Vascular Plants of Williamson County

Mimosa borealis [Fabaceae]
pink mimosa, fragrant mimosa

Mimosa borealis A. Gray, pink mimosa, fragrant mimosa. Shrub, fall–deciduous, somewhat spinescent, with open, ascending principal branches, in range to 170 cm tall; shoots with only cauline leaves clustered closely to young twigs, stem internode sometimes with a prickle compressed side–to–side (not in pairs), 3.5—8 mm long, often purple–red to purple.


Stems broadly ridged, with 3 ridges descending from each leaf or 1 from each prickle; bud scales short–tomentose.


Leaves helically alternate, even pinnately–2–compound with 1—2 pairs of primary leaflets (pinnae) diverging at ca. 60° from axis, flat, with 4—6 pairs of secondary leaflets, petiolate, glabrous; stipules 2, attached below petiole on leaf base, acuminate, 3—5 × 0.4 mm, green often purple at base, inner surface short–hairy near base; petiole hemicylindric flattened on upper side, to 26 mm long, glabrous; primary leaflet arising on upper side of petiole or somewhat channeled rachis, oblong in outline, to 38 × 16.5 mm, with constriction at base of rachilla, rachis continued past the upper pair of primary leaflets (positioned to the lower side), awl–shaped, ca. 1.5 mm long; petiolules of secondary leaflets (pulvinus) ca. 0.6 mm long, light green, glabrous, with minute colorless glands at the base; blades of secondary leaflets oblong or narrowly ovate to narrowly obovate, 2.5—5 × 1—1.2 mm, somewhat asymmetrically tapered at base, entire, obtuse to acute (rounded) at tip, pinnately veined with midrib not along midline, surfaces nearly smooth and minutely dotted (stomates), glabrous and ± glaucous.


Inflorescence headlike spike, axillary to leaf scar on twigs from previous year (periderm), forming on condensed lateral shoot with 1—several leaves, hemispheroid, 16—18 mm across, ± 18–flowered, flowers helically alternate without internodes, bracteate, glabrous; peduncle cylindric, at flowering 14—25 × 0.6—0.7 mm, broader at top and lacking pulvinus at base, yellow–green and somewhat glaucous; rachis subspheroid, 1.5 × 1 mm, glabrous; bractlet subtending flower awl–shaped to oblanceolate–linear, 1.2—1.9 × 0.4 mm, green or upper portion reddish, short–ciliate on margins and with several short hairs at tip.


Flower bisexual, radial, 5—7 mm across, 9—10 mm long; pleasantly fragrant; nectary on receptacle around stalk of pistil, glossy green; calyx inconspicuously 5–toothed, 1.3—1.5 mm long; tube ± funnel–shaped (bell–shaped), 1 mm wide at top, green or pinkish above midpoint, teeth (lobes) short and broad, < 0.25 mm long, with ephemeral glandular hairs on margin; petals 5, ascending, obovate to oblong, 2.8—3.2 × 1.1—1.4 mm, often twisted or somewhat wavy, inrolled to lower side, midvein green and margins rose–pink; stamens 8—11, free; filaments ascending–radiating, subequal to unequal, cylindric, 6.5—8.5 mm long, dark pink to rose aging lighter, glabrous; anthers dorsifixed, dithecal, 0.55—0.7 mm long, light yellow or blushed with rose, longitudinally dehiscent; pollen pale yellow; pistil 1, short–stalked (stipe), stalk 0.2—0.35 mm long, greenish, pistil 9—9.5 mm long and slightly > stamens; ovary superior, ellipsoid to narrowly obovoid, 1 × 0.4 mm, green, glabrous, 1–chambered with 5 ovules attached to upper side; style pale pink at base to darker pink or rose–pink above (same color as filaments), tapered to a minute terminal stigma.


Fruit pod (legume), schizocarpic, lax on peduncle, 1—4(—5)–seeded, breaking into 1–seeded joints after valves separating from sutures, an upward–curved series of bulging seeds with constricted valves and indented margins between seeds, 15—55 × 8—11 mm, brownish, convex on 1 surface and concave on other surface, commonly with 1—3 prickles 1—3.5 mm long on lower edge; stalk cylindric, to 2.5 mm long, glabrous; peduncle in fruit to 24 mm long.


Seed enclosed within joint, ovoid, ca. 5—5.5 × 4 × 3 mm, brown, with well–developed raphe and short hilum at top of raphe; surface irregularly pitted.

A. C. Gibson