Melilotus albus Medik., white sweetclover. Annual (biennial herb), taprooted, several–stemmed at base, with ascending branches throughout, ascending to erect, 50—200 cm tall; shoots sparsely short–strigose becoming glabrescent, freshly scented (like alfalfa, Medicago sativus); roots nodulated.
Stems 5—6–ridged, to 8 mm diameter, with 3 conspicuous ridges descending from each leaf, tough, green and finely striped; pith white, stem aging hollow.
Leaves helically alternate, pinnately 3–foliolate with paired lateral leaflets, petiolate with pulvinus, with stipules; stipules 2, fused to petiole base and stem at node on a thickened flap, free portion linear to narrowly awl–shaped, 5—12.7 mm long, green, linear portion with 1 vein; petiole with pulvinus above stipules, channeled (including pulvinus), 4—25 mm long, the pulvinus ca. 1 mm long, not swollen, above pulvinus with fine appressed hairs or glabrous; rachis channeled, 1.5—8 mm long; petiolule = pulvinus, short, ± 1(—several) mm long, finely hairy; blades of leaflets elliptic or oblong to ovate, (10—)15—37.5(—40) × 2.5—20 mm, terminal leaflet 2—7 mm > lateral leaflets, broadly tapered at base, regularly short–serrate on margins, rounded or truncate at tip, pinnately veined with midrib raised on lower surface, dull, lower surface typically sparsely short–strigose.
Inflorescence raceme, axillary, cylindric, to 170 × 10—12 mm, typically 20—60–flowered, flowers helically alternate, bracteate, puberulent; peduncle ≥ subtending leaf, to 100 mm long, peduncle and raceme axis conspicuously ridged, with each ridge descending from a bractlet (pedicel) and having a swelling on ridge just below bractlet; bractlet subtending pedicel threadlike, to 1 mm long, sparsely hairy, somewhat persistent; pedicel 1—2 mm long, arching or often recurved.
Flower bisexual, bilateral and pealike (papilionaceous), 2 mm across, with a faint, sweet fragrance; nectary nectar–producing receptacle below ovary, nectar present within stamen sheet; calyx 5–lobed, ± 2(—3) mm long, green; tube 1.5—2 mm wide, 5–veined (inconspicuously 10–veined), short–strigose; lobes subequal, acuminate, 0.8—1(—2) mm long; petals 5, all ± white; banner lacking claw, broadly ovate, 3.5—5 × 2.5—4 mm, with margins initially folded over wings but at anthesis with margins and above midpoint spreading upward and outward; wings 2, clawed at base, ± oblong, 2.5—4 mm long, claws ca. 1 mm long and upwardly curved, with a backward–pointing, fleshy tooth at base of upper margin, adhering to keel below tooth; keel of 2 petals partially fused on lower edge of limbs, short–clawed at base, 2—3.8 × 1 mm; stamens 10, diadelphous (9 filaments fused and 1 free), 2.3—3.4 mm long; filaments sheet to about midpoint, whitish; anthers basifixed, dithecal, 0.25—0.3 mm long, light orange or bright to creamy yellow, longitudinally dehiscent; pollen whitish, discharged before flower opens; pistil 1, 2.5—3.3 mm long and slightly > stamens, short–stalked (stipe); ovary superior, narrowly fusiform, green, 1–chambered with 3 ovules attached to upper side; style curved, pale green to whitish, glabrous; stigma obliquely capitate.
Fruit legume (pod), indehiscent, 1–seeded, ovoid, 3(—5) × 2 mm, conspicuously net–veined, pointed at tip.
Seed ± ovoid, 1.7—2 × 1.3—1.5 mm, dull orange–yellow to light yellow–brown, smooth.
A. C. Gibson & B. A. Prigge