2. Senna obtusifolia (L.) H.S. Irwin & Barneby (sickle pod, coffee weed)

Cassia obtusifolia L.

C. tora L.

Map 1697, Pl. 386 f

Plants annual (sometimes short-lived perennials farther south), producing a disagreeable odor when bruised or crushed. Stems 1 to several, (5–)30–100 cm long, erect or ascending, usually unbranched, with scattered, minute glandular hairs and sometimes also sparse, short, appressed or incurved hairs toward the tip. Leaves with the petiole 2–4 cm long, the petiolar gland positioned between the lowermost leaflets (rarely immediately below or somewhat above the lowermost pair), 1.5–2.0 mm long, narrowly columnar or slightly tapered from near the base, appearing sessile or more commonly short-stalked and angled toward the leaf tip. Leaf blades 5–8 cm long, with (2)3 pairs of opposite leaflets. Leaflets 2–6 cm long, 2–3 cm wide, broadly obovate to broadly obovate, oblique at the base, rounded or occasionally broadly angled to a very bluntly pointed tip, the margins with a pale, narrow band and short, ascending hairs, the surfaces glabrous or the undersurface with a few minute glandular or longer, nonglandular hairs toward the base. Inflorescences of solitary or paired flowers, sometimes also appearing as small clusters at the stem tip, the flower stalks 10–25 mm long, becoming elongated to 40 mm at fruiting. Calyces zygomorphic, the sepals variously 5–10 mm long, 2–3 mm wide, oblong-obovate to broadly elliptic, rounded to bluntly pointed at the tip, the margins with short, spreading hairs. Petals 7–14 mm long, 4–7 mm wide, oblong-obovate to obovate. Stamens with the anthers purplish brown. Ovary 4–6 mm long, with appressed or ascending hairs, the style 3–4 mm long. Fruits 9–16 cm long, 2–4 mm wide, arched downward at maturity, more or less circular to more commonly bluntly rectangular in cross-section, sparsely to moderately hairy when young, becoming glabrous at maturity, only slightly impressed between the seeds but with a pair of longitudinal ridges near the margins on each surface, greenish brown to brown at maturity. Seeds 3–5 mm long, 2.0–2.5 mm wide, somewhat rhomboidal to trapezoidal in outline, slightly flattened, the surface often developing a fine network of cracks toward the margins at maturity, reddish brown to dark brown, shiny, the pleurogram usually dull and slightly lighter than the rest of the seed. 2n=24, 26, 28. July–September.

Uncommon in southeastern and southwestern Missouri, north sporadically to the city of St. Louis (native range poorly understood, probably originally native to tropical and warm-temperate regions of the New World, now widely distributed in both hemispheres; in the U.S., present in the eastern states west to Nebraska and Texas, also California, Hawaii). Banks of streams and rivers, sloughs, and upland prairies; also railroads and open, disturbed areas.

The natural range of S. obtusiflora prior to the European colonization of North America is not fully understood. Steyermark (1963) considered at least some of the Missouri populations native and noted that Nicholas Riehl had collected the species as early as 1838 in a prairie in St. Louis. The species has a long history of association with humans, and farther south it can become a rank weed of pastures, farms, and waste places. As with many widespread, weedy species, there exists considerable genetic and morphological variation. The leaves have been used for food, as an adulterant of coffee, as a laxative and purgative, in poultices, and for dying cloth (Irwin and Barneby, 1982). The commercially available laxatives produced from anthraquinone extracts of Senna involve other tropical species. Burrows and Tyrl (2001) reviewed the toxicity of the genus, noting that both S. obtusifolia and S. occidentalis (but usually not S. marilandica) have been implicated in livestock poisoning, primarily when cattle have been fed fresh chopped forage containing relatively high concentrations of Senna, when cattle ingest wilted plants after first frost in the autumn, or when pigs and other livestock ingest grain contaminated by Senna seed.