Megathyrsus (Pilg.) B.K. Simon & S.W.L. Jacobs

Plants perennial; cespitose, with short, thick rhizomes. Culms usually erect, sometimes geniculate and rooting at the lower nodes; nodes pubescent or glabrous. Sheaths glabrous or pubescent, sometimes with papillose-based hairs, margins sometimes ciliate; ligules of hairs; blades flat, erect or ascending, glabrous or with hairs, sometimes with appressed papillose-based hairs, margins sometimes ciliate basally. Panicles 10-65 cm, rachises smooth or scabrous, branches single or whorled at the lowest nodes, lower axils with a tuft of hairs, base, upper axils glabrous; branches usually spikelike, secondary and tertiary branches usually appressed when present; pedicels unequal, straight or curved, glabrous or with a single setaceous hair near the apex. Spikelets 2.7-4.3 mm long, usually glabrous, rarely densely covered with papillose-based hairs, solitary, paired or in triplets, usually appressed to the branch axes. Glumes appresed to the florets, rachilla between the glumes not pronounced; lower glumes to 1/3 the length of the spikelet, 1-3-veined, obtuse or truncate, glabrous; upper glumes subequal to the upper floret, 5-veined, glabrous; lower lemma subequal to the upper floret, glabrous, 5-veined, without cross venation, acute, muticous or mucronate; lower florets staminate; upper lemmas ellipsoid, pale, glabrous, transversely rugose, apices acute or mucronulate. x = 9.

Simon and Jacobs (2003) transferred Urochloa maxima to a new genus, Megathyrsus, citing its morphological distinction from the taxa indicated as its close relatives in molecular-based trees. Gomez-Martinez and Culham (2000), Zuloaga et al. (2000), Giussani et al. (2001), and Aliscioni et al. (2003) all demonstrated that its only species M. maximus should not be included in Panicum sensu stricto. The support for its recognition as a distinct genus, although reflected here, is more equivocal. In the consensus trees of Giussani et al. (2001) and Aliscioni et al. (2003), the species is shown as being in a clade with Urochloa plantinginea, Chaetium bromoides, and Eriochloa punctata. The majority rule tree of Duvall et al. (2001) shows it as being, together with Eriochloa punctata, part of the unresolved base of a clade that otherwise contains a clade of Brachiaria mutica [=Urochloa mutica], U. reptans, U. acuminata, and Chaetium bromoides and is sister to a clade containing two species of Melinis [one treated in the article as a species of Rhyelytrum]. Simon and Jacobs (2003) noted that expanding Urochloa to include the other taxa in the same clade would not be widely accepted. For that reason, they proposed recognition of Megathyrsus which at present includes two species, M. maximus and M. infestus. Only one species is known from North America.

1.   Megathyrsus maximus (Jacq.) B.K. Simon & S.W.L. Jacobs
Guinea Grass

Culms (60)100-250 cm tall, about 10 mm thick, mostly erect, sometimes geniculate and rooting at the lower nodes; nodes pubescent or glabrous. Sheaths usually shorter than the internodes, glabrous or pubescent, sometimes with papillose-based hairs, margins sometimes ciliate; collars densely pubescent, hairs appressed or divergent; ligules 1-3 mm; blades (15)30-75(100) cm long, 10-35 mm wide, flat, erect or ascending, glabrous or with hairs, sometimes with appressed papillose-based hairs, margins scabrous, sometimes ciliate basally, midveins conspicuous, sunken, whitish. Panicles 20-65 cm, about 1/3 as wide as long, open, rachises smooth or scabrous; primary branches usually more than 20, 12-40 cm, axes 0.4-0.6 mm wide, not winged, ascending, those of the lower node(s) verticillate and pilose at the base, upper axils glabrous, lower branches naked basally; secondary and tertiary branches well-developed; pedicels unequal, 0.5-1.5 mm, straight or curved, glabrous or with a single setaceous hair near the apex. Spikelets 2.7-3.6 mm long, 0.9-1.1 mm wide, oblong-ellipsoid, usually glabrous, rarely densely covered with papillose-based hairs, usually solitary or paired, sometimes in triplets, usually appressed to the branch axes. Glumes appressed to the florets, rachilla between the glumes not pronounced; lower glumes 0.8-1.2 mm, 1-3-veined, obtuse or truncate, glabrous; upper glumes 2.1-3.5 mm, 5-veined, glabrous; lower lemmas subequal, 2.1-3.5 mm, glabrous, 5-veined, without cross venation, acute, muticous or mucronate; lower florets staminate; upper lemmas 1.9-2.4 mm, ellipsoid, pale, glabrous, transversely rugose, apices acute, mucronulate; anthers 1.2-2.2 mm. 2n = 18, 32, 36, 44, 48.

Megathyrsus maximums is an important forage grass that is native to Africa. In the Flora region, it grows in fields, waste places, stream banks, and hammocks. It is cultivated widely as a forage grass at low elevations, especially near the coast, and often escapes.
There are usually two varieties recognized. Only Megathyrsus maximus (Jacq.) B.K. Simon & S.W.L. Jacobs var. maxima, which has glabrous spikelets, is known in the Flora area. Specimens with densely pubescent spikelets belong to U. maxima var. pubiglumis (K. Schum) B.K. Simon & S.W.L. Jacobs