| # The grass genera of the world |

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

### Diectomis Kunth

~ Andropogon (A. fastigiatus)

Habit, vegetative morphology. Annual (becoming red on drying); caespitose. Culms 30–150 cm high; herbaceous; branched above. Culm nodes exposed; glabrous (dark). Culm internodes solid. The shoots not aromatic. Leaves not basally aggregated; non-auriculate. Leaf blades linear; narrow; 1–4 mm wide; flat (rather flaccid); without cross venation. Ligule an unfringed membrane (tough); not truncate; up to 10–20 mm long.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets. The spikelets of sexually distinct forms on the same plant; hermaphrodite and sterile. The male and female-fertile spikelets mixed in the inflorescence. The spikelets overtly heteromorphic (the pedicelled member much larger); all in heterogamous combinations.

Inflorescence. Inflorescence compound paniculate; open; spatheate; a complex of ‘partial inflorescences’ and intervening foliar organs. Spikelet-bearing axes ‘racemes’; the spikelet-bearing axes with more than 10 spikelet-bearing ‘articles’ (‘many jointed’); solitary (one per spathe); disarticulating; disarticulating at the joints. ‘Articles’ non-linear (trumpet-shaped); without a basal callus-knob; appendaged (the appendage terminal, small, bidentate); densely long-hairy (long-villous, with white hairs sometimes to 7 mm long). Spikelets paired; sessile and pedicellate; consistently in ‘long-and-short’ combinations; in pedicellate/sessile combinations. Pedicels of the ‘pedicellate’ spikelets free of the rachis (trumpet-shaped). The ‘shorter’ spikelets hermaphrodite. The ‘longer’ spikelets sterile.

Female-sterile spikelets. The pedicel translucent down the middle and expanded at the tip, the pedicellate spikelet much the larger, up to 8 mm long, its lower glume large, flat, papery, reddish, with a 5–7 mm awn.

Female-fertile spikelets. Spikelets 4–5 mm long; compressed laterally (between the rachis and the pedicel); falling with the glumes (and with the ‘article’ and the accompanying pedicelled member). Rachilla terminated by a female-fertile floret. Hairy callus present. Callus short.

Glumes two; more or less equal; exceeding the spikelets; long relative to the adjacent lemmas; the upper awned (with a long, fine bristle from the sinus); very dissimilar (the lower two-keeled, membranous in the sulcus, with firm keels and awnless or mucronate, the upper deeply naviculate, with a long slender awn from its apical sinus). Lower glume two-keeled; sulcate on the back (deeply and narrowly grooved between the keels, cf. Andropogon Sect. Piestium etc.); not pitted; relatively smooth; 2 nerved (without a median nerve, the sulcus membranous). Upper glume 3 nerved. Spikelets with incomplete florets. The incomplete florets proximal to the female-fertile florets. The proximal incomplete florets 1; epaleate; sterile. The proximal lemmas ciliolate; awnless; 2 nerved; more or less equalling the female-fertile lemmas; similar in texture to the female-fertile lemmas (hyaline); not becoming indurated.

Female-fertile florets 1. Lemmas broad, deeply naviculate-complanate; less firm than the glumes (hyaline); not becoming indurated; incised; 2 lobed; not deeply cleft (bidentate); awned. Awns 1; median; from a sinus; geniculate; hairless (glabrous); much longer than the body of the lemma. Lemmas ciliolate; non-carinate; without a germination flap; 2 nerved. Palea present; conspicuous but relatively short; entire; awnless, without apical setae (ciliate); textured like the lemma (hyaline); nerveless; keel-less. Palea back glabrous. Lodicules present; 2; fleshy; glabrous. Stamens 3. Anthers relatively long; not penicillate; without an apically prolonged connective. Ovary apically glabrous. Styles free to their bases. Stigmas 2.

Fruit, embryo and seedling. Fruit ellipsoid; not grooved; compressed laterally; glabrous. Hilum short. Embryo large (about half the grain length).

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae present; intercostal (at least, absent over the midrib). Intercostal papillae not over-arching the stomata (but associated with them); consisting of one oblique swelling per cell (each approaching one end of a stoma). Long-cells fairly similar in shape costally and intercostally; differing markedly in wall thickness costally and intercostally (the costals thicker walled, at least over the midrib). Intercostal zones with typical long-cells. Mid-intercostal long-cells rectangular; having markedly sinuous walls and having straight or only gently undulating walls (the long-cell walls thin and hard to observe). Microhairs present; elongated; clearly two-celled. Stomata common. Subsidiaries non-papillate; dome-shaped (medium domes). Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells common; not paired (solitary, associated with prickle bases); not silicified. Prickles abundant costally and intercostally, variable in size. Crown cells absent. Costal short-cells conspicuously in long rows. Costal silica bodies present and well developed; ‘panicoid-type’; dumb-bell shaped.

Transverse section of leaf blade, physiology. C4; XyMS–. PCR sheath outlines even. PCR sheath extensions absent. Mesophyll with non-radiate chlorenchyma; not Isachne-type; without ‘circular cells’; not traversed by colourless columns. Leaf blade with distinct, prominent adaxial ribs; with the ribs very irregular in sizes. Midrib conspicuous; having a conventional arc of bundles (with one median primary and one to several minor bundles on either side); with colourless mesophyll adaxially. The lamina symmetrical on either side of the midrib. Bulliforms present in discrete, regular adaxial groups (but these infrequent), or not present in discrete, regular adaxial groups (the bulliform cells mostly irregular, occupying much of the intercostal zones); in simple fans (in places). Many of the smallest vascular bundles unaccompanied by sclerenchyma. Combined sclerenchyma girders present (with the primary bundles); forming ‘figures’ (anchors, I’s and T’s). Sclerenchyma all associated with vascular bundles.

Special diagnostic feature. The pedicelled member of the sessile/pedicellate spikelet pairs much the larger, very striking, with a broad, flat, papery, reddish, long-awned lower glume.

Cytology. Chromosome base number, x = 5, or 10. 2n = 20. 2 ploid.

Classification. Watson & Dallwitz (1994): Panicoideae; Andropogonodae; Andropogoneae; Andropogoninae. Soreng et al. (2015): Panicoideae; Andropogonodae; Andropogoneae; Andropogoninae. 1 species (D. fastigiata (Sw.) Kunth).

Distribution, phytogeography, ecology. Pantropical.

Mesophytic; species of open habitats; glycophytic. Savanna in sandy soils, on rocky slopes or near streams in grassland, open forest etc.

Economic aspects. Important native pasture species: D. fastigiata provides good fodder until the awns form.

References, etc. Leaf anatomical: studied by us.

Special comments. Fruit data wanting.

Illustrations. • Diectomis fastigiata: Hitchcock (1936).

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: 15th June 2025. delta-intkey.com’.

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