) is discussed under 1. E. palustris. (2) The E. tenuis complex (species 16–21) is discussed under 21. E. tenuis. (3) The four species (species 57–60) of 8c. Eleocharis subg. Zinserlingia that occur in North America belong to the E. quinqueflora (= E. pauciflora) complex, discussed under subg. Zinserlingia. (4) All six species of 8a2b. E. ser. Ovatae (species 42–47) constitute a complex discussed under ser. Ovatae.

The supraspecific classification of Eleocharis used here is that of M. S. González-E. and P. M. Peterson (1997), which was based on a study of most species worldwide. Other recent classifications are based on more or less regional studies (H. K. Svenson 1957; T. V. Egorova 1981; I. Kukkonen 1990). A study using limited DNA data from 30 species (E. H. Roalson and E. A. Friar 2000), mostly from North America, indicates that the following supraspecific taxa are probably monophyletic: 8a2a. ser. Maculosae, 8a2b. ser. Ovatae, and 8d. subg. Limnochloa, whereas the following taxa are probably para- or polyphyletic: 8a1. sect. Eleocharis, 8a1a. ser. Eleocharis, 8a1d. ser. Tenuissimae, and 8a2. sect. Eleogenus.

Users of this treatment should be aware of the following: culms that are smooth when fresh are often ridged when dry; culms of pressed specimens are often flattened and must be carefully rehydrated and sectioned to determine the original cross-section shape; culm widths given here are usually for culms pressed flat; floral scale widths are measured on flattened scales or by doubling the width measured on scales folded along the midrib; and achene length does not include the tubercle, which is often included in descriptions published elsewhere. In this treatment, I describe the tubercle (style base) after the achene in consecutive sentences to stress the separate nature of the two structures.

Some species, mostly of 8a1d. Eleocharis ser. Tenuissimae, often proliferate from spikelets, often on arching or horizontal culms, especially when growing as submerged or floating aquatics. Because many such plants reproduce entirely asexually and have no normal spikelets or achenes, it is often impossible to identify them to species. The invalid name E. prolifera Torrey has sometimes been used for these plants. Species of ser. Tenuissimae in which the spikelets may be proliferous and which are easily confused with each other are E. baldwinii, E. brittonii, E. microcarpa, E. nana, E. retroflexa, and E. vivipara. Aquatic forms of at least some of those species are very hard to distinguish from Websteria confervoides (Poiret) S. S. Hooper. Other species in which the spikelets often poliferate or the culm tips root are E. pachycarpa of ser. Tenuissimae, E. melanocarpa of 8a1b. ser. Melanocarpae, and E. rostellata of 8a1c. ser. Rostellatae. When submersed, plants of E. acicularis of 8b. subg. Scirpidium and E. elongata and E. robbinsii of 8d. subg. Limnochloa may be entirely vegetative, the latter two species sometimes forming whorls of flaccid stems without spikelets.

(Discussion copyrighted by Flora of North America; reprinted with permission.)