26.28   TRIPSACUM L.
Mary E. Barkworth

Plants perennial; monoecious, staminate and pistillate spikelets evidently distinct, located in the same inflorescences, pistillate spikelets below the staminate spikelets. Culms 0.7-5 m. Leaves not aromatic; sheaths open; ligules membranous, erose to ciliate. Inflorescences terminal and axillary, panicles of 1-several subdigitate to racemose rames; rames with pistillate spikelets proximally and staminate spikelets distally; disarticulation in the rames, beneath the pistillate spikelets and at the base of the staminate portions. Pistillate spikelets exposed, solitary, embedded in the indurate rame axes; lower glumes coriaceous, closing the hollows in the rachises and concealing the florets; upper glumes similar but smaller; lower florets sterile; upper florets pistillate; lemmas and paleas hyaline, unawned; styles 2, not fused. Staminate spikelets paired, both sessile or both subsessile, or 1 sessile and the other pedicellate; glumes coriaceous, chartaceous, or membranous; lemmas and paleas hyaline, unawned. Pedicels (when present) not fused to the rame axes. x = 9. The origin of the name is unknown.

Tripsacum is a genus of 12 species, all of which are native to tropical and subtropical regions of the Western Hemisphere; three are native to the Flora region. They are good forage grasses, but are rarely sufficiently abundant in the Flora region to be important in this regard. The genus is of interest to plant breeders because of its relationship to Zea.

Measurements of the pistillate spikelets are based on measurements of the lower glumes of the sessile spikelets, the remainder of the spikelet being concealed between the rachis and the lower glume.

SELECTED REFERENCES Gray, J.R. 1974. The genus Tripsacum L. (Gramineae): Taxonomy and chemosystematics. Ph.D. dissertation, University of Illinois at Urbana-Champaign, Urbana, Illinois, U.S.A. 191 pp.; de Wet, J.M.J., D.E. Brink, and C.E. Cohen. 1983. Systematics of Tripsacum section Fasciculata (Gramineae). Amer. J. Bot. 70:1139-1146; de Wet, J.M.J., J.R. Harlan, and D.E. Brink. 1982. Systematics of Tripsacum dactyloides. Amer. J. Bot. 69:1251-1257; Li, Y.G., C.L. Dewald, and V.A. Sokolov. 2000. Sectional delineation of sexual Tripsacum dactyloides-T. maizar allotriploids. Ann. Bot. (London), n.s., 85:845-850.

Staminate spikelets in sessile-pedicellate pairs, the pedicels almost flat to plano-convex in cross section, 2-5 mm long, less than 0.3 mm wide; glumes usually membranous (sect. Fasciculata) ..... 1. T. lanceolatum
Staminate spikelets sessile, subsessile, or in sessile-pedicellate pairs, the pedicels triangular in cross section, to 2 mm long and about 0.5-0.8 mm wide; glumes somewhat coriaceous (sect. Tripsacum) (2)
Blades 9-35(45) mm wide, flat; culms 1-2(4) m tall ..... 2. T. dactyloides
Blades 1-7(15) mm wide, involute or folded; culms to 1 m tall ..... 3. T. floridanum

Tripsacum sect. Fasciculata Hitchc.

Staminate spikelets in sessile-pedicellate pairs, the pedicels slender and relatively flexible.

1.   Tripsacum lanceolatum Rupr. ex E. Fourn.
Mexican Gamagrass

Plants rhizomatous. Culms 1-2 m tall, 2-4 mm thick. Lower sheaths hispid; upper sheaths essentially glabrous; ligules erose, not ciliate; blades to 100 cm long, 8-30 mm wide, glabrous or slightly pubescent. Terminal inflorescences with 4-7(10) rames. Pistillate spikelets 2-3 mm wide, beadlike in appearance. Staminate spikelets in sessile-pedicellate pairs; glumes 5-10 mm long, 1.5-2 mm wide, usually membranous, acute; pedicels 2-5 mm long, less than 0.3 mm wide, almost flat to plano-convex in cross section, flexible. 2n = 72.

Tripsacum lanceolatum grows in moist soil (often in canyon bottoms) of mountains from southeastern Arizona and southwestern New Mexico through Mexico to Guatemala. It has not been found in New Mexico since the 1800s.

Tripsacum L. sect. Tripsacum

Staminate spikelets in sessile pairs or 1 member of each pair on a short, stout pedicel.

2.   Tripsacum dactyloides (L.) L.
Eastern Gamagrass

Plants with short, knotty rhizomes. Culms 1-2(4) m tall, 3-5 mm thick, clumped. Sheaths usually glabrous, occasionally slightly pilose; ligules ciliate; blades 30-75(120) cm long, 9-35(45) mm wide, flat, usually glabrous, tapering to attenuate apices. Terminal inflorescences erect, with (1)2-3(6) rames; rames 12-25 cm. Pistillate spikelets 6-8 mm long, 3-5.5 mm wide. Staminate spikelets all sessile or subsessile; glumes 5-12 mm, coriaceous, blunt, acute, or bifid; pedicels, when present, about 1 mm long, 0.5-0.8 mm wide, triangular in cross section, rigid. 2n = 36, 54, 72.

Tripsacum dactyloides grows in water courses and limestone outcrops from the central and eastern United States through Mexico to northern South America. Plants from the United States and northern Mexico belong to Tripsacum dactyloides (L.) L. var. dactyloides. They differ from those of the other two varieties in their erect stems and sessile staminate spikelets. Narrow-bladed plants of T. dactyloides from Texas resemble T. floridanum, but on transplanting to favorable conditions develop the wider blades characteristic of T. dactyloides. The two species can hybridize; the hybrids are partially sterile.

Growing Tripsacum dactyloides for forage has proven practical only in South America. It is also used as an ornamental grass, the chief attraction being its foliage.

3.   Tripsacum floridanum Porter ex Vasey
Florida Gamagrass

Plants with short, thick rhizomes. Culms to 1 m tall, to 2 mm thick, usually solitary or in small clumps. Sheaths glabrous; blades to 60 cm long, 1-7(15) mm wide, involute or folded, glabrous. Terminal inflorescences erect, with 1-2 rames. Pistillate spikelets 3.5-4.5 mm wide. Staminate spikelets sessile-pedicellate; spikelets 5-7 mm; glumes coriaceous, acute; pedicels to 2 mm long, to 0.5 mm wide, triangular in cross section. 2n = 36.

Tripsacum floridanum grows along roadsides and in pine woods, often in wet soils, of Florida and Cuba. It is grown as an ornamental, but it reseeds rather too readily under some conditions. Reports of T. floridanum from Texas are based on narrow-bladed specimens of T. dactyloides.