10.11 xACHNELLA Barkworth
Mary E. Barkworth

Plants perennial; cespitose, not rhizomatous. Culms to 90 cm, erect, glabrous, not branching at the upper nodes; prophylls shorter than the sheaths. Sheaths mostly glabrous, margins sparsely ciliate, hairs longer distally; cleistogenes not present in the basal sheaths; collars without tufts of hair at the sides; auricles absent; ligules scarious, glabrous; blades of basal leaves to 40 cm long, 1–1.5 mm wide, convolute when dry, tapering to the narrowly acute apices, flag leaf blades longer than 10 mm, bases about as wide as the top of the sheaths. Inflorescences panicles. Spikelets 6–8.5 mm, with 1 floret; rachillas not prolonged beyond the base of the floret; disarticulation above the glumes, beneath the floret. Glumes saccate-lanceolate, tapering from above midlength into elongate apices, midveins extending to or nearly to the apices, apices narrowly acute to acuminate; lower glumes 6–8.5 mm; upper glumes 6.5–7.5 mm, slightly narrower than the lower glumes; florets 4–5 mm, lengths more than 3 times the widths; calluses about 0.7 mm, blunt; lemmas coriaceous, evenly hairy throughout, hairs 1–2 mm, apical hairs not longer than those below; awns twisted, not or once-geniculate, readily deciduous; paleas more than 2/3 as long as the lemmas, veins ending at the apices; anthers poorly developed, indehiscent, sometimes with 1 or 2 penicillate hairs.

xAchnella comprises sterile hybrids between Achnatherum and Nassella. Only one such hybrid is known; it is restricted to the Flora region.


SELECTED REFERENCE Johnson, B.L. and G.A. Rogler. 1943. A cytotaxonomic study of an intergeneric hybrid between Oryzopsis hymenoides and Stipa viridula. Amer. J. Bot. 30:49–56.


1. xAchnella caduca (Beal) Barkworth

Culms 50–90 cm, glabrous. Basal sheaths mostly glabrous, margins and throats usually ciliate; ligules 0.5–1.7 mm, truncate-rounded; blades 1–3.5 mm wide. Panicles 15–18 cm; branches ascending. Lower glumes 6–8.5 mm long, 1–1.6 mm wide; florets 4–5 mm; calluses about 0.7 mm; lemmas 7-veined, hairy, hairs 1–2 mm, apices awned; awns 9–16 mm, readily deciduous; paleas 2.5–3.3 mm, 2/3–3/4 the length of the lemmas, hairy; anthers 2, 1.7–2.3 mm, differing in length within the florets, indehiscent. 2n = 65.

xAchnella caduca is a sterile hybrid between Achnatherum hymenoides and Nassella viridula that occurs infrequently in Montana and Wyoming. It differs from Achnatherum hymenoides in its longer glumes and florets; from Nassella viridula in its more saccate glumes, longer lemma hairs, and well-developed palea; and from other hybrids involving Achnatherum hymenoides in its readily deciduous awn.