Desmodium
tick trefoil
erect to diffusely spreading or decumbent to prostrate, pubescent, glabrous, or glabrescent.
alternate (rarely mostly clustered near base of stem), trifoliolate or unifoliolate;
stipules present, persistent or caducous, striate, ciliate, sometimes amplexicaul or subamplexicaul;
petiolate;
stipels usually present;
leaflets 1 or 3, blade margins usually entire, main lateral veins arcuate along margin, rarely reaching margin, surfaces usually pubescent, sometimes glabrous, abaxially paler, sometimes glaucous;
terminal leaflet petiolulate, larger than laterals, lateral leaflets 1-stipellate, blades usually oblique.
10–50+-flowered, axillary or terminal, usually pseudoracemes, usually 2–5 flowers per node, unbranched when axillary, branched and paniclelike (compound pseudoracemes) when terminal;
bracts present, primary bracts 1, subtending flower pairs, sometimes collectively conspicuous before flowering, secondary bracts usually 2, subtending each flower, sometimes absent;
bracteoles absent.
usually uncinate-puberulent, sometimes glutinous-pubescent.
papilionaceous;
calyx campanulate, lobes 5, usually appearing 4-lobed, adaxial pair connate except apically, with 2 minute teeth, lateral lobes equal to or slightly longer than tube, abaxial lobe longer than tube and other lobes, or sometimes as long as lateral lobes;
corolla usually pink to purple, blue-purple, or purple, sometimes lavender to greenish blue, ochroleucous, or white, banner clawed or tapering proximally, blade usually broadly obovate, often with a pair of dark spots (false nectar guides) at base;
wings and keel petals clawed;
stamens 10, usually monadelphous, sometimes diadelphous;
anthers dorsifixed.
loments, usually stipitate, rarely sessile, compressed, straight or ± incurved, sometimes twisted or contorted, oblong, narrowly oblong, or linear, splitting between indehiscent segments, usually conspicuously uncinate-pubescent throughout, rarely sutures glabrous;
segments (articles) (1 or)2–10, lateral faces reticulate (rugose in D. scorpiurus);
sutures constricted at joints, abaxial incised deeper than adaxial or both equally incised, connections between segments (isthmi) adaxial or central.
1 per segment, usually brown, yellowish brown, or tan, asymmetric, usually oblong, 4-sided, or reniform, rim-arillate.
= [10] 11.
Desmodium
Species ca. 120 (36, including 1 hybrid, in the flora).
Desmodium, as traditionally circumscribed, had its greatest diversity in southeastern Asia at infrageneric level and Mexico to northern South America at specific level (H. Ohashi 2005). More recent data (K. Ohashi et al. 2018) showed that Desmodium is polyphyletic. The traditional North American Desmodium (B. G. Schubert 1950; D. Isely 1990, 1998) was divided into two genera: Desmodium in the narrow sense adopted here and Hylodesmum, the previous Desmodium ser. Americana B. G. Schubert (H. Ohashi and R. R. Mill 2000). North American species of Desmodium are geographically separated into two groups: those of the central and eastern United States, and those of the Southwest (trans-Pecos Texas to Arizona), which mostly represent the northern peripheries of Mexican species (Isely 1990, 1998); they are otherwise absent west of the Rocky Mountains.
Most of the North American species of Desmodium are well established as natural species except for three species complexes that D. Isely (1990, 1998) designated as species groups due to the frequency of intermediate morphology among the traditional species. Each species complex, D. ciliare Group, D. paniculatum Group, and D. procumbens Group, is here regarded as a single species: D. marilandicum, D. paniculatum, and D. procumbens, respectively.
Few natural hybrids in Desmodium are known. Only D. × humifusum is demonstrated to have a hybrid origin in nature in North America (J. A. Raveill 2002). Forms intermediate between different species have been presumed to be natural hybrids (D. Isely 1990, 1998). B. G. Schubert (1970) stated that hybridization is rare, and the morphological characters are relatively stable; her concepts are followed here except as previously noted.
Mature loments are helpful in identifying Desmodium species; they consist of multiple segments that break into one-seeded units or articles at maturity. The shape of the segments and location of the connections between segments (isthmi) vary widely among species. Fruits typically have uncinate (hooked) hairs, facilitating dispersal via animal fur and human clothing.
(Discussion copyrighted by Flora of North America; reprinted with permission.)
1. Inflorescences fascicles, terminal (appearing axillary); leaflet blades broadly obovate or cuneate-obovate, terminal blade 5–10 mm; stems prostrate. | D. triflorum |
1. Inflorescences racemes or panicles, terminal or axillary; leaflet blades usually ovate, elliptic, or oblong, terminal blade (9–)10–120(–150) mm; stems erect, ascending, decumbent, or prostrate. | → 2 |
2. Leaves unifoliolate. | → 3 |
3. Leaflet blades linear, 2–5 mm wide, length 10+ times width; loments: stipe to 1 mm, sutures equally crenate, moderately uncinate-puberulent, segments 3–6, each (2–). | → 3–3 |
3–3. 5 mm, connections central. | D. gramineum |
3. Leaflet blades ovate to narrowly ovate, 10–30 mm wide, length (2–)2.5–5 times width; loments: stipe 1.5–2.5 mm, sutures crenate abaxially, sinuate adaxially, glabrous, segments 3–5, each 2.5–6 mm, connections adaxial. | D. psilophyllum |
2. Leaves usually trifoliolate, rarely unifoliolate proximally and/or distally. | → 4 |
4. Stems decumbent, decumbent-assurgent, procumbent, climbing, or prostrate (sometimes vinelike, rarely erect in D. procumbens). | → 5 |
5. Stipules persistent, amplexicaul or subamplexicaul. | → 6 |
6. Loment sutures equally shallow-undulate abaxially and adaxially, segments narrowly oblong or narrowly elliptic, 1.5 mm wide, faces rugose; stipules amplexicaul, 2–3.5 mm, base auriculate. | D. scorpiurus |
6. Loment sutures deeply crenate abaxially, crenate adaxially, segments subglobose or subrhombic to elliptic, 4–8 mm wide, faces reticulate; stipules subamplexicaul, 5–12 mm, base obliquely cordate. | → 7 |
7. Terminal leaflet blades ovate, length 1.2–2 times width; corollas white or ochroleucous, 7–8 mm; loments glabrous, sutures uncinate- pubescent. | D. ochroleucum |
7. Terminal leaflet blades elliptic, orbiculate, or broadly ovate, length 0.8–1.2 times width; corollas pink, fading blue-purple, 9–11 mm; loments uncinate-puberulent throughout. | D. rotundifolium |
5. Stipules caducous or persistent, not amplexicaul. | → 8 |
8. Leaves trifoliolate, often unifoliolate proximally and/or distally; leaflets polymorphic between proximal and/or distal ones and median ones. | D. procumbens |
8. Leaves trifoliolate, rarely unifoliolate proximally; leaflets monomorphic. | → 9 |
9. Terminal leaflet blades ovate-elliptic to narrowly so, 8–10 mm wide. | D. batocaulon |
9. Terminal leaflet blades ovate, elliptic, broadly ovate to rhombic, suborbiculate, or orbiculate, 10–60 mm wide. | → 10 |
10. Stems angular (3-sided); loment segments 5–9; primary bracts densely imbricate before anthesis, narrowly ovate, 7–10 mm. | D. intortum |
10. Stems terete; loment segments 2–6; primary bracts not imbricate, ovate, 1–6.5 mm. | → 11 |
11. Terminal leaflet blades ovate or rhombic-ovate, 30–70 mm, length 1.4–2 times width; loment segments obtusely angled abaxially. | Desmodium × humifusum |
11. Terminal leaflet blades orbiculate to broadly ovate or broadly rhombic-ovate, 15–45 mm, length 1–1.7 times width; loment segments symmetrically rounded abaxially. | → 12 |
12. Leaflets: adaxial surface appressed bulbous-pilose; inflorescence rachis densely patent uncinate-pubescent and usually bulbous-pilose; primary bracts 4.5–6.5 mm; calyces 5–6 mm; corollas 7–8 mm. | D. grahamii |
12. Leaflets: adaxial surface glabrous; inflorescence rachis uncinate-puberulent; primary bracts 1–2 mm; calyces 2 mm; corollas 3.5–5 mm. | D. lineatum |
4. Stems usually erect or ascending, rarely procumbent or sprawling. | → 13 |
13. Stipules persistent or partly persistent, amplexicaul or subamplexicaul. | → 14 |
14. Loments: connections adaxial, sutures dentate or crenate abaxially, sinuate adaxially. | D. canescens |
14. Loments: connections central, sutures equally or subequally crenate, or deeply crenate abaxially, shallowly dentate adaxially. | → 15 |
15. Loments sparsely pubescent, at least on sutures. | D. psilocarpum |
15. Loments densely uncinate-pubescent, at least on sutures. | → 16 |
16. Loment segments glabrous except uncinate-puberulent on sutures; leaflet blades densely villosulous abaxially. | D. lindheimeri |
16. Loment segments uncinate-puberulent throughout; leaflet blades subappressed-villous or uncinate-puberulent and strigose abaxially, or uncinate-puberulent only on veins. | → 17 |
17. Leaflets uncinate-puberulent on veins; calyces 4–5 mm; loments: segment margins not twisted, not rolled, connections 1/2–4/5 as broad as segments. | D. illinoense |
17. Leaflets uncinate-puberulent and strigose or subappressed-villous; calyces 1.5–3 mm; loments twisted conspicuously when young, margins alternately involute and revolute, connections 1/4 as broad as segments. | D. tortuosum |
13. Stipules persistent or caducous, not amplexicaul. | → 18 |
18. Inflorescence rachis and/or stems pilose or villous and uncinate-pubescent, sometimes glabrescent. | → 19 |
19. Terminal leaflet blades 5–7 mm wide, linear, length 8–10 times width. | D. tenuifolium |
19. Terminal leaflet blades 15–90 mm wide, elliptic, narrowly to broadly ovate, or broadly rhombic, length 1–4 times width. | → 20 |
20. Loments: adaxial sutures straight or slightly sinuate, connections 1/2–2/3 as broad as segments; herbs stoloniferous or rhizomatous. | D. incanum |
20. Loments: adaxial sutures usually sinuate, sometimes repand, connections 1/4–1/2 as broad as segments; herbs not stoloniferous. | → 21 |
21. Corollas 8–11 mm; primary bracts 6–7 mm; loment stipe 2(–3) mm; leaflet blades slightly or evidently strigulose abaxially, sparsely puberulent, almost glabrescent adaxially. | D. canadense |
21. Corollas 6–8 mm; primary bracts 2–4 mm; loment stipe 3–6 mm; leaflet blades spreading-villous or velvety abaxially, uncinate-puberulent on veins adaxially. | → 22 |
22. Corollas 6–7 mm; leaflet blades ovate to narrowly ovate, bases usually rounded; loment segments 2–4, rounded abaxially. | D. nuttallii |
22. Corollas 7–8 mm; leaflet blades broadly ovate or broadly rhombic, bases acute to cuneate or truncate; loment segments (3 or)4 or 5, symmetrically angled abaxially. | D. viridiflorum |
18. Inflorescence rachis and stems uncinate-pubescent, sometimes also sparsely pilose, glutinous, or glutinous-villous. | → 23 |
23. Stems conspicuously angled; leaflet blades usually with pale-blotched along midrib. | D. tweedyi |
23. Stems usually striate, not conspicuously angled (angled in D. metcalfei); leaflet blades without pale blotching along midrib. | → 24 |
24. Stipules usually caducous, sometimes moderately persistent. | → 25 |
25. Loments: distal segment much larger than proximal segments. | D. scopulorum |
25. Loments: distal segment size similar to proximal segments. | → 26 |
26. Loment segments angled abaxially. | → 27 |
27. Leaflets thick, ± leathery, adaxial surface glabrous or sparsely puberulent; pedicels 10–20 mm in fruit. | D. laevigatum |
27. Leaflets usually thin, rarely ± thick, papery, adaxial surface sparsely appressed-puberulent and pilose; pedicels to 12(–20) mm in fruit. | D. paniculatum |
26. Loment segments rounded abaxially. | → 28 |
28. Terminal leaflet blades narrowly oblong-elliptic to linear, 4–10 mm wide. | D. arizonicum |
28. Terminal leaflet blades mostly ovate or elliptic-ovate, 6–33 mm wide. | → 29 |
29. Corollas 4–6 mm; loment segments 1–4. | D. marilandicum |
29. Corollas 8–12 mm; loment segments (2 or)3–8. | → 30 |
30. Leaflet blades narrowly elliptic-oblong, apex obtuse, lateral veins reaching margin, margins flat; loments villosulous or pubescent and uncinate-puberulent, connections central. | D. cinerascens |
30. Leaflet blades narrowly ovate-oblong, apex acute, lateral veins looped within margin, margins revolute; loments uncinate-pubescent, connections slightly adaxial. | D. metcalfei |
24. Stipules persistent or moderately so. | → 31 |
31. Loment connections central, sutures ± equally crenate. | → 32 |
32. Loment margins involute, segments usually angled abaxially, sometimes rounded. | D. procumbens |
32. Loment margins flat (sometimes slightly involute in D. rosei), segments rounded abaxially. | → 33 |
33. Terminal leaflet blades 8–10 mm wide, usually oblong-ovate to narrowly so, sometimes broadly elliptic to oblong, length 1–3.5 times width; corollas 4–5 mm; herbs perennial, with woody rootstock. | D. retinens |
33. Terminal leaflet blades 2–5 mm wide, linear to narrowly oblong, length 7+ times width; corollas 3–3.5 mm; herbs annual, with slender taproot. | D. rosei |
31. Loment connections adaxial, sutures crenate or dentate abaxially, sinuate or straight adaxially (dentate in D. cuspidatum). | → 34 |
34. Terminal leaflet blades linear to narrowly oblong or narrowly elliptic, length 4–10 times width. | → 35 |
35. Petioles 1–5 mm; pedicels 2–5 mm; corollas 5 mm. | D. sessilifolium |
35. Petioles 6.5–18 mm; pedicels 5–13 mm; corollas 4 mm. | D. strictum |
34. Terminal leaflet blades ovate, broadly ovate, or rhombic, length 1.3–3 times width. | → 36 |
36. Leaves trifoliolate proximally, leaflet blade apex sharply acuminate to shortly cuspidate; loment segments (7–)9–11 mm; corollas 8–12 mm; calyces glabrate, margins sparsely ciliate; stems usually glabrous, sometimes sparsely uncinate-puberulent or pilose. | D. cuspidatum |
36. Leaves unifoliolate proximally, leaflet blade apex acute or obtuse; loment segments 4–8 mm; corollas 5–7 mm; calyces puberulent to pubescent; stems usually densely uncinate- puberulent to -pubescent and villous. | D. floridanum |
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