Flora of North America species comparison

Eleocharis acicularis is abundant and ecologically important throughout much of its range. It occurs in a wide variety of habitats, including acid waters. I have not seen voucher specimens for reports from Alabama and Florida. I have not seen vouchers for the reported chromosome numbers of 2n = 30–38 or 50–58.

Eleocharis acicularis often forms large rooted mats or floating masses, which when submerged, are often non-flowering. Submerged, usually nonflowering plants are abundant throughout much of the range of the species (H. K. Svenson 1929; P. E. Rothrock and R. H. Wagner 1975). They have been called E. acicularis forma fluitans (Doellinger) Svenson; E. acicularis forma inundata Svenson; E. acicularis forma longicaulis (Desmazières) Hegi; E. acicularis forma submersa (Nilsson) Norman; and E. acicularis var. submersa (Nilsson) Svenson. The culms of the submerged plants are terete, smooth, soft to flaccid, translucent, and the partitions of the air cavities within are clearly visible. Submerged plants may closely resemble aquatic forms of some other species, especially Eleocharis parvula, E. robbinsii, and Schoenoplectus subterminalis (Torrey) Sojak (N. C. Fassett 1957; E. G. Voss 1967, 1972–1996, vol. 3).

Although E. acicularis is very variable, recognition of varieties is premature pending a worldwide taxonomic revision of subg. Scirpidium. Much of the variation is apparently due to phenotypic plasticity in response to environmental factors, especially water depth (P. E. Rothrock and R. H. Wagner 1975). The named varieties intergrade extensively, and achenes, which are important in defining the varieties, are often absent. H. K. Svenson (1929) recognized four varieties and two forms for North America, but later (1957) did not recognize infraspecific taxa.

Most plants from the Arctic to cool-temperate North America, including higher elevations in the Southwest, are very similar to plants from northern Europe and presumably belong to typical E. acicularis, which was described from Europe. Those plants have culms cylindric or three- to four-angled; leaf sheaths mostly obscure and closely sheathing; and achenes about two times longer than wide.

Eleocharis acicularis var. gracilescens Svenson, type from St. Louis, Missouri, applies to plants with flowering culms unusually long (to 60 cm), smooth or sometimes with five to twelve fine ridges, their bases not cormlike; spikelets often linear-lanceoloid, often unusually long (to 8 mm); floral scales often unusually long (to 3.5 mm), often stramineous; and achenes much less than two times longer than wide. Plants of E. acicularis var. gracilescens are probably usually emergent in shallow water; they are known from scattered collections from Alabama, Florida, Kansas, Missouri, North Dakota, Oklahoma, South Carolina, Tennessee, and Texas. Robust plants of E. acicularis var. gracilescens are sometimes misidentified as E. wolfii. H. K. Svenson (1957) stated that var. gracilescens may be distinct from E. acicularis. Some specimens from California and Oregon that resemble E. acicularis var. gracilescens may be ecologic forms of E. acicularis var. occidentalis.

Eleocharis acicularis var. occidentalis Svenson, type from Santa Barbara, California (illustrated in H. L. Mason 1957; H. K. Svenson 1939, plate 539), applies to plants with culms only 2–6 cm, smooth or usually some prominently four-angled or finely to ca. eight-ridged, their bases often persistent, swollen, and cormlike (illustrated herein); distal leaf sheath summits mostly evident, often markedly inflated; achenes much less than two times longer than wide. This variety often grows in vernal pools. As here defined, it is known only from Arizona, California, and Nevada. Most of the specimens H. K. Svenson (1929) cited in the protologue and identified on herbarium specimens as E. acicularis var. occidentalis are probably referable to typical E. acicularis.

Eleocharis acicularis var. porcata S. G. Smith (S. G. Smith 2001), type from Texas, applies to plants that are similar to E. acicularis var. occidentalis except that their culms (cross section illustrated herein) reach 22 cm × 0.5 mm, some or all are prominently six- to twelve-ridged and mostly distinctly compressed, and their bases are not cormlike. This variety grows in mud and shallow water of marshes, ponds, and stream margins; it is known from Alberta, Arizona, Colorado, Illinois, Kansas, Louisiana, Nebraska, New Mexico, North Dakota, and Texas. Because of their compressed, prominently-ridged culms, robust plants of E. acicularis var. porcata, including a paratype of E. wolfii, are sometimes misidentified as E. wolfii (S. G. Smith 2001).

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

The flowers are mostly cleistogamous, with stigmas and stamens remaining enclosed in the floral scales except sometimes on the shortest culms. We have not seen specimens to confirm literature reports of E. intermedia from Iowa.

Eleocharis intermedia perhaps should be placed in sect. Intermediae (Svenson) Zinserling following I. Kukkonen (1990). H. K. Svenson (1929) placed it in his E. ser. Intermediae (name invalid) but later (1937, 1957) placed it in his E. subser. Truncatae (name invalid), and M. S. González-E. and P. M. Peterson (1997) placed it in E. subser. Truncatae. However, E. intermedia differs from all other members of ser. Eleocharis in its annual, tufted habit without evident, creeping rhizomes, culms varying in one tuft from long to very short, often with some spikelets subsessile, all spikelets of a tuft fruiting simultaneously, and the proximal scale often subtending a flower. It was placed in ser. Tenuissimae because of a resemblance to some other species of that series; closer observation reveals that it differs substantially in its achenes or habit from other members of ser. Tenuissimae. It rather closely resembles E. flavescens var. olivacea and E. geniculata in 8a2a. ser. Maculosae as well as small plants of species in 8a2b. ser. Ovatae.

Eleocharis macounii Fernald had been treated as a distinct species, or included within E. intermedia, E. obtusa, or E. ovata in the broad sense (including E. obtusa). P. M. Catling and S. G. Hay (1993) presented morphologic evidence suggesting that E. macounii Fernald is a hybrid between E. intermedia and E. obtusa, and named it E. ×macounii (Fernald) P. M. Catling & S. G. Hay. It is known only from the type collection from 1893 and a second collection from 1987, both from along the Gatineau River north of Ottawa in southwestern Quebec. Its habit is very similar to that of E. intermedia, from which it differs mainly in its achenes lenticular to trigonous in the same plant, the two or three angles prominent, keeled, costate, the surface smooth or very finely reticulate or pitted at 30X, and tubercles 0.2–0.5 mm wide. It produces few achenes, whereas E. intermedia and E. obtusa usually produce abundant achenes. In the protologue M. L. Fernald (1899) compared E. macounii with the Eurasian E. carniolica W. D. J. Koch. H. K. Svenson (1929, 1937) compared it with both E. intermedia and the Eurasian E. multicaulis (Smith) Smith and later (1957) placed it in synonymy under E. obtusa. Pending further research, it seems best to treat the two known collections of E. macounii as probable hybrids between E. intermedia and either E. obtusa or E. ovata; all three species are sympatric in the region where the collections of E. macounii were made (P. M. Catling and S. G. Hay 1993; B. M. H. Larson and P. M. Catling 1996).

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