Acacia farnesiana (L.) Willd., sweet acacia, huisache. Small tree to large shrub, winter–deciduous, spinescent, several—many–stemmed at base, “trunks” ascending (never erect), with many spreading young branches in canopy, in range 200—550 cm tall, ≤ canopy width (mature specimen); shoots with long shoot–short shoot organization, having most leaves and inflorescences produced on unexpanded short shoots, usually 1—3 leaves per short shoot during a season, inconspicuously short–hairy mostly along axes soon aging glabrescent; bark brown, cracked on principal lower stems.
Stems of long shoot with 1 poorly defined ridge from each leaf, aging ± angled and soon woody, slightly zigzagged, internodes mostly 13—35 mm long (canopy branches), young stems sparsely short–hairy and soon appearing glabrous, green soon turning brownish and later with tight periderm, lenticels initially blisters aging elongate ⊥ axis; internodes of short shoots absent.
Leaves helically alternate, even–2–pinnately compound with (1—)2—6 pairs of primary leaflets (pinnae; rarely, a short leaf having only 1 pair with 1 primary leaflet producing secondary leaflets and 1 primary leaflet axislike without secondary leaflets), in range 13—60 mm long, petiolate with pulvinus, with stipules; stipules 2, sharp–tipped spines attached to base of petiole and divergent, mostly to 13 mm long, green rapidly aging reddish eventually becoming dry and whitish; petiole 5—15 mm long, pulvinus at base cylindric and 1—3 mm long, not thickened more than rest of petiole, with minute hairs, petiole above pulvinus channeled (2–ridged), short–hairy, on upper side usually with an elliptic–shaped nectary (gland) mostly 3—4 mm from pulvinus (± midpetiole; on small leaves often absent); rachis 2–ridged with primary leaflets ascending in opposite pairs from ridges, the pairs mostly 4—6 mm apart and diverging from rachis at ca. 45°, axis, often having an elliptic–shaped nectary (gland) between the last pair of primary leaflets and rarely a nectary between the first and second pair, rachis axis extending several mm beyond the last pair of primary leaflets, axis short–hairy; stipels subtending primary and secondary leaflets obscure; primary leaflets to 25 mm long, most primary leaflets per blade ± same length, with 10—21 pairs of secondary leaflets (pinnules) along rachilla; petiolule of primary leaflets ascending, 1.1—1.6 mm long, mostly a pulvinus; rachilla 4–sided, short–hairy; petiolules of secondary leaflets 0.2—0.3 mm long, permitting sleep movements, essentially glabrous; blades of secondary leaflets overlapping, oblong, 1.7—4 × 0.4—0.8 mm, subtruncate and slightly oblique at base, entire and sometimes short–ciliate on trailing margin, rounded to obtuse (acute) at tip, obscurely pinnately veined with midrib eccentric to leading margin, glabrous and somewhat glaucous
Inflorescence headlike spike, axillary and < leaf, head spheric, fully open ± 5 mm across, flowers helically alternate and densely packed, ca. 80–flowered, bracteate, puberulent; peduncle straight, < 10 mm long, with constriction at base; bract subtending spike, shieldlike and strongly reflexed, ± 1.2 mm long, purple–red, hard, puberulent; bractlet subtending flower oblanceolate, ca. 1.5 mm long, often with yellowish, flat axis and a reddish blade, short–ciliate tip, puberulent.
Flower bisexual, radial, 0.6—0.7 mm across; nectary absent; calyx 3—5–lobed; tube V–shaped, ± 1 mm long and 0.7—0.8 mm wide at orifice, transparent–orange; lobes obtuse, reddish, faintly 5–veined; corolla short 5–lobed, erect and fitting tightly within corolla, ± 2.3 mm long, transparent yellow; tube ± 2 mm long; lobes deltate, 0.3—0.5 mm long, with thickened margins and often purple blotch at tip, short–strigose on outer (lower) surface, in bud and at anthesis bent over orifice; stamens 30+, exserted; filaments threadlike, 4—5 mm long, golden yellow, not smooth; anthers dithecal, 0.25 mm long, golden yellow, longitudinally dehiscent; producing pale yellow pollinia (permanently united pollen); pistil 1, 6.5—7 mm long, short–stalked, the stalk 0.3—0.35 mm long; ovary superior, fusiform, 0.9—1.1 mm long, light green, glabrous, 1–chambered with ca. 20 ovules attached to upper side; style exserted ca. 2 mm above anthers, ascending, unevenly thickened, ± straight, light yellow; stigma terminal, minute.
Fruit legume (pod), indehiscent, several—19–seeded, variously curved to nearly straight, in range 60—80 × 8—10 × 8—10 mm (fully loaded with seeds), dark brown, bulging at seeds, having 2 rows of seeds and 2+ seeds per bulge, wall thick with pulp filling chamber; stalk < 4 mm long, woody.
Seed ovoid to ellipsoid slightly compressed side–to–side, 4—7 × 3.6—5 mm, dull olive–brown, smooth, usually with an ovate mark on compressed faces and a small terminal hilum.
A. C. Gibson