| # The grass genera of the world |

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

### Agenium Nees

Habit, vegetative morphology. Perennial; caespitose. Culms 50–80 cm high; herbaceous; branched above. Leaves not basally aggregated. Leaf blades linear to linear-lanceolate (acuminate); narrow; to 3 mm wide; without cross venation; disarticulating from the sheaths (at least on the lower culms). Ligule an unfringed membrane (minutely ciliolate only); truncate; 0.5 mm long. Contra-ligule absent.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets. The spikelets of sexually distinct forms on the same plant; hermaphrodite and male-only, or hermaphrodite, male-only, and sterile, or female-only, male-only, and sterile. The male and female-fertile spikelets mixed in the inflorescence. The spikelets overtly heteromorphic (the upper pedicellate spikelets being awnless); in both homogamous and heterogamous combinations (with several conspicuous homogamous pairs at the raceme bases, these being awnless and male or neuter).

Inflorescence. Inflorescence of spicate main branches (of andropogonoid racemes), or a single raceme; usually digitate, or subdigitate. Primary inflorescence branches 1–3. Inflorescence espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes ‘racemes’; the spikelet-bearing axes several-jointed; solitary, or paired, or clustered (the raceme bases, when digitate, filiform and flexuous); with very slender rachides; disarticulating; disarticulating at the joints. ‘Articles’ non-linear; without a basal callus-knob; not appendaged; disarticulating obliquely; densely long-hairy (along the margins). Spikelets paired; not secund; sessile and pedicellate; consistently in ‘long-and-short’ combinations; in pedicellate/sessile combinations. Pedicels of the ‘pedicellate’ spikelets free of the rachis. The ‘shorter’ spikelets male-only to sterile (at the base of the racemes), or hermaphrodite to female-only (in the upper pairs). The ‘longer’ spikelets male-only, or sterile.

Female-sterile spikelets. The (upper) pedicelled spikelets male, larger. The male spikelets with glumes (the G1 villous). The lemmas awnless.

Female-fertile spikelets. Spikelets 5 mm long; compressed dorsiventrally (subterete); falling with the glumes (and with the adjacent joint and pedicel). Rachilla terminated by a female-fertile floret. Hairy callus present (the hairs shorter than in the pedicelled spikelet). Callus pointed.

Glumes two; more or less equal; long relative to the adjacent lemmas; dorsiventral to the rachis; hairy (G1 more so); without conspicuous tufts or rows of hairs; not pointed (G1 truncate, G2 obtuse); awnless; very dissimilar (G1 hairier, sulcate). Lower glume not two-keeled; sulcate on the back (with a median, translucent groove); not pitted (but the sulcus looks like an extended pit); 6 nerved (without a median). 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 awnless (ciliate); 0 nerved; decidedly exceeding the female-fertile lemmas (the latter being reduced to its awn); not becoming indurated (hyaline, much shorter than the glumes).

Female-fertile florets 1. Lemmas consisting almost entirely of awn; entire; awned. Awns 1 (up to 40 mm long); geniculate; hairless to hairy (scabrid to shortly hairy); much longer than the body of the lemma (which is virtually non-existent); entered by one vein; deciduous. Lemmas 1 nerved (in the awn). Palea absent. Lodicules present; 2; free; fleshy; glabrous. Ovary apically glabrous. Stigmas 2; red pigmented.

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae present; intercostal. Intercostal papillae over-arching the stomata; consisting of one oblique swelling per cell, or consisting of one symmetrical projection per cell (large, rough). Intercostal zones with typical long-cells to exhibiting many atypical long-cells (and the interstomatals very short). Mid-intercostal long-cells having markedly sinuous walls. Microhairs present; panicoid-type; 25.5–27–27 microns long; 4–4.5(–5) microns wide at the septum. Microhair total length/width at septum 5–7.5. Microhair apical cells 15 microns long. Microhair apical cell/total length ratio 0.56–0.59. Stomata common; 18–21–21 microns long. Subsidiaries non-papillate; dome-shaped (mostly), or triangular (a few). Guard-cells overlapped by the interstomatals (sometimes, slightly), or overlapping to flush with the interstomatals. Intercostal short-cells absent or very rare (very scarce, ignoring the microhairs and a few small prickle bases). Costal short-cells conspicuously in long rows. Costal silica bodies ‘panicoid-type’; mostly short dumb-bell shaped.

Transverse section of leaf blade, physiology. C4; XyMS–. Leaf blade adaxially flat. Midrib conspicuous (via a large bundle with an I-shaped combination girder); limits not clearly defined. Bulliforms not present in discrete, regular adaxial groups (the epidermis extensively bulliform). Many of the smallest vascular bundles unaccompanied by sclerenchyma. Combined sclerenchyma girders present (with all the primaries); forming ‘figures’ (most notably the mid-vein I). Sclerenchyma all associated with vascular bundles.

Classification. Watson & Dallwitz (1994): Panicoideae; Andropogonodae; Andropogoneae; Andropogoninae. Soreng et al. (2015): Panicoideae; Andropogonodae; Andropogoneae; Anthistiriinae. 4 species.

Distribution, phytogeography, ecology. Brazil to Argentina.

Species of open habitats. Dry savanna.

References, etc. Leaf anatomical: studied by us - A. villosum Nees.

Special comments. Description based mainly on A. villosum. Fruit data wanting.

Illustrations. • Agenium villosum, as Heteropogon: Hackel, Fl. Bras. 2 (1895). • A. villosum: Nicora & Rúgolo de Agrasar (1987). • Agenium villosum, abaxial epidermis of leaf blade: this project. R.S.B.S. Taxonomy Laboratory (Canberra) slide. Slides and full details now housed at the Western Australian Herbarium.

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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