Sorghastrum* Britton, Man. Fl. North. States 71 (1901).
Derivation:. From Sorghum (another grass genus, q.v.) and Latin suffix astrum (a poor imitation of).
Key references (keys and floras):. B.K.Simon, Key to Australian Grasses 157 (1993); D.Sharp and B.K.Simon, AusGrass (2002).
W.D.Clayton & S.A.Renvoize, Genera Graminum (1986), genus (586).
Naturalised. About 20 species, from mainly tropical and subtropical Africa and America. 1 species in Australia, WA.
Habit. Annual or perennial, tufted. Ligule an unfringed membrane to a fringed membrane.
Inflorescence. Inflorescence paniculate (narrowly elongated, more or less unilateral panicles of much-reduced, capillary racemes), subdigitate to non-digitate. Spikelet-bearing axes much reduced (the ultimate units with very few spikelets, often only one accompanied by the sterile pedicel), with heteromorphic spikelets (in that the sterile spikelets are reduced to pedicels) or with homomorphic spikelets (rarely the pedicellate spikelets are well developed and similar to the sessile ones), falling entire (when reduced to one joint) or disarticulating at joints.
Spikelets. Spikelets dorsally compressed (plump), 2 flowered, with 1 fertile floret, nearly always paired (but ostensibly solitary, by virtue of the pedicelled spikelet being reduced to its pedicel, by contrast with Sorghum), sessile and pedicelled (but usually ostensibly solitary by suppression), in pedicelled/sessile combinations; with rachilla terminating in a floret; sessile spikelet with lower incomplete floret. Fertile spikelets dorsally compressed (plump), falling with glumes (and the joint).
Glumes. Glumes more or less equal, long relative to adjacent lemmas, hairy (lower glume), pointed, awnless, dissimilar (the lower flattened and often hairy on the back, the upper glabrous and slightly keeled above). Lower glume convex on back to flattened on back, relatively smooth, 9 nerved. Upper glume 5 nerved.
Florets. Lower incomplete floret(s) sterile. Lemmas 2-lobed, awnless, 2 nerved, more or less equalling fertile lemmas to exceeding fertile lemmas, not becoming indurated (hyaline). Fertile florets 1. Lemmas less firm than glumes, linear, almost reduced to the awn, the margins narrow and thin, incised, awned, 1 nerved, not keeled. Awns 1, from a sinus, geniculate, hairless (glabrous), much longer than body of lemma. Palea present or absent, when present, conspicuous and relatively short or very reduced. Lodicules 2. Stamens 3. Stigmas 2. Grain small. Hilum short. Embryo large. Endosperm hard. Pedicels free of rachis.
Kranz Anatomy. C4, biochemical type NADP-ME.
2n = 20, 40, and 60, 2, 4, and 6 ploid, commonly adventive.
Habitat. Hydrophytic to helophytic. Savanna and woodland margins, often in wet places. Shade species, or species of open habitats.
Classification. Panicoideae; Andropogoneae.
Notes. A relative of Sorghum with characteristic barren pedicels, also with a strong link to Saccharum (Clayton and Renvoize, 1986). Sorghastrum has variously been placed as a subgenus within Sorghum or into a distinct genus. Preliminary molecular phylogenetic studies did not include Sorghastrum in the classical subtribe Sorghinae (Spangler et al, 1999).
Types Species. S. avenaceum (Michx.) Nash = S. nutans (L.) Nash.
Biogeographic Element. Clifford & Simon 1981, Simon & Jacobs 1990: Naturalised.