8.01 DIARRHENA P. Beauv.
David M. Brandenburg
Plants perennial; rhizomatous, rhizomes 1.5–5 mm thick, scaly. Culms 48–131 cm tall, 1–3 mm thick, slender and arching, unbranched, usually clumped, rarely solitary. Leaves basally concentrated or proximal; sheaths open, longer than the internodes, margins narrowly hyaline, entire, sometimes ciliate; collars cartilaginous, thickened, light green or yellowish, somewhat flared marginally; auricles sometimes present; ligules stiffly membranous, rounded, ciliolate; blades flat, tapering basally, long-tapering apically, midveins usually eccentric. Inflorescences panicles, contracted, exserted, arching, racemose distally; branches 1 or 2 per node, ascending or appressed, terminating in a spikelet. Spikelets cylindrical when young, laterally compressed at maturity, with (2)3–5(7) florets, distal floret reduced and sterile, sometimes including an additional rudimentary floret; disarticulation above the glumes and beneath the florets. Glumes unequal, chartaceous, lanceolate, glabrous, keeled, sometimes scabridulous near the keels distally, margins entire or ciliolate, apices acute; lower glumes 1/3–2/3 shorter than the upper glumes, less than 1/3 as long as the adjacent lemmas, 1–3(5)-veined; upper glumes (3)5-veined; calluses glabrous or with a few hairs, hairs about 0.5 mm; lemmas mostly chartaceous, veins 3, prominent, convergent, margins hyaline, entire, sometimes ciliate, apices sharply cuspidate, cusps 1–2 mm; paleas from 1/2 as long as to slightly shorter than the lemmas, chartaceous, keeled, sides narrowly hyaline; lodicules about 1.5 mm, lanceolate to elliptic, apices ciliolate; anthers 2, yellow. Caryopses prominently beaked, style bases usually persistent, pericarp loose, at least partially. x = 10. Name from the Greek dias, ‘twice’, and arren, ‘male’, alluding to the 2 anthers.
Diarrhena is an odd and distinctive genus whose relationships are not clear. Two of its approximately six species grow in the woodlands of eastern North America; the remainder, which are sometimes placed in the segregate genus Neomolinia Honda, occupy similar habitats in eastern Asia. The Asian species have x = 19. The above description pertains to the North American species.
Although Diarrhena americana and D. obovata grow in similar habitats and overlap in their ranges, no intermediates have been found. Earlier reports of intermediate specimens are based on the use of less reliable characters for distinguishing between the two species.
SELECTED REFERENCES Brandenburg, D.M., J.R. Estes, and S.L. Collins. 1991. A revision of Diarrhena (Poaceae) in the United States. Bull. Torrey Bot. Club 118:128–136; Koyama, T. and S. Kawano. 1964. Critical taxa of grasses with North American and eastern Asiatic distribution. Canad. J. Bot. 42:859–864; Tateoka, T. 1960. Cytology in grass systematics: A critical review. Nucleus (Calcutta) 3:81–110.
For an interactive dichotomous key, click here; the interactive, multientry key is not yet available.
1. Calluses pubescent on all but the lowest mature lemma; lemma of the lowest floret in each spikelet (6)7.1–10.8 mm long, widest below the middle, tapering gradually to the apex; mature fruits 1.3–1.8 mm wide, gradually tapering to a blunt beak ... D. americana
1. Calluses glabrous on all mature lemmas; lemma of the lowest floret in each spikelet 4.6–7.5 mm long, widest near or above the middle, abruptly contracted to the apex; mature fruits 1.8–2.5 mm wide, abruptly contracted to a bottlenose-shaped beak ... D. obovata
1. Diarrhena americana P. Beauv.
Culms 60–131 cm, glabrous or pubescent. Sheaths often pubescent; collars usually pubescent; auricles pubescent; ligules 0.5–1.8 mm; blades 25–51 cm long, 7–20 mm wide, glabrous or scabridulous on both surfaces, margins scab-ridulous or ciliate. Panicles 9–30 cm, with 4–23 spikelets. Spikelets 10–20 mm, oblong to elliptic, with (2)4–5(7) florets. Glumes green to stramineous; lower glumes 1.7–4.2 mm long, 0.3–0.7 mm wide in profile, (1)3(5)-veined; upper glumes 2.8–6.4 mm long, 0.6–1.2 mm wide in profile, (3)5-veined; calluses pubescent on all but the lowest mature lemma; lemmas (3.8)5.3–10.8 mm, widest below the middle, lanceolate in profile, tapering gradually, apices sharply cuspidate; paleas glabrous or scabridulous, apices usually bifid, sinuses to 0.7 mm deep; anthers (1.7)2–2.9(3.5) mm. Caryopses 4.6–5.8 mm long, 1.3–1.8 mm wide, narrowly lanceolate in outline, gradually tapering to a blunt beak, wrinkled or smooth, usually blackish-brown to black, except for the straw-colored beak, rarely orange-brown. 2n = unknown.
Diarrhena americana is restricted to the United States, where it grows in rich, moist woods from Missouri to Maryland and south to Oklahoma and Alabama. Its range is primarily to the east of the range of D. obovata.
2. Diarrhena obovata (Gleason) Brandenburg
Culms 48–131 cm, glabrous. Sheaths, collars, and auricles glabrous or pubescent; ligules 0.2–1 mm; blades 24–72 cm long, 6–18 mm wide, abaxial surfaces glabrous or scabrid-ulous, adaxial surfaces similar or pubescent, margins usually scabridulous, rarely smooth. Panicles 5–30 cm, with 4–33 spikelets. Spikelets 7–17 mm, oblong to ovate, with (2)3–5(7) florets. Glumes green to stramineous; lower glumes 1.7–3.7 mm long, 0.3–0.6 mm wide in profile, 1–3-veined; upper glumes 2.2–5.2 mm long, 0.75–1.5 mm wide in profile, (3)5-veined; calluses glabrous; lemmas 3.7–7.5 mm, widest near or above the middle, obovate or elliptic in profile, abruptly tapering, apices sharply cuspidate; paleas glabrous, keeled, apices usually bifid, rarely truncate, sinuses 0.05–0.3(0.5) mm deep; anthers 1.4–2 mm. Caryopses 4.1–6 mm long, 1.8–2.5 mm wide, broadly elliptic to obovate in outline, narrowing abruptly to a bottlenose-shaped beak, usually smooth, occasionally wrinkled, shiny, usually straw-colored, with occasional brown areas basally. 2n = 60.
Diarrhena obovata is restricted to the Flora region, growing in rich woodlands from South Dakota to Ontario and New York and south to Texas, Tennessee, and Virginia. It is most common in the prairie states.