17.10   TRIDENS Roem. & Schult.
Jesús Valdés-Reyna

Plants perennial; usually cespitose, often with short, knotty rhizomes, occasionally with elongate rhizomes, never stoloniferous. Culms 5-180 cm, erect, mostly glabrous, lower nodes sometimes with hairs. Sheaths shorter than the internodes, open; ligules membranous and ciliate or of hairs; blades 6-25 cm long, 1-8 mm wide, flat or involute, margins not thick and cartilaginous. Inflorescences terminal, usually panicles (sometimes reduced to racemes), 5-40 cm, exceeding the upper leaves, exserted. Spikelets 4-10(13) mm, laterally compressed, with 4-11(16) florets, more than 1 floret bisexual; sterile florets distal to the fertile spikelets; disarticulation above the glumes. Glumes from shorter than to equaling the distal florets; lower glumes 1(3)-veined; upper glumes shorter than or about equal to the lower glumes, 1-3(9)-veined, unawned; calluses usually glabrous, sometimes pilose; lemmas hyaline or membranous, 3-veined, veins usually shortly hairy below, apices rounded to truncate, emarginate to bilobed, midvein often excurrent to 0.5 mm, lateral veins not or more shortly excurrent; paleas glabrous or shortly pubescent on the lower back and margins, veins glabrous or ciliolate; lodicules 2, free or adnate to the palea; anthers 3, reddish-purple. Caryopses dorsiventrally compressed and reniform in cross section, dark brown; embryos about 2/5 as long as the caryopses. x = 10. Name from the Latin tres, three, and dens, tooth, referring to the three shortly excurrent veins of Tridens flavus, the type species.

Tridens, a genus of 14 species, is native to the Americas; all ten species described here are native to the the Flora region. Hitchcock (1951) included both Erioneuron and Dasyochloain Tridens; Tateoka (1961) demonstrated that they should be excluded. One of the differences between Tridens and the other two genera lies in their chromosome bases numbers, 10 in Tridens and 8 in Erioneuron and Dasyochloa. Tridens albescens is exceptional within Tridens in having chromosome numbers that suggest two base numbers, 10 and 8.


SELECTED REFERENCES Burbidge, N.T. 1953. The genus Triodia R. Br. (Gramineae). Austral. J. Bot. 1:121-184; Gould, F.W. 1975. The grasses of Texas. Texas A&M University Press, College Station, Texas, U.S.A. 653 pp.; Hitchcock, A.S.1951 [title page 1950]. Manual of the Grasses of the United States, ed. 2, rev. A. Chase. U.S.D.A. Miscellaneous Publication No. 200. U.S. Government Printing Office, Washington, D.C., U.S.A. 1051 pp.; Tateoka, T. 1961. A biosystematic study of Tridens (Gramineae). Amer. J. Bot. 48:565-573.

1
Primary panicle branches appressed to strongly ascending; panicles 0.3-4 cm wide, dense and spikelike (2)
Primary panicle branches ascending to reflexed or drooping; panicles 1-20 cm wide, open, not spikelike (6)
2
Lateral veins of the lemmas glabrous or pubescent only at the base ..... 1.T. albescens
Lateral veins of the lemmas pilose to well above the base (3)
3
Glumes evidently longer than the adjacent lemmas, often twice as long, usually equaling or exceeding the distal florets ..... 2. T. strictus
Glumes from shorter than to equaling the adjacent lemmas, often exceeded by the distal florets (4)
4
All 3 lemma veins shortly excurrent; calluses pilose ..... 3. T. carolinianus
Lateral lemma veins not excurrent, often terminating before the distal margin, the midvein sometimes excurrent; calluses glabrous or shortly pilose (5)
5
Panicles 7-25 cm long, 0.3-0.8 cm wide; lemma midveins rarely excurrent ..... 4. T. muticus
Panicles 5-8(10) cm long, 1.2-2.5 cm wide; lemma midveins always shortly excurrent ..... 5. T. congestus
6
All pedicels shorter than 1 mm ..... 6. T. ambiguus
Some pedicels longer than 1 mm (7)
7
Lateral veins of the lemmas rarely excurrent (8)
Lateral veins of the lemmas commonly excurrent as short points (9)
8
Lemmas 4-6 mm; ligules 0.4-1 mm ..... 7. T. buckleyanus
Lemmas 2-3.2 mm; ligules 1.2-3 mm ..... 8. T. eragrostoides
9
Blades 1-5 mm wide; panicles 5-16 cm long ..... 9. T. texanus
Blades mostly 3-10 mm wide; panicles 15-40 cm long ..... 10. T. flavus


1.   Tridens albescens (Vasey) Wooton & Standl.
White Tridens

Plants cespitose, often with hard, knotty, shortly rhizomatous bases. Culms 30-100 cm; lower nodes sometimes sparsely bearded. Sheaths glabrous, not or obscurely keeled; ligules to 0.5 mm, membranous, ciliate; blades 1-4 mm wide, folded or involute, glabrous, apices sharp. Panicles 8-25 cm long, 0.5-1.3 cm wide, dense; branches appressed, lowest branches 2-6 cm; pedicels 1-2 mm. Spikelets 4-10 mm, with 4-11 florets. Glumes about as long as the adjacent lemmas, thin, 1-veined, acute or apiculate; lower glumes 4-4.5 mm; upper glumes 4-4.5 mm; lemmas 3-4(5) mm, thin, papery, mostly white, often purple distally, glabrous or the lateral veins with a few short hairs towards the base, all veins ending before the distal margin; paleas 3-3.5 mm, glabrous, bowed-out at the base; anthers 1-1.5 mm. Caryopses 1.5-1.8 mm. 2n = 60, 64, 72.

Tridens albescens grows in plains and open woods, often in clay soils that periodically receive an abundance of water. Its range extends into northern Mexico.


2.   Tridens strictus (Nutt.) Nash
Longspike Tridens

Plants with hard, knotty, shortly rhizomatous bases. Culms 50-170 cm, stiffly erect. Sheaths rounded, glabrous except for a few hairs on either side of the collar; ligules about 0.5 mm, membranous, ciliate; blades 2-8 mm wide, flat or loosely infolded, glabrous, tapering to the apices. Panicles 10-30(36) cm long, 1-2 cm wide; branches to 6 cm, erect or appressed; pedicels 1-1.5 mm, glabrous. Spikelets 4-7 mm, with 5-11 florets. Glumes 4-7 mm, always conspicuously exceeding and often twice as long as the adjacent lemmas, usually equaling or exceeding the distal florets, glabrous, 1-veined, tapering to acuminate apices; calluses pilose; lemmas (2)3-3.5 mm, veins pilose to well above midlength, lateral veins often excurrent; paleas 2-3 mm, bases not bowed-out; anthers 1-1.5 mm. Caryopses 1-1.5 mm. 2n = 40.

Tridens strictus grows in open woods, old fields, right of ways, and coastal grasslands. It is endemic to the United States.


3.   Tridens carolinianus (Steud.) Henrard
Creeping Tridens

Plants rhizomatous; rhizomes elongate, 2.5-5 mm thick, scaly. Culms 80-120 cm. Lower sheaths pilose; ligules about 0.5 mm, membranous, ciliate; blades 2-7 mm wide, flat, both surfaces sparsely pilose basally, margins smooth or scabridulous; upper leaves with glabrous sheaths and blades. Panicles 9-15 cm long, 1-4 cm wide, nodding, purplish; branches appressed or narrowly ascending; pedicels 2-3(3.5) mm. Spikelets 7-10 mm, with 3-5 florets. Glumes glabrous, 1-veined; lower glumes 3.5-4.5 mm; upper glumes 4-5 mm; calluses sparsely pilose; lemmas 4-5 mm, veins pilose at least to midlength, all 3 veins excurrent as short points; paleas 3-3.5 mm, glabrous, bases bowed-out; anthers 1-2(2.5) mm. Caryopses 2-2.5 mm. 2n = unknown.

Tridens carolinianus grows in pinelands and open sandy woods along the coastal plain from North Carolina to Louisiana.


4.   Tridens muticus (Torr.) Nash
Slim Tridens

Plants cespitose, with knotty, shortly rhizomatous bases. Culms 20-80 cm; nodes often with soft, 1-2 mm hairs. Sheaths rounded, lower sheaths often strigose or pilose, upper sheaths glabrous or scabrous; ligules 0.5-1 mm, membranous, ciliate; blades 1-4 mm wide, usually involute or loosely infolded, glabrous, scabrous, or sparsely pilose, attenuate distally. Panicles 7-20(25) cm long, 0.3-0.8 cm wide; branches erect, spikelets imbricate but usually not crowded; pedicels 1-2 mm. Spikelets 8-13 mm, with 5-11 florets. Glumes glabrous, usually purple-tinged; lower glumes 3-8(10) mm, 1-3-veined; upper glumes 4-10 mm, 1-7-veined; lemmas 3.5-7 mm, usually purple-tinged, midveins pilose on the basal 1/3-1/2, rarely excurrent, lateral veins pilose to well above midlength, never excurrent; paleas 1-2 mm shorter than the lemmas, margins pubescent; anthers 1-1.5 mm. Caryopses 1.5-2.3 mm. 2n = 40.

1
Upper glumes 4-5(6) mm long, 1-veined ..... var. muticus
Upper glumes usually 5.5-10 mm long, 3-7-veined ..... var.elongatus


Tridens muticus var. elongatus (Buckley) Shinners

Culms usually 40-80 cm. Blades often 3-4 mm wide. Upper glumes usually 5.5-10 mm, 3-7-veined.

Tridens muticus var. elongatus grows on well-drained, clayey and sandy soils from Colorado to Missouri and from Arizona to Louisiana. It is not known from Mexico.


Tridens muticus (Torr.) Nash var. muticus

Culms usually 20-50 cm. Blades 1-2 mm wide. Upper glumes 4-5(6) mm, 1-veined.

Tridens muticus var. muticus is a common species on dry, sandy or clay soils in the arid southwestern United States and adjacent Mexico.


5.   Tridens congestus (L.H. Dewey) Nash
Pink Tridens

Plants cespitose, with shortly rhizomatous bases. Culms 30-75 cm. Sheaths glabrous, rounded; ligules to 0.5 mm, membranous, ciliate; blades 1.5-5 mm wide, tapering and involute distally. Panicles 5-8(10) cm long, 1.2-2.5 cm wide, dense; branches 0.5-3 cm, erect to ascending; pedicels 1-3 mm. Spikelets 5-10 mm, with 5-12 florets. Glumes glabrous, usually tinged with pink, 1-veined; lower glumes 3-4.5 mm; upper glumes 4-4.5(5) mm; lemmas 3-5 mm, usually tinged with pink, midveins and margins pubescent on the basal 1/3-1/2, midveins shortly excurrent, lateral veins pilose to midlength, terminating before the distal margins; paleas 0.5-1 mm, shorter than the lemmas, scabrous on the veins, broadened below, bases bowed-out at maturity; anthers 1-1.5 mm. Caryopses (1)1.5-2 mm. 2n = unknown.

Tridens congestus grows in moist depressions, ditches, and low flats of otherwise dry hills in Texas. It resembles T. albescens, but usually has shorter panicles, spikelets that are more or less evenly pink rather than purple-tipped, and more deeply cleft lemma apices.


6.   Tridens ambiguus (Elliott) Schult.
Pine-Barren Tridens

Plants cespitose, with knotty, shortly rhizomatous bases. Culms 60-125 cm. Sheaths rounded or the basal sheaths keeled, glabrous, except for a few hairs on either side of the collar; ligules 1-2 mm, membranous, ciliate; blades 2-5 mm wide, elongate, usually involute distally. Panicles 8-16(20) cm long, 1.5-4 cm wide, not dense; branches to 8(10) cm, erect to divergent, stiff; pedicels shorter than 1 mm. Spikelets 4-6 mm long, pale to dark purple, with 4-6 florets. Glumes 1-veined; lower glumes 4-4.5 mm; upper glumes about 5 mm; lemmas 3-4 mm, veins pubescent to midlength or beyond, midveins excurrent, lateral veins often excurrent; paleas 3-3.5 mm, veins ciliolate, bases bowed-out; anthers 1-1.5 mm. Caryopses 1.5-1.8 mm. 2n = 40.

Tridens ambiguus grows on the southeastern coastal plain, from North Carolina to Texas. It is usually found in mesic to perennially moist soils of pine flatwoods and pine-oak savannahs, in seasonally inundated depressions, and at the margins of pitcher plant bogs, often in disturbed sites.


7.   Tridens buckleyanus (L.H. Dewey) Nash
Buckley's Tridens

Plants cespitose. Culms 30-60 cm, erect; lower nodes sometimes hispid. Sheaths scabridulous, rounded; ligules 0.4-1 mm, membranous, ciliate; blades 1-4 mm wide, flat, apices attenuate. Panicles 10-28 cm long, 1-6 cm wide; branches (2)4-13 cm, widely-spaced, ascending to spreading; pedicels 1-2 mm. Spikelets 7-10 mm, pale to dark purple, with 2-5 florets. Glumes 1-veined; lower glumes 4-5(6) mm; upper glumes slightly shorter; lemmas 4-6 mm, midveins and back pubescent below midlength, lateral veins pubescent to well above midlength, midveins occasionally excurrent, lateral veins usually ending before the distal margins, rarely excurrent; paleas 3.5-4 mm, veins pubescent basally; anthers 1.5-2 mm. Caryopses about 2.5 mm. 2n = 32, but this may be erroneous (Gould 1975).

Tridens buckleyanus is endemic to the southeastern portion of the Edwards Plateau, Texas. It grows on rocky slopes along shaded stream banks and the borders of woodlands.


8.   Tridens eragrostoides (Vasey & Scribn.) Nash
Lovegrass Tridens

Plants cespitose, with knotty, shortly rhizomatous bases. Culms 50-100 cm; nodes sometimes sparsely bearded. Sheaths glabrous, scabrous, or sparsely pilose, rounded; ligules 1.2-3 mm, glabrous, membranous, usually lacerate; blades 10-15 cm long, 1.5-5 mm wide, scabrous (occasionally sparsely pilose), apices long-attenuate. Panicles 10-30 cm long, to 20 cm wide, open; branches 5-10(12) cm, lax, ascending to reflexed at maturity, proximal internodes longer than the distal internodes; pedicels (1.5)3-5 mm. Spikelets 3-7 mm, with 5-12 florets. Glumes glabrous, 1-veined, purple; lower glumes 2-2.5 mm; upper glumes 2-3.5 mm; lemmas 2-3.2 mm, veins puberulent to well above midlength, midveins sometimes excurrent, lateral veins rarely reaching the distal margins; paleas 1.5-2 mm, glabrous or scabrous basally, neither enlarged nor bowed-out; anthers 1-1.5 mm. Caryopses 1-1.3 mm. 2n = 40.

Tridens eragrostoides grows in brush grasslands, generally in partial shade. Its range extends from the southern United States into Mexico and Cuba.


9.   Tridens texanus (S. Watson) Nash
Texas Tridens

Plants cespitose, with knotty, shortly rhizomatous bases. Culms 20-75 cm, slender, strictly erect; nodes glabrous; internodes often pilose. Sheaths mostly glabrous or pilose throughout, collar and distal portion of the margins densely pilose; ligules to 0.5 mm, membranous, ciliate; blades 1-3(5) mm wide, flat or becoming inrolled, hispid, with long hairs on the adaxial surface just above the ligule, apices attenuate. Panicles 5-16 cm long, 2-9 cm wide, open or loosely contracted; branches (2)4-7 cm, slender, lax, strongly divergent to drooping, basal portion naked, spikelets confined to the distal portion; pedicels (2)3-6 mm. Spikelets 6-13 mm, with 6-12 florets. Glumes glabrous, 1-veined; lower glumes 3 mm; upper glumes 3.5-4 mm, veins bright green; lemmas 3-4.5 mm, usually purple or rosy-purple at maturity, veins pilose to midlength, lateral veins often excurrent as short points; paleas 3-3.5 mm, glabrous, abruptly broadened and bowed-out below; anthers 1-1.5 mm. Caryopses 1.5-2 mm. 2n = 40.

Tridens texanus grows in clayey and sandy loam soils, often in the protection of shrubs and along fenced road right of ways. Its range extends from southern Texas into northern Mexico.


10.   Tridens flavus (L.) Hitchc.
Purpletop Tridens

Plants with firm, knotty, shortly rhizomatous bases. Culms 60-180 cm. Sheaths keeled, mostly glabrous, collars pubescent; ligules to 0.5 mm, membranous, ciliate; blades 3-10 mm wide, glabrous or sparsely hispid, apices attenuate and involute. Panicles 15-40 cm long, 3-5 cm wide, erect or nodding; branches 10-15(25) cm, strongly divergent to drooping, stiff or lax, lower branches naked for 1/3-1/2 of their length; pedicels 3-8 mm. Spikelets 5-10 mm long, with 4-8 florets. Lower glumes 2.5-3 mm, often mucronate; upper glumes 3.5-4 mm; lemmas 3-5 mm, lateral veins puberulent or ciliate to well above midlength, midveins and lateral veins usually excurrent, midveins extending to 0.5 mm, lateral vein extensions shorter; paleas as long as the lemmas, widened below; anthers 1-1.5 mm. Caryopses 1.8-2 mm. 2n = 40.

1
Panicles nodding; pulvini inconspicuously or conspicuously hairy, the hairs confined to the adaxial side of the branches ..... var. flavus
Panicles erect throughout; pulvini always conspicuously hairy, the hairs extending around the base of the branches ..... var. chapmanii


Tridens flavus var. chapmanii (Small) Shinners

Panicles usually erect throughout; branches stiff; pulvini conspicuously hairy, hairs extending around the base of the branches.

Tridens flavus var. chapmanii grows in pine and oak woods of the southeastern United States from Missouri to Virginia and south from eastern Texas to Florida.


Tridens flavus (L.) Hitchc. var. flavus

Panicles nodding distally; branches flexible; pulvini inconspicuously or conspicuously hairy, hairs confined to the adaxial side of the branches.

Tridens flavus var. flavus grows in old fields and open woods. Its range extends to Nuevo Léon, Mexico. It was discovered for the first time in Canada in 1976, growing along a railway track in southern Ontario.