Senna roemeriana (Scheele) H. S. Irwin & Barneby, twoleaf senna. Perennial herb, thick–rooted, 1—several–stemmed at base, with spreading to ascending principal shoots, in range 20—50 cm tall; shoots with only cauline leaves, foliage and inflorescence strigose and short–strigose, flowering canopy ± foul–smelling produced from inconspicuous, yellowish, fleshy hairs on axes and in axils.
Stems initially angled and weakly ridged aging with subtly flattened faces, straight, aging brownish (orange + green), internodes 10—30 mm long, puberulent and short–hairy with hairs arching upward.
Leaves helically alternate, even–2–foliolate and between leaflets initially having an ascending, stalked, lanceolate, orange–yellow gland ca. 1 mm long, petiolate with pulvinus, with stipules; stipules 2, attached to stem at base of petiole, linear, 6—9.5 × 0.3—0.4 mm, short–hairy with mostly appressed hairs; petiole channeled above pulvinus, 4—26 mm long, with pulvinus initially hidden beneath pubescence, 1—1.5 mm long, axis grayish green and short–hairy; stipel subtending leaflet absent; petiolule = pulvinus < 1 mm long; blades of leaflets asymmetrically narrowly lanceolate and straight to somewhat sickle–shaped (falcate), 25—60 × 5—14 mm, with midrib closer to margins of facing leaflets, oblique–rounded at base, entire, acute at tip, pinnately veined with the midrib prominent and raised on lower surface base–to–tip. with somewhat appressed, upward–pointing hairs, glaucous, lower surface paler.
Inflorescence raceme, axillary, ascending, (2—)3—5–flowered, axis at anthesis < 45 mm long, flowers helically alternate, bracteate, mixed strigose, short–hairy, and puberulent, axes also initially having scattered yellowish glandular hairs; peduncle stemlike, 20—25 mm long increasing in fruit; bractlet subtending pedicel linear, to 9 mm long, short–hairy with upward–pointing hairs, deciduous; pedicel at anthesis 8—12 mm long increasing to 17 mm long in fruit.
Flower bisexual, ± radial, ca. 25 mm across; nectary nectar–producing receptacle below ovary, cup–shaped, 1 × 2 mm, receptacle green and ± short–strigose on outer side; sepals 5, subequal, with 1 outside of others, elliptic, 5—5.5 × 3—3.5 mm, broad at base, yellow–green or with wide yellow margins, short–ciliate on margins, obtuse to rounded at tip, lower surface strigose and short–strigose, abscised only after fruit begins to grow; petals 5, cupped, short–clawed, ± subequal with banner somewhat wider, bright yellow aging with more orange and having reddish veins, claws 1—1.3 mm long; limbs obovate, ca. 14 × 6.5—8 mm, tapered at base, pinnately veined, glabrous; stamens 10, free, heteromorphic, 7 fertile stamens and 3 sterile staminodes; staminodes on upper side appearing fused at base, paddlelike with upper portion flattened front–to–back, lateral staminodes ca. 2.5 mm long and center staminode 2 mm long, yellow, staminode anthers absent; fertile stamens ± monomorphic with filaments of different lengths exserted and slightly arching upward, 2 on each side of ovary, 2 lateral on lower side, and 3 on lower side; filaments 2—4.5 mm long, yellow aging orange–yellow; anthers basifixed, dithecal, curved upward, 2—3 mm long, orange–brown, dehiscing by lateral pore at top of each anther sac with pores toward stigma, pores 0.25—0.3 mm diameter; pollen pale orange; pistil 1, short–stalked (stipe), pubescent at base; ovary superior, oblong–cylindric not compressed side–to–side, 2 × 1 mm, green, densely sericeous, 1–chambered with to 12 ovules attached on upper side; style ca. 2.5 mm long, yellow, with minute terminal stigma.
Fruit legume (pod), dry, spreading, dehiscent, to 12–seeded, mostly straight, oblong not compressed side–to–side, in range ca. 26—32 × 5 × 5 mm, orange–brown, with prominent ridge on top and bottom, valves somewhat short–hairy with hairs arching upward and scattered puberulent hairs; stalk below pod (stipe) 1—1.4 mm long, somewhat orange before drying.
Seed ± pear–shaped flattish on 1 face, 4.5—5 × 4.5—5 × 1.8—2.4 mm, thick–walled and hard, glossy brown, having an elaborate system of low ridges (verrucose), with nob having lateral hilum.
A. C. Gibson