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See list of 9 species in this genus

A single example of an intergeneric hybrid with Dulichium is known from New England and described under D. arundinaceum. Reference: Kral (2002b).

  • 1a. Achenes 15–25 mm long including the 10–20 (–21) mm long tubercles; leaf blades 3–10 (–15) mm wide; style undivided or bifid only at tip
    • 2a. Stems not cespitose, produced from slender rhizomes, achene body 4–5 mm long; achene tubercle 10–15 mm long; spikes solitary or in clusters of 2–6
    • 2b. Stems cespitose, without rhizomes; achene body 5–6 mm long; achene tubercle (15–) 18–20 (–21) mm long; spikes mostly in dense clusters of 10–30
  • 1b. Achenes 1–3 mm long including the 0.1–1.7 mm long tubercle; leaf blades 0.2–3 (–5) mm wide; style deeply bifid
    • 3a. Plants annual; perianth bristles absent [Fig. 124]; inflorescences open, diffuse, cyme-like or panicle-like; spikes brown to nearly black, stalked
      • 4a. Tubercle depressed-triangular, 0.1–0.3 mm tall; achene body distinctly but irregularly transverse-rugose, with the marginal edges interrupted at the tubercle base
      • 4b. Tubercle narrow-triangular, 0.5–0.9 mm tall; achene body obscurely transverse-rugose, with the marginal edges flowing into the tubercle base
    • 3b. Plants perennial; perianth bristles present (though sometimes caducous in 
 R. torreyana) [Figs. 122,123]; inflorescences of remote to congested clusters of spikes; spikes white to pale brown or red-brown to deep brown, usually clustered
      • 5a. Perianth bristles retrorsely barbellate (smooth in rare forms of R. capillacea 
 and R. capitellata, antrorsely barbellate in rare forms of R. capitellata) [Fig. 122]; base 
of achene narrowed to a stipe-like process
        • 6a. Flowers with 10–12 perianth bristles, these often antrorsely setose near base; floral scales white to light yellow-brown
        • 6b. Flowers with 6 perianth bristles, without antrorsely oriented hairs at the base [Fig. 122]; floral scales pale brown to red-brown or deep brown
          • 7a. Leaf blades filiform, 0.2–0.4 mm wide; spikes all ascending, in ovoid to ellipsoid clusters; achene body elliptic in outline, 1.5–2 mm long
          • 7b. Leaf blades ± flat, 1.5–3 mm wide; basal spikes of each cluster spreading, 
the spikes in turbinate to hemispherical clusters; achene body obovate in outline [Fig. 122], 1.2–1.5 mm long
      • 5b. Perianth bristles antrorsely barbellate (sometimes caducous, and therefore 
absent in fruit, in R. torreyana) [Fig. 123]; base of achene cuneate, not narrowed to a stipe-like process
        • 8a. Perianth bristles firmly attached to achene, reaching to base of tubercle or longer [Fig. 123]; tubercle (0.7–­) 1–1.3 (–1.5) mm long; achene bodies very finely transversely rugose (i.e., pattern not evident without high magnification)
        • 8b. Perianth bristles weakly attached to achene, sometimes caducous, not extending more than the length of achene body; tubercle 0.2–0.3 (–0.5) mm long; achene bodies finely transversely rugose with irregular horizontal ridges separated by intervals of narrow, vertical alveolae

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