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Key to Desmodium

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1 Leaves all unifoliolate
1 Leaves (all, or at least the upper leaves) trifoliolate.
..2 Longest calyx lobes shorter than the calyx tube; stipe of the loment 4-20 mm long, about 3× or more as long as the calyx; mature leaves without stipels at the base of the petiolules of the leaflets; leaves subverticillate at the top of the stem (alternate in D. pauciflorum); stamens monadelphous; lower margin of the loment incised to the upper suture
..2 Longest calyx lobes longer than the calyx tube; stipe of the loment absent or nearly so, included within the calyx; mature leaves retaining stipels at the base of the petiolules of the leaflets; leaves alternate; stamens diadelphous; lower margin of the loment not incised to the upper suture.
....3 Leaflets narrow, the terminal leaflet < 14 mm wide, and also 4-12× as long as wide, typically thick and strongly reticulate; petioles of midstem leaves 1-10 (-15) mm long; [primarily of the Coastal Plain and lower Piedmont (rarely Mountains) in our area]
....3 Leaflets broader, the terminal leaflet > 15 mm wide, or < 4× as long as wide, typically thin and not reticulate; petioles of midstem leaves various, but > 15 mm long if leaflet proportions are narrow; [collectively widespread in our area].
......4 Stems trailing vinelike along the ground, and/or the plants stoloniferous or rhizomatous
......4 Stems erect or ascending, not vinelike.
........5 Stipules (5-) 7-20 mm long, > 1 mm wide at the base, clasping or truncate at the base, persistent (most or all of the stipules persisting through the year), 4-20 mm long; leaflets 1.5-3× as long as wide
........5 Stipules 2-6 (-9.5) mm long, < 1 mm wide at the base, narrowed or parallel-sided to the base, caducous (most or all of the stipules falling soon after expansion of the leaves); leaflets 1-8 (-10)× as long as wide

Key A: Desmodium with very narrow leaflets

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1 Petioles (0-) 1-3 (-4) mm long, the leaves thus subsessile; leaflets 5-10 mm wide, strongly pubescent on the lower surface
1 Petioles 3-15 mm long, the leaves thus obviously petiolate; leaflets 2-5 (-8) mm wide, glabrate or inconspicuously puberulent on the lower surface.
..2 Loment segments flat to distinctly concave along the upper (suture) margin; [of dry to mesic habitats]
..2 Loment segments slightly convex along the upper (suture) margin; [of boggy, wet, or mesic habitats]

Key B: Desmodium with trailing stems or stoloniferous-rhizomatous habit

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1 Flowers in axillary clusters; leaflets broadly obovate, 0.5-1 cm long
1 Flowers in axillary or terminal racemes; leaflets rotund, broadly ovate, broadly elliptic, or narrowly elliptic, the larger > 1 cm long.
..2 Loments linear, ca. 1.5 mm wide, the sinuses symmetrical above and below, only slightly constricted relative to the width of the loment
..2 Loments elongate, > 3 mm wide, the sinuses assymmetrical, notched more deeply below, and obviously constricted to < half of the width of the loment.
....3 Flowers in simple terminal racemes; leaflets mostly > 2.5× as long as wide
....3 Flowers either in axillary racemes or in terminal panicles; leaflets mostly 0.9-2.0× as long as wide.
......4 Stipules ovate, persistent, slightly to strongly clasping at the base, 6-12 mm long.
........5 Leaflets ovate, 1.2-1.9× as long as wide; flowers white to yellowish; loment uncinate-puberulent only along the sutures
........5 Leaflets ovate, 0.8-1.1× as long as wide; flowers blue-purple; loment uncinate-puberulent over the surface
......4 Stipules lanceolate to linear (or deltate in D. humifusum), usually quickly deciduous, not clasping at the base, 2-8 mm long.
..........6 Terminal leaflet 1.4-2.0× as long as wide, 3.0-7.0 cm long; loment segments 6-8 mm long; stipules 4-8 mm long, ovate to lance-acuminate
..........6 Terminal leaflet 0.9-1.2× as long as wide, 1.5-2.3 cm long; loment segments 4-5 mm long; stipules lanceolate, 1-5 mm long

Key C: Subkey in Desmodium

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1 Loment segments 3-5.5 mm long, nearly symmetrical along the axis of the loment (the isthmi more or less equal above and below, thus each segment diamond-shaped, rounded-diamond-shaped, or essentially elliptical).
..2 Leaflet undersurfaces with hooked hairs, mainly or only on the veins; loment segments 4-5.5 mm long; perennial; [OH, IN, IL, AR, OK, and TX plains northward]
..2 Leaflet undersurcaes villous, strigose, or also with hooked hairs; loment segments 3-3.5 mm long; annual from a taproot; [NC south to s. FL, west to AR and TX, mainly Coastal Plain]
1 Loment segments 5-11 mm long, asymmetrical along the axis of the loment (the isthmi deeper below than above, thus each segment triangular, rounded-triangular, or semi-circular).
....3 Leaflets with conspicuous pale variegation along the midvein; corollas 7-8 mm long, white ('fading' light pink); [Crosstimbers and Blackland Prairies of TX and OK]
....3 Leaflets uniformly green; corollas 6-13 mm long, bright pink-purple; [collectively widespread]
......4 Corolla 6-7 mm long; loment with 2-4 segments, each 5-7 mm long; lower leaves often 1-foliolate; [of se. SC and southward]
......4 Corolla 8-13 mm long; loment with 4-6 segments, each 6.5-11 mm long; lower leaves usually 3-foliolate; [collectively widespread in our area].
........5 Stem densely spreading pilose (at least the upper stem) and also uncinate-puberulent; loment segments 6.5-10 mm long
........5 Stem glabrous or uncinate-puberulent; loment segments 9-11 mm long

Key D: Subkey in Desmodium

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1 Corolla 8-11 mm long; petioles mostly 1-8 mm long; [DE, MD, VA, KY, MO, OK northwards]
1 Corolla 3-8 (-9) mm long (or 8-10 mm long in D. laevigatum); petioles mostly longer, mostly 10-60 mm long (except D. ciliare and D. obtusum); [collectively widespread in our area].
..2 Loment with 1-3 segments, rounded below.
....3 Leaflets cinereous on the lower surface; corolla 6-7 mm long; loment with 3 (-4) segments
....3 Leaflets not cinereous on the lower surface; corolla 3.5-6 mm long; loment with 1-2 (-3) segments; [“Desmodium ciliare group”].
......4 Leaflets 3-5.5× as long as wide
......4 Leaflets 1.2-3.5× as long as wide.
........5 Terminal leaflet usually distinctly longer and narrower than the lateral leaflets; stem (near the middle) sparsely to densely uncinate-pubescent
........5 Terminal leaflet similar to the lateral leaflets; stem (near the middle) glabrous to pilose, or also with some uncinate-pubescence.
..........6 Petioles 1-3 (-5) mm long; pedicels 3-8 mm long; stem usually pilose; leaflets sub-appressed pubescent (to glabrate)
..........6 Petioles 10-25 mm long; pedicels 8-15 mm long; stem glabrous (to sparsely uncinate-puberulent); leaflets glabrous or with only a few scattered hairs
..2 Loment with 2-5 segments, mostly obtusely angled below.
............ 7 Leaves densely villous on the lower surface; stem densely pubescent with uncinate or non-uncinate hairs.
............ ..8 Leaflets 1.5-2.0 (-2.2)× as long as wide; loment usually curved (the upper margin convex); loment with 2-4 segments; loment segments 4-7 mm long
............ ..8 Leaflets 1.0-1.5 (-1.9)× as long as wide; loment straight; loment with (3-) 4-5 (-6) segments; loment segments 5-8 (-9) mm long
............ 7 Leaves glabrous to moderately appressed-villous on the lower surface; stem glabrate, pilose or uncinate pubescent.
............ ....9 Bracts (subtending clusters of 2-3 flowers) usually villous; plants moderately to densely villous; loment usually incurved (the upper margin convex); loment with 2-4 segments, each segment 4-5 mm long
............ ....9 Bracts (subtending clusters of 2-3 flowers) not villous; plants glabrous or slightly to moderately villous or pilose; loment usually nearly straight; loment with 3-5 segments, each segment 4-8.5 mm long.
............ ......10 Corolla 8-10 mm long; pedicels usually 10-15 (-20) mm long; stems and leaves glabrous; leaflets distinctly pale on the lower surface
............ ......10 Corolla 6-8 (-9) mm long; pedicels 3-12 mm long; stems and leaves pubescent or glabrate (but pubescent at least on the leaves); leaflets green or slightly pale on the lower surface; [“Desmodium paniculatum group”].
............ ........11 Leaflet lower surface glabrous, except for the conspicuous uncinate puberulence on the veins; stems and petioles glabrous or uncinate-puberulent; [plant of the Coastal Plain and possibly lower Piedmont]
............ ........11 Leaflet lower surface strigose to conspicuously sub-appressed-villous, and sometimes also uncinate-puberulent; stems and petioles glabrate to conspicuously pilose or uncinate-puberulent; [plants collectively widespread in our area].
............ ..........12 Leaflets (2.5-) 3-8 (-10)× as long as wide; leaflet pubescence usually sparse, of straight, appressed hairs < 0.5 mm long (or sometimes of longer spreading hairs); leaflets usually lacking uncinate pubescence on either surface; mid-stems glabrous or glabrate, the pubescence usually uncinate puberulence.
............ ............ 13 Loment segments rounded on the lower margin (thus semicircular to gibbous); leaves subsessile to short-petiolate; [plant restricted to Coastal Plain]
............ ............ 13 Loment segments angled on the lower margin (thus triangular to sub-rhombic); leaves long-petiolate; [plant widespread in our area]
............ ..........12 Leaflets 1.5-3 (-4)× as long as wide; leaflet pubescence usually evident, of spreading hairs > 0.5 mm long; leaflets usually with uncinate pubescence on the veins of the upper surface; mid-stems pubescent, either pilose or with uncinate pubescence (if not, evidently pubescent on the petioles).
............ ............ ..14 Each side (from the isthmus on one side of the article to the broadest point) of the ventral margin of most articles straight to slightly convex; leaves gradually but noticeably reduced in size and petiole length distally along the stem, often with small leaves extending well onto the flowering branches; terminal leaflets lanceolate to broadly ovate (most specimens, especially of full sun habitats, are on the narrow end of this range), broadest nearer the base than the middle
............ ............ ..14 Each side (from the isthmus on one side of the article to the broadest point) of the ventral margin of most articles concave; leaves mostly all the same size and not or little extending onto the flowering branches (the few leaves that do extend into the inflorescence branches will also be reduced in size but more abruptly so); terminal leaflets narrowly ovate to broadly elliptic-ovate (most specimens are on the wider end of this range), broadest nearer the middle than the base
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