1 Bristles absent (or apparently so at 10×); achene 0.5-0.7 mm wide; tubercle 0.1-0.2 mm long, skull-cap like; [section Pusillae].

  2 Achene including tubercle 1.0-1.2 mm long, the achene surface evidently reticulate and obscurely transversely ridged, the body ellipsoid; bristles present, white, barely visible at 20×, the longest shorter than the achene body

  2 Achene including tubercle 0.6-0.9 mm long, the body obovoid; bristles absent.

    3 Achene surface smooth, faintly reticulate, not transversely ridged

    3 Achene surface rough, distinctly transversely ridge

1 Bristles present or absent; if absent, then the achene > 1 mm long or > 0.7 mm wide, and tubercle triangular to subulate.

      4 Culms and leaves filiform.

        5 Achene including tubercle 1.0-1.2 mm long; tubercle minute, skullcap-like; [section Pusillae]

        5 Achene including tubercle 1.5-2.9 mm long; tubercle triangular to triangular-acuminate; [section Rariflorae].

          6 Bristles subequaling to exceeding the tubercle; tubercle 0.75-1.4 mm long

          6 Bristles shorter than the achene body; tubercle 0.3-1.0 mm long.

             7 Achene 1.3-1.6 mm long, 0.9-1.4 mm wide; tubercle 0.3-0.6 (-0.75) mm long; bristles 1/3-1/2(-4/5) as long as achene body

             7 Achene 1.6-1.8 mm long, 1.35-1.5 mm wide; tubercle 0.6-1.0 mm long; bristles 2/3 to nearly as long as achene body

      4 Culms stouter; leaves wider, not filiform.

               8 Achene faces flat or concave; when one face is concave, the opposite face is sometimes slightly convex (slightly biconvex R. decurrens and R. microcarpa are keyed here for convenience).

                 9 Achene at least twice as long as wide, elliptic-oblong; tubercle subulate, 0.8-1.2 mm long; [section Mixtae]

                 9 Achene < twice as long as wide, obovate; tubercle triangular, 0.2-0.9 mm long.

                   10 Longer bristles exceeding the achene body.

                     11 Achene ±2.2 mm long, ±1.8 mm wide; tubercle ±0.9 mm long; [section Globulares]

                     11 Achene 0.8-1.2 mm long, 0.7-1.2 mm wide; tubercle 0.2-0.5 mm long; [section Mixtae].

                       12 Larger leaves (3-) 4-6 mm wide; bristles exceeding tubercle; achene faces flattened

                       12 Larger leaves 1-3 (-4) mm wide, bristles half as long as achene to equaling tubercle; achene faces slightly convex

                   10 Longer bristles shorter than to equaling achene body, or absent.

                          13 Larger leaves 3-5 mm wide; achene 1.4-1.6 mm wide; tubercle 0.6-0.8 mm long, abruptly rising from a flaring basal collar; [section Globulares]

                          13 Larger leaves 1-3 (-4) mm wide; achene 0.7-1.3 mm wide; tubercle 0.15-0.5 mm long, without a flaring basal collar; [section Mixtae].

                            14 Bristles evident, at least 1/4 as long to exceeding achene.

                              15 Culms 20-50 cm long; leaves 1-2 mm wide; bristles ½ or less as long as achene body

                              15 Culms 50-110 cm long; leaves (1-) 2-4.5 mm wide; longer bristles more than ½ as long to exceeding achene body.

                                16 Achene 1.3-1.8 mm long, 0.9-1.2 mm wide, the faces flat with 10-12 transverse ridges

                                16 Achene 0.8-1.4 mm long, 0.7-1.2 mm wide, the faces slightly biconvex with 6-12 transverse ridges.

                                  17 Clusters elongate; achene 1.0-1.4 mm long, 0.8-1.0 mm wide, narrowly obovate to elliptic, averaging 8-12 transverse ridges; most tubercle bases convexly seated on the achene summit and somewhat decurrent along the achene margins, the tubercle surface often whitish-waxy

                                  17 Clusters usually compact; achene 0.8-1.2 mm long, 0.7-1.2 mm wide, suborbicular to elliptic, averaging 6-7 transverse ridges; most tubercle bases flat across the achene summit, not decurrent, the tubercle surface usually dark, not waxy

               8 Achenes biconvex or tumid.

                                    18 Achene 1.4-4.2 mm long, 1.2-3.6 mm wide, the summit with a thickened bony to crustaceous rim surrounding the base of the tubercle; [section Harveyae].

                                       19 Achene lenticular and transversely ridge, ±1.4 mm long, ±1.2 mm wide

                                       19 Achene tumid, lightly pitted or cancellate in a honeycomb pattern, 1.5-4.2 mm long, 1.4-3.6 mm wide.

                                         20 Leaves 4-8 mm wide; achene 3.0-4.2 mm long, 3.0-3.6 mm wide

                                         20 Leaves 2-4 mm wide; achene < 2.7 mm long and < 2.5 mm wide.

                                           21 Achene 2.0-2.7 mm long, 2.0-2.5 mm wide

                                           21 Achene 1.5-1.9 mm long, 1.4-1.7 mm wide

                                    18 Achene 0.7-1.8 mm long, 0.7-1.5 mm wide, the summit without a textured rim surrounding the base of the tubercle (if the base of the tubercle is rim-like, then it is distinguished from the summit of the achene by a constriction or articulation).

                                             22 Bristles absent; achene 0.7-1.0 mm long; [section Psilocarya].

                                               23 Tubercle triangular-lanceolate, as long as broad or longer; achene weakly transversely ridged

                                               23 Tubercle depressed, broader than long; style not persistent; achene strongly transversely ridged

                                                 24 Spikelets (5-) 6-10 mm long, tan or brown, with an acuminate tip; tubercle ends upturned away from the achene at about a 90 degree angle

                                                 24 Spikelets 4-6 (-8) mm long, dark brown to black, with an acute tip; tubercle ends accrescent to the achene

                                             22 Bristles present (occasionally detached in R. decurrens and R. miliacea with achenes 1.0-1.4 mm long); achene 0.8-1.9 mm long.

                                                    25 Bristles not exceeding the achene body.

                                                      26 Cluster branches flexuous (the branches curved), not uniformly erect and stiff; bristles one-half as long to equaling the achene (or longer in R. microcarpa); [section Mixtae].

                                                        27 Clusters elongate; spikelets 2.5-3.5 mm long; achene narrowly obovate to elliptic, averaging 8-12 transverse ridges; most tubercle bases convexly seated on the achene summit and somewhat decurrent along the achene margins, the tubercle surface often whitish-waxy

                                                        27 Clusters usually compact, branches ascending to spreading (sometimes widely so in R. vernalis); spikelets 1.5-3.0 mm long; achene suborbicular to elliptic, averaging 5-8 (-9) transverse ridges, most tubercle bases flat across the achene summit, not or only slightly decurrent, the tubercle surface usually dark, not waxy.

                                                          28 Inflorescence occupying the upper 2/3-3/4 of the length of the culm, the lowest lateral panicle at the first or second node above the base; spikelets 1.5-2.25 mm long, broadly ovoid to suborbicular, apically rounded; achene body averaging 5-6 (-7) horizontal ridges; tubercle 0.2-0.4 mm long

                                                          28 Inflorescence occupying the upper ¼-½ of the culm, the lowest 2-4 nodes barren; spikelets 2.0-3.0 mm long, ovoid, acute; achene body averaging 6-9 horizontal ridges; tubercle 0.25-0.85 mm long.

                                                            29 Inflorescence cluster branches ascending, the clusters mostly tightly congested; achene 1.0–1.7 times as long as wide, dark brown, shallowly nearly horizontally ridged or with nearly isodiametric alveolae, ridges 7–9; bristles of each achene nearly equal in length, from 0.5 to 1.5 times as long as the achene body; plants slender, mostly less than 1 meter tall, with leaves typically less than 40 cm long and 3 mm wide; [NC s. to s. FL, w. to s. MO and s. TX]

                                                            29 Inflorescence cluster branches widely spreading in flower, spreading to ascending in fruit, the clusters rather open, not congested; achene 1.0–1.2 times as long as wide, light brown, strongly transversely ridged with 6–8 irregular ridges and narrow rectangular alveolae; bristles unequal in length, from 0.9–1.5 times as long as the achene body; plants robust, typically more than 1.2 meters tall, with leaves often over 50 cm long and 4 mm wide; [FL]

                                                      26 Cluster branches stiff, erect, bristles < 1/3 to ¾ (-1) × as long as achene body; [section Globulares].

                                                              30 Leaves to 2 mm wide; longer bristles no more than ½ as long as achene; achene body lenticular, one side often flattened

                                                              30 Leaves to 5 mm wide; longer bristles > ½ as long to equaling achene; achene biconvex.

                                                                 31 Larger culm leaves to 5 mm wide; achenes (1.2-) 1.4-1.6 (-1.9) mm long, (1.1-) avg. 1.4 (-1.75) mm wide; achene surface alveoli longitudinally narrow; tubercle 0.3-0.7 mm long, base 0.6-1.0 mm wide

                                                                 31 Larger culm leaves to 3 mm wide; achenes (1.0-) avg. 1.3 (-1.5) mm long and wide; if achene surface alveoli longitudinally narrow, then tubercle 0.2-0.4 mm long and base 0.5-0.7 mm wide (R. globularis).

                                                                   32 Longer bristles ⅓-½ (-¾)× the length of the achene; achene surface alveoli longitudinally narrow (typically 0.02-0.05 mm wide between the longitudinal walls), the latitudinal walls raised into horizontal ridges; tubercle 0.2-0.4 mm long, the base 0.5-0.7 mm wide

                                                                   32 Longer bristles ⅔-1× the length of the achene; achene surface alveoli nearly as wide as long (typically 0.05-0.1 mm wide between the longitudinal walls), the latitudinal walls obscurely or not at all raised into horizontal ridges; tubercle 0.35-0.7 mm long, the base 0.7-0.9 mm wide

                                                    25 Bristles equaling or longer than the tubercle.

                                                                     33 Primary branches of the inflorescence spreading at right angles from the culm, each spikelet or small cluster on slender spreading or reflexed stalks; [section Mixtae]

                                                                     33 Primary branches of the inflorescence ascending.

                                                                       34 Spikelets (4-) 6-9 mm long; achene (body) 1.4-1.7 mm long (including a stipe 0.3-0.6 mm in length); [section Mixtae]

                                                                       34 Spikelets < 5 mm long; achene (body) 1.0-1.5 (-1.6) mm long (if ≥ 1.4 mm long, then the achene stipe either absent (R. saxicola) or the stipe ≤ 0.3 mm (R. caduca); [section Mixtae, except R. saxicola in section Globulares].

                                                                         35 Tubercle 0.4-0.8 mm long, the edges setose or uneven with waxy or crusty irregular protuberances; [section Mixtae].

                                                                           36 Achene obovate to suborbicular, 1.2-1.6 mm wide, latitudinal alveoli walls strongly raised into transverse ridges; achene base with a stipe to 3 mm long

                                                                           36 Achene slenderly obovoid, 0.8-1.0 mm wide, latitudinal alveoli walls weakly or not at all raised into transverse ridges; achene stipe diminuitive or virtually absent

                                                                         35 Tubercle 0.2-0.5 (-0.85) mm long, the edges smooth.

                                                                              37 Spikelets 3.5-4 mm long; bristles (usually) exceeding the tubercle; achene 1.3-1.5 (-1.6) mm long, 1.2-1.3 mm wide; [section Globulares]

                                                                              37 Spikelets 1.5-3 mm long; longer bristles nearly equaling the tubercle; achene 0.8-1.2 mm long, 0.7-1.2 mm wide; [section Mixtae].

                                                                                38 Inflorescence occupying the upper 2/3-3/4 of the length of the culm, the lowest lateral panicle at the first or second node above the base; spikelets 1.5-2.25 mm long, broadly ovoid to suborbicular, apically rounded; achene body averaging 5-6 (-7) horizontal ridges; tubercle 0.2-0.4 mm long

                                                                                38 Inflorescence occupying the upper ¼-½ of the culm, the lowest 2-4 nodes barren; spikelets 2.0-3.0 mm long, ovoid, acute; achene body averaging 6-9 horizontal ridges; tubercle 0.25-0.85 mm long.

                                                                                  39 Inflorescence cluster branches ascending, the clusters mostly tightly congested; achene 1.0–1.7 times as long as wide, dark brown, shallowly nearly horizontally ridged or with nearly isodiametric alveolae, ridges 7–9; bristles of each achene nearly equal in length, from 0.5 to 1.5 times as long as the achene body; plants slender, mostly less than 1 meter tall, with leaves typically less than 40 cm long and 3 mm wide; [NC s. to s. FL, w. to s. MO and s. TX]

                                                                                  39 Inflorescence cluster branches widely spreading in flower, spreading to ascending in fruit, the clusters rather open, not congested; achene 1.0–1.2 times as long as wide, light brown, strongly transversely ridged with 6–8 irregular ridges and narrow rectangular alveolae; bristles unequal in length, from 0.9–1.5 times as long as the achene body; plants robust, typically more than 1.2 meters tall, with leaves often over 50 cm long and 4 mm wide; [FL]