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.)

Varieties 3 (3 in the flora).

The name Eleocharis capitata (Linneaus) R. Brown was long misapplied to E. tenuis (H. K. Svenson 1939). Although the extremes of the three varieties are very different, intermediates are fairly common. Some plants are intermediate with E. elliptica.

Eleocharis tenuis belongs to the E. tenuis complex, which comprises species 16–21 and is restricted to North America, where it is widely distributed except for the Southeast and Southwest. Eleocharis occulta, E. bifida, and E. nitida are very distinct, have little variation, and have relatively restricted ranges; E. tenuis, E. elliptica, and E. compressa are difficult to separate, are highly variable, and have relatively large ranges (see also discussions under E. compressa and E. elliptica). Eleocharis occulta and E. bifida are evidently closely related to E. compressa; they are known only from unglaciated areas south of the limits of Pleistocene glaciation, while E. compressa has a broad range in glaciated regions. The cytotaxonomic study by L. J. Harms (1972) of E. tenuis, E. elliptica, and E. compressa included artificial interspecific and infraspecific hybrids as discussed under those species.

Eleocharis tenuis var. pseudoptera might also be treated as E. elliptica var. pseudoptera following L. J. Harms (1972) because it is intermediate between E. tenuis var. tenuis and E. elliptica var. elliptica in most characters except for the 4-angled, usually deeply sulcate culms. It appears to intergrade with E. elliptica. It is placed in E. tenuis because many plants, including an isotype, are more like E. tenuis than E. elliptica; because most plants key more easily to E. tenuis; and to continue the use of the traditional name. The achenes of the holotype (from Lancaster county, Pennsylvania) and some other specimens closely resemble those of E. elliptica but are not obviously persistent after the scales fall. The usually diagnostic tooth on the distal leaf sheath in E. tenuis var. pseudoptera is also characteristic of E. elliptica var. elliptica; but in E. tenuis var. pseudoptera it is more often present and usually longer and stouter than in E. elliptica. The culms of the holotype of E. tenuis var. pseudoptera and many other specimens are 4- to 5-angled, and in some specimens are very irregularly to 6-angled and often rigid and compressed. L. J. Harms (1972) transferred E. tenuis var. pseudoptera to E. elliptica because he counted the same 2n = 38 chromosome number in both E. elliptica and E. tenuis var. pseudoptera and produced artificial E. elliptica × E. tenuis var. pseudoptera hybrids in which meiotic pairing and pollen stainability were very high. A. E. Schuyler (1977) reported 2n = 39 for E. tenuis var. pseudoptera as well as 2n = 34 and 68 for a putative E. tenuis var. tenuis × E. tenuis var. pseudoptera hybrid.

Putative hybrids between Eleocharis compressa and E. erythropoda of the E. palustris complex in Ontario have been reported (P. M. Catling and S. G. Hay 1993), and I have observed putative E. compressa × E. erythropoda and E. elliptica × E. erythropoda hybrids in the field in southeastern Wisconsin. It seems possible that introgression from E. erythropoda is responsible for some of the variation of both E. compressa and E. elliptica, especially the frequent presence of some 2-fid styles and lenticular achenes in both species and some (rarely all) entire floral scales in E. compressa (S. G. Smith 2001).

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