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Key to Eleocharis, Key D: spikerushes with achenes bearing several distinct longitudinal ribs with very narrow horizontal cells between the ribs (subgenus Scirpidium)

Cyperaceae

Eleocharis

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1 Culms compressed, rectangular or C-shaped in cross-section, to 1.2 (-1.5) mm wide; some or all of culm edges with acute or serrulate ridges
1 Culms not compressed, without sharp ridges; instead terete or obscurely angled (or only slightly compressed and not bearing sharpened ridges); culms < 0.5 mm wide.
  2 Achenes < 0.6 mm long, much less than 2× longer than wide; [Texas].
    3 Distal leaf sheaths translucent, acute apically; scales orange-brown to stramineous, spreading, 10-30 per spike; floral scales spreading; plants annual, densely cespitose; [s. TX and Tamaulipas, possibly extinct]
    3 Distal leaf sheaths red proximally, blunt apically; scales bright red-brown, 5-15 per spike; plants perennial, with evident or obscured rhizomes; [primarily TX Gulf Coastal Plain, extant]
  2 Achenes 0.7-1.1 mm long, ca. 2× (or more) longer than wide; [collectively widespread].
      4 Culms capillary, firm, to 20 cm tall, 0.2-0.4 (-0.5) mm wide, not wrinkling in drying; spikelets 3-6 mm long; achenes 0.7-1.1 mm long, 0.35-0.6 mm wide; [widespread in our region]
      4 Culms stout and spongy, to 8 cm tall, 0.6-1.0 mm wide, becoming wrinkled in drying; spikelets 2-4 mm long; achenes 0.75-0.9 mm long, 0.34-0.4 mm wide; [primarily scattered across Gulf Coastal Plain, known historically from VA Beach in 1934]