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The grass genera of the world

L. Watson, T.D. Macfarlane, and M.J. Dallwitz

Sorghastrum Nash

From Sorghum (another grass genus, q.v.) and Latin suffix astrum (a poor imitation of).

Including Dipogon Steud., Poranthera Raf.

Habit, vegetative morphology. Annual, or perennial; caespitose. Culms 70–150 cm high; herbaceous; branched above, or unbranched above. The branching simple. Leaves not basally aggregated; auriculate, or non-auriculate. Leaf blades without cross venation; rolled in bud. Ligule an unfringed membrane to a fringed membrane.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets. The spikelets of sexually distinct forms on the same plant; hermaphrodite and sterile; overtly heteromorphic (in that the sterile spikelets are reduced to pedicels), or homomorphic (rarely the pedicellate spikelets are well developed and simiar to the sessile ones). Plants outbreeding.

Inflorescence. Inflorescence paniculate (narrowly elongated, more or less unilateral panicles of much-reduced, capillary ‘racemes’); open (usually narrow); with capillary branchlets; subdigitate to non-digitate. Inflorescence axes not ending in spikelets (in that the axis of each inflorescence unit ends in a bristle resembling a sterile pedicel). Inflorescence espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes very much reduced (the ultimate units with very few spikelets, often only one accompanied by the sterile pedicel); disarticulating (but the disarticulating units much reduced); falling entire (when reduced to one joint), or disarticulating at the joints. Spikelets nearly always paired (but ostensibly solitary, by virtue of the ‘pedicelled’ member being reduced to its pedicel - by contrast with Sorghum); secund (the inflorescence one-sided), or not secund; sessile and pedicellate (but usually ostensibly solitary by suppression); consistently in ‘long-and-short’ combinations (but the sterile member of each combination is nearly always reduced to its pedicel). Pedicels of the ‘pedicellate’ spikelets free of the rachis. The ‘shorter’ spikelets hermaphrodite. The ‘longer’ spikelets sterile (reduced to pedicels).

Female-fertile spikelets. Spikelets 5–8 mm long; compressed dorsiventrally (plump); falling with the glumes (and the joint). Rachilla terminated by a female-fertile floret. Hairy callus present.

Glumes two; more or less equal; long relative to the adjacent lemmas; free; hairy (G1); without conspicuous tufts or rows of hairs; pointed; awnless; very dissimilar (the lower flattened and often hairy on the back, the upper glabrous and slightly keeled above). Lower glume not two-keeled; convex on the back to flattened on the back; not pitted; relatively smooth; 9 nerved. Upper glume 5 nerved. Spikelets with incomplete florets. The incomplete florets proximal to the female-fertile florets. The proximal incomplete florets 1; epaleate; sterile. The proximal lemmas 2-lobed; awnless; 2 nerved; more or less equalling the female-fertile lemmas to decidedly exceeding the female-fertile lemmas; not becoming indurated (hyaline).

Female-fertile florets 1. Lemmas less firm than the glumes; linear, almost reduced to the awn, the margins narrow and thin; incised; awned. Awns 1; median; from a sinus; geniculate; hairless (glabrous); much longer than the body of the lemma. Lemmas hairy (sometimes long-ciliate), or hairless; non-carinate; 1 nerved (?). Palea present, or absent; when present, conspicuous but relatively short, or very reduced; not indurated. Lodicules present; 2; free; fleshy; glabrous. Stamens 3; with free filaments. Anthers not penicillate; without an apically prolonged connective. Ovary apically glabrous. Styles fused, or free to their bases. Stigmas 2.

Fruit, embryo and seedling. Fruit free from both lemma and palea; small. Hilum short. Embryo large. Endosperm hard. Embryo without an epiblast; with a scutellar tail; with an elongated mesocotyl internode. Embryonic leaf margins overlapping.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae present; costal, or intercostal. Mid-intercostal long-cells rectangular; having markedly sinuous walls. Microhairs present; panicoid-type. Stomata common. Subsidiaries triangular. Intercostal short-cells common. Costal short-cells conspicuously in long rows, or predominantly paired. Costal silica bodies horizontally-elongated crenate/sinuous, or tall-and-narrow, or ‘panicoid-type’; not sharp-pointed.

Transverse section of leaf blade, physiology. C4; biochemical type NADP–ME (S. nutans); XyMS–. PCR sheath outlines uneven. PCR sheath extensions present, or absent. Maximum number of extension cells 1. PCR cell chloroplasts with reduced grana; centrifugal/peripheral. Mesophyll with radiate chlorenchyma. Leaf blade with distinct, prominent adaxial ribs, or ‘nodular’ in section, or adaxially flat; when ribbed with the ribs more or less constant in size, or with the ribs very irregular in sizes. Midrib conspicuous; having a conventional arc of bundles; with colourless mesophyll adaxially. Bulliforms present in discrete, regular adaxial groups; associated with colourless mesophyll cells to form deeply-penetrating fans and combining with colourless mesophyll cells to form narrow groups penetrating into the mesophyll; associating with colourless mesophyll cells to form arches over small vascular bundles (all the bulliforms involved in these configurations). Combined sclerenchyma girders present; forming ‘figures’. Sclerenchyma all associated with vascular bundles.

Phytochemistry. Tissues of the culm bases with abundant starch.

Special diagnostic feature. Spikelets ostensibly solitary, each accompanied by a barren pedicel.

Cytology. Chromosome base number, x = 10. 2n = 20, 40, and 60. 2, 4, and 6 ploid.

Classification. Watson & Dallwitz (1994): Panicoideae; Andropogonodae; Andropogoneae; Andropogoninae. Soreng et al. (2015): Panicoideae; Andropogonodae; Andropogoneae; Sorghinae. About 20 species.

Distribution, phytogeography, ecology. Mainly tropical and subtropical Africa and America.

Commonly adventive. Helophytic to mesophytic; shade species, or species of open habitats; glycophytic. Savanna and woodland margins, often in wet places.

Economic aspects. Important native pasture species: S. arundinaceum (sometimes cyanogenic when young); S. nutans.

Rusts and smuts. Rusts — Puccinia. Taxonomically wide-ranging species: ‘Uromycesclignyi. Smuts from Ustilaginaceae. Ustilaginaceae — Sphacelotheca and Tolyposporella.

References, etc. Leaf anatomical: photos of S. friesii and S. stipoides provided by R.P. Ellis.

Illustrations. • S. incompletum, as S. bipennatum: Rose Innes, Ghana Grasses (1977). • General aspect (S. friesii): Gibbs Russell et al., 1990. • S. elli˛ttii and S. nutans: Britton (1913), Ill. Flora of Northern U.S.A., 1

We advise against extracting comparative information from the descriptions. This is much more easily achieved using the DELTA data files or the interactive key, which allows access to the character list, illustrations, full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, distributions of character states within any set of taxa, geographical distribution, and classifications. See also Guidelines for using data taken from Web publications.

Cite this publication as: ‘Watson, L., Macfarlane, T.D., and Dallwitz, M.J. 1992 onwards. The grass genera of the world: descriptions, illustrations, identification, and information retrieval; including synonyms, morphology, anatomy, physiology, phytochemistry, cytology, classification, pathogens, world and local distribution, and references. Version: 11th December 2017.’.