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The grass genera of the world

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

Homozeugos Stapf

From the Greek homos (alike, same) and zugon (a pair, a yoke), alluding to homomorphic spikelet pairs.

Habit, vegetative morphology. Perennial; caespitose. Culms 60–200 cm high; herbaceous; sometimes unbranched above. Culm nodes hairy. Young shoots intravaginal. Leaves auriculate (with sheath auricles), or non-auriculate. Leaf blades narrowly linear (or junciform); narrow; without cross venation. Ligule an unfringed membrane; truncate, or not truncate.

Reproductive organization. Plants bisexual, all with bisexual spikelets; with hermaphrodite florets. The spikelets all alike in sexuality (in homogamous pairs).

Inflorescence. Inflorescence of spicate main branches (of few fascicled ‘racemes’, or a solitary ‘raceme’); digitate (usually), or non-digitate (when the ‘raceme’ solitary). Primary inflorescence branches 1–6. Inflorescence espatheate; not comprising ‘partial inflorescences’ and foliar organs. Spikelet-bearing axes slender ‘racemes’; clustered; with very slender rachides; disarticulating; disarticulating at the joints. ‘Articles’ linear; not appendaged; disarticulating obliquely; densely long-hairy. Spikelets paired; consistently in ‘long-and-short’ combinations; in pedicellate/sessile combinations. Pedicels of the ‘pedicellate’ spikelets free of the rachis. The ‘shorter’ spikelets hermaphrodite. The ‘longer’ spikelets hermaphrodite.

Female-fertile spikelets. Spikelets 6–14 mm long; not noticeably compressed (almost cylindrical); falling with the glumes. Rachilla terminated by a female-fertile floret. Hairy callus present (spiny). Callus long; pointed.

Glumes two; more or less equal; long relative to the adjacent lemmas; free; hairy; without conspicuous tufts or rows of hairs; awnless; very dissimilar. Lower glume not two-keeled (except towards the tip); convex on the back (leathery); relatively smooth; 7–8 nerved (these evenly spaced). Upper glume 3 nerved. Spikelets with incomplete florets. The incomplete florets proximal to the female-fertile florets. Spikelets with proximal incomplete florets. The proximal incomplete florets 1; sterile. The proximal lemmas awnless; less firm than the female-fertile lemmas (hyaline); not becoming indurated.

Female-fertile florets 1. Lemmas linear; less firm than the glumes (scarious below, leathery above); not becoming indurated; incised; 2 lobed; not deeply cleft (bidentate); awned. Awns 1; median; from a sinus, or apical; geniculate; hairy to long-plumose; much longer than the body of the lemma. Lemmas hairy (ciliate).

Abaxial leaf blade epidermis. Costal/intercostal zonation conspicuous. Papillae present; intercostal. Intercostal papillae over-arching the stomata (slightly, at one end); consisting of one oblique swelling per cell (mostly on the interstomatals, in the form of one large, thick-walled papilla to each cell). Long-cells markedly different in shape costally and intercostally (the costals much narrower); of similar wall thickness costally and intercostally (walls medium thickness). Mid-intercostal long-cells rectangular; having markedly sinuous walls. Microhairs present; elongated; clearly two-celled; panicoid-type; 36–41–45 microns long; 4.5–4.8–5.4 microns wide at the septum. Microhair total length/width at septum 8–8.8. Microhair apical cells 16.5–20–22.5 microns long. Microhair apical cell/total length ratio 0.46–0.5. Stomata common; 27(–30) microns long. Subsidiaries non-papillate; low dome-shaped and triangular. Guard-cells overlapping to flush with the interstomatals. Intercostal short-cells absent or very rare (ignoring the prickles). Small prickles abundant. Crown cells absent (but with abundant costal prickles). Costal short-cells conspicuously in long rows. Costal silica bodies ‘panicoid-type’; mostly nodular.

Transverse section of leaf blade, physiology. C4; XyMS–. PCR sheath outlines even. PCR sheath extensions absent. Leaf blade with distinct, prominent adaxial ribs (over the primary bundles); with the ribs more or less constant in size (round-topped). Midrib very conspicuous; having a conventional arc of bundles (several small bundles on either side, between the large median and two large laterals, all the bundles towards the abaxial surface); with colourless mesophyll adaxially. The lamina symmetrical on either side of the midrib. Bulliforms present in discrete, regular adaxial groups; in simple fans and associated with colourless mesophyll cells to form deeply-penetrating fans; associating with colourless mesophyll cells to form arches over small vascular bundles. All the vascular bundles accompanied by sclerenchyma (the numerous small bundles each with an abaxial strand). Combined sclerenchyma girders present (with all the primary laterals, and some of the lower order bundles); forming ‘figures’ (especially T’s). Sclerenchyma all associated with vascular bundles.

Classification. Watson & Dallwitz (1994): Panicoideae; Andropogonodae; Andropogoneae; Andropogoninae. Soreng et al. (2015): Panicoideae; Andropogonodae; Andropogoneae; Saccharinae. 5 species.

Distribution, phytogeography, ecology. Tropical Africa.

Mesophytic. Savanna.

References, etc. Morphological/taxonomic: Stapf 1915. Leaf anatomical: studied by us - H. eylesii C.E. Hubb.

Special comments. Fruit data wanting. Illustrations. • H. fragile: Hook. ic. Pl. 31 (1922)

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.’.