Desmodium paniculatum (L.) DC. (tall tick trefoil)
Pl. 394 e, f; Map 1745
Stems 50–150 cm long, erect or
ascending, sometimes arched, rarely spreading with an ascending tip, often
branched, the median portion sparsely pubescent with appressed hairs or with
scattered straight, spreading and/or hooked hairs, rarely completely glabrous
or densely covered with straight, spreading and/or hooked hairs. Petioles of
the median leaves 1–5 cm long, only slightly reduced toward the stem tip, those
of the upper leaves 1–5 cm long. Stipules 2–6 mm long, 0.3–1.0 mm wide,
narrowly triangular, tapered to a sharply pointed tip, at maturity brown and
appressed, shed early or less commonly semipersistent. Leaflets flat or nearly so,
the undersurface sparsely pubescent with straight, appressed or rarely
spreading hairs, the network of raised veins relatively inconspicuous. Central
leaflet (1.5–)3.5–8.0(–10.0) cm long, (0.2–)0.5–2.5 cm wide (generally 3–8
times as long as wide), narrowly oblong, or lanceolate, the tip bluntly to
sharply pointed. Lateral leaflets (1.5–)2.5–7.0(–8.0) cm long, (0.2–)0.5–2.0 cm
wide. Stipels 1–4 mm long. Inflorescences terminal and from the upper leaf
axils, branched, the axis with hooked and sometimes straight, spreading hairs.
Primary bracts, 1.5–3.0 mm long, lanceolate, tapered to a sharply pointed tip,
mostly shed early. Secondary bracts 1.0–1.5 mm long, linear or hairlike, shed
early or semipersistent. Flower stalks 4–11 mm long. Calyces green, with scattered
appressed hairs and dense very short hairs, the tube 1.0–1.5 mm long, the lobes
2–3 mm long. Corollas 5–8 mm long, pink, often fading to bluish purple or blue,
the nectar guides green and white outlined in purple. Fruits straight in
outline, the stalklike base 2–5 mm long (extending past the persistent calyx),
consisting of (1–)4 or 5 segments, each 5–7 mm long and 3–5 mm wide, bluntly
angled to more or less rounded on the upper margin, bluntly angled on the lower
margin, with deeper indentation below (rarely rounded below and with nearly
equal indentations above and below), the margins and faces uniformly covered
with hooked hairs. 2n=22.
July–September.
Scattered to common nearly throughout
the state (eastern U.S. west to Nebraska and Texas; Canada). Bottomland
forests, openings and edges of mesic to dry upland forests, tops of bluffs,
glades, savannas, upland prairies, banks of streams and rivers, edges of
marshes and fens, and margins of ponds and lakes; also old fields, pastures,
levees, railroads, and roadsides.
This species, along with the
broader-leafleted D. perplexum and D. glabellum, presents one of the most
difficult complexes in the Missouri flora. Although many specimens can be
readily classified as one of the three species, a substantial proportion seems
to bridge the morphological gaps between them (Isely, 1983b). Isely (1990)
suggested that plants of intermediate morphology might represent hybrids
between D. paniculatum and one or
both of these other two species. Some other authors have chosen to combine all
three taxa into D. paniculatum and to
recognize only two varieties: the nominate variety with narrow leaflets and
less conspicuous pubescence; and the other variety with broader leaflets and
conspicuous pubescence (Steyermark, 1963). The latter has been called var. dillenii (Darl.) Isely and encompasses
both D. glabellum and D. perplexum. The present recognition of
three species is tentative and further research is needed. Adding to the
complexity of the group is a tendency for some collections, especially from
northeastern Missouri, to have larger flowers, longer stipules, and rounded
fruit segments, somewhat similar to those of D. canadense. Isely (1990) noted the existence in the Southeast of
occasional plants resembling D. paniculatum,
but with very short petioles. He suggested that such plants might represent
hybrids between this species and D.
ciliare. Such plants eventually may be found in Missouri, as the two
putative parents frequently grow in proximity. Evidence for hybridization with D. rotundifolium is discussed under that species.