Monographs Details: Abarema microcalyx (Spruce ex Benth.) Barneby & J.W.Grimes
Authors:Rupert C. Barneby
Authority: Barneby, Rupert C. & Grimes, James W. 1996. Silk tree, guanacaste, monkey's earring: a generic system for the synandrous Mimosaceae of the Americas. Part I. Abarema, Albizia, and allies. Mem. New York Bot. Gard. 74: 1-292.
Description:Species Description - Macrophyllidious trees 5-25 m with trunk attaining 3.5 dm dbh and grayish annotinous branches, the young stems, lf-stks, and peduncles densely minutely brown-puberulent when young but early glabrescent, only the axillary buds permanently puberulent, the broad chartaceous bicolored lfts facially glabrous, often microscopically ciliolate, lustrous dark brown-olivaceous above, paler brown beneath, the short dense capituliform racemes of exceptionally small, greenish white fls arising solitary and paired from the axil of coeval lvs, immersed in foliage. Stipules linear to subulate 1-2 mm, early caducous from base of developing lvs, absent from fruiting specimens. Lf- formula i-iii/1-4, lfts 8-32(-36) per lf; lf-stalks of larger lvs 0.5-8.5 cm, the petiole 0.5-2.6 cm, the one or the longer of 2 interpinnal segments (0.7-) 1-3.5 cm; petiolar nectary at or shortly below pulvinules of first pinna-pair verruciform, hemispherical or sometimes vertically elongated, 1—2.5(—3) cm diam at base, as or only half as tall, minutely pored, commonly livid and sometimes microscopically puberulent, similar but smaller nectaries sometimes at further nodes of lf- stk and at tip of pinnae, but the last often lacking; pinnae distally accrescent, the rachis of furthest pair 2.7-8 cm, the longer interfoliolar segments 12-32 mm; pulvinules in dorsal view 1.5-2.5 x 0.8-1.7 mm, closely cross-wrinkled; lfts strongly accrescent distally, the blades obliquely obovate, asymmetrically ovate or incipiently rhombic-ovate, somewhat incurved, at base scarcely or moderately inequilateral, broad-cuneate or rounded, obtuse muticous, the distal pair (27-)40-85 x (20-)28-45 mm, 1.3-1.9 times as long as wide; the subcentric midrib gently arched forward, the widely ascending secondary veins brochidodrome well within the loosely revolute margin, all these together with random tertiary venules prominulous on both faces but more sharply so beneath. Peduncles 3.7-11.5 cm; racemes ±(20-)30-50-fld, the slenderly clavate receptacle 2-15 mm; bracts not over 1 x 0.5 mm, linear-subulate or subulate, caducous; fls subequiform as to size and shape of 5-merous perianth, but the terminal 1(—2—3) sometimes sessile and with modified androecium; PERIPHERAL FLS: pedicel 0.6-2.5 mm; calyx campanulate 0.7-1.2 mm, the low-deltate teeth 0.1-0.3 mm; corolla greenish 3.2-5 mm, the ovate lobes 1-1.7 mm, microscopically puberulent externally; androecium 10-14-merous, 9-17 mm, the tube including short stemonozone 1.6-2.4 mm; ovary glabrous, conical at apex; style ±2 cm, the stigma scarcely 0.1 mm diam, not dilated; TERMINAL FL: sessile or almost so, the perianth as in peripheral fls, the androecial tube ±2-5 mm. Pods mostly solitary, subsessile, in profile broad-linear, evenly incurved through almost to somewhat more than a full circle, in profile ±5.5-14 x 1.1-2.9 cm, (6-)7-12-seeded, the seminiferous suture either evenly recurved or undulately impressed between seeds, the coriaceous or subligneous valves at first piano-compressed, often but not consistently becoming turgid over each seed, fuscous or nigrescent and coarsely but not prominently venulose when mature, smooth and orange internally, the microscopically puberulent sutures dorsally plane and ±2 mm wide to dorsally ridged to 5 mm wide, the valves in cross-section 0.2-2 mm thick, the exocarp and pale crustaceous endocarp always very thin, the pitchlike mesocarp highly variable (as described under each variety); dehiscence of Abarema, in thick-walled pods tardy, the valves when of thin texture cracking and irregularly separating, when of thick texture continuous; funicles strap-shaped, distally coiled; seeds not seen.

Discussion:Among lowland abaremas with few large leaflets A. microcalyx is distinguished by the combination of mounded leaf-nectaries and compactly racemose capitula of very small flowers. It is variable in leaf-formula, the leaflets of the largest leaves of a plant varying in number from 8 to 36, and in both width and texture of the pod. Narrower, thinner-textured pods have relatively narrow sutures, the seminiferous one undulately impressed between seeds, and mesocarp <0.5 mm thick over each seed, whereas broader thicker pods have wider sutures not or obscurely constricted between seeds and mesocarp about 1-2 mm thick in cross-section. We have found some variation in morphology of the capitulum, involving length of pedicels and development of a modified central flower, but whereas the characters of leaf-formula and fruit are seemingly well correlated with dispersal and thereby indicate racial differentiation, flowering specimens are as yet too few to establish any significant pattern of variation. Three varieties are here recognized.