Desmodium

The characteristic fruit immediately sets mature material of this genus and Hylodesmum apart in our flora. The modified legume is a loment, consisting of (1) 2 or more 1-seeded, flat, indehiscent segments which are ultimately dispersed as separate units. These are covered with tiny hooked hairs and readily attach to fur or clothes.

The stipules in many species are deciduous, but they and the flowers may provide helpful characters when the fruit is not yet ripe. The flowers of all species are some shade of pink or purple, or whitish; they sometimes fade (or dry) to a greenish shade. This is a difficult genus, with a number of characters often having to be considered in order to make a sure identification of species.

1. Plant prostrate, trailing; leaflets suborbicular; stipules ovate, persisting and conspicuous, becoming ± reflexed.

D. rotundifolium

1. Plant erect; leaflets longer than broad; stipules various.

2. Leaflets suborbicular to ovate-oblong, rounded-obtuse at the apex, the terminal one about the same size as the lateral ones, up to 2.5 (–3) cm long.

3. Stem and leaves lightly hairy; mature pedicels ca. 4–9 (–10) mm long; petioles (3–) 6–9 (–12) mm long.

D. ciliare

3. Stems and leaves essentially glabrous; mature pedicels mostly 7–15 mm long; petioles (8–) 12–22 (–24) mm long.

D. marilandicum

2. Leaflets ± ovate to lanceolate and acute at the apex, or narrowly linear-oblong, the terminal one usually larger than the lateral ones and over 3.5 cm long.

4. Leaves sessile or nearly so, the petioles less than 3 mm long; leaflets narrowly oblong or linear.

D. sessilifolium

4. Leaves with petioles of middle (or all) leaves at least 4 mm long; leaflets ± ovate to lanceolate.

5. Lower surface of leaflets (at least on midvein and other main veins) with tiny hooked hairs (besides any other kinds); stipules persistent, ovate (short- or long-acuminate).

6. Axis of inflorescence with numerous spreading eglandular straight-tipped hairs longer than its diameter; stem or inflorescence branched, the plant with more than 1 raceme; leaflets weakly reticulate-veiny beneath; segments of mature fruit ca. 7–11 mm long.

D. canescens

6. Axis of inflorescence with all hairs shorter than its diameter, the longest ones glandular or hooked; stem often simple, the plant then with a single raceme; leaflets very strongly reticulate-veiny beneath; segments of mature fruit ca. 4–6.5 mm long.

D. illinoense

5. Lower surface of leaflets without hooked hairs (or a very few scattered ones present); stipules deciduous to somewhat persistent, lanceolate to linear.

7. Calyx less than 3 mm long (including the longest tooth); flowers less than 6 mm long; segments of fruit (1–) 2–3, rounded (not at all angled) along both sutures (more strongly on one than on the other).

D. obtusum

7. Calyx or flowers (or both) usually longer; segments of fruit mostly 2–4, rounded or triangular in aspect (triangular in species with small flowers).

8. Pubescence on lower surface of leaflets denser on the midvein than on the branch veins, and denser on these than on the smaller veins; petiole shorter than the width of the terminal leaflet; segments of fruit rounded in aspect, not triangular.

D. canadense

8. Pubescence (if any) on lower surface of leaflets quite uniform in density on all veins (and surface); petiole (at least on middle leaves) equaling or exceeding the width of the terminal leaflet (sometimes shorter in D. paniculatum); segments of fruit distinctly triangular in aspect (or even unequally diamond-shaped).

9. Stipules lanceolate, broadest just above the base, (9–) 11–18 (–21) mm long; terminal leaflet broadly acuminate, (9–) 10–15 cm long; segments of fruit 7–11 mm long; flowers ca. 10–13 mm long.

D. cuspidatum

9. Stipules narrowly triangular, broadest at the very base, 3–8 mm long; terminal leaflet elliptic-lanceolate, or less than 10 cm long, or both, not acuminate; segments of fruit (5.5–) 6–7.5 mm long; flowers (5–) 6–7.5 (–9.5) mm long.

10. Leaflets mostly (2.7–) 3–7 times as long as wide; middle and upper portions of stems glabrous or with scattered minute curved hairs.

D. paniculatum

10. Leaflets mostly 1.3–2.9 times as long as wide; middle and upper portions of stems usually with elongate straight or hooked hairs (or both).

11. Stem and petiole pubescence mostly of long straight or wavy hairs, often with an underlayer of fine puberulence, hooked hairs absent or sparse.

D. perplexum

11. Stem and petiole pubescence of sparse to dense hooked hairs, often with an underlayer of fine puberulence, long straight or wavy hairs absent or very sparse.

D. glabellum