Acacia allenii D. H. Janzen

David S. Seigler & John E. Ebinger, 1995, Taxonomic Revision of the Ant-Acacias (Fabaceae, Mimosoideae, Acacia, Series Gummiferae) of the New World, Annals of the Missouri Botanical Garden 82, pp. 117-138 : 121-122

1. Acacia allenii D. H. Janzen , Smithsonian Contr. Bot. 13: 53. 1974. TYPE: Costa Rica. Puntarenas: Osa Peninsula, bank of tributary of Rio Agua Buena, 3.5 mi. SW of Rincon , 9 Mar. 1967, D. H. Janzen 1769 (holotype, US; isotypes, F, GH, K, MEXU, MICH, MO, NY, UC) .

Tree to 25 m tall with widely spreading branches, young twigs dark brown to dark reddish brown, lightly puberulent. Stipular spines black to dark brown, commonly grooved and with low, rounded longitudinal ridges, glabrous to lightly puberulent, nearly terete in cross section, sometimes slightly reflexed, symmetrical, broadly V-shaped with an angle of 90-120°, to 45 mm long, 8-12 mm wide near the base. Leaves 150-400 mm long, pinnae 10-22 pairs per leaf, 60-110 mm long, 15-22 mm between pinna pairs; rachis grooved, lightly puberulent, rachis glands puberulent, lightly striate, narrowly volcano-shaped, 1 or 2 located at the node of each pinna pair, sometimes scattered along the rachis between the pinna pairs; petiole grooved, lightly puberulent, 14-30 mm long. Petiolar glands narrowly volcano-shaped, usually 4-10, scattered along the petiole, puberulent, lightly striate, apex 0.4-0.7 mm across, base 0.8-1.2 mm across. Leaflets 15-25 pairs per pinna, glabrous, linear, 6-12 mm long, 1.8-3.9 mm wide, lateral veins obvious, 2-3 veins from the base, apex obtuse to rarely acute. Inflorescence a densely flowered globose head, 5-7 mm across, in axillary clusters of 5-15 in the axil of reduced leaves, commonly on short axillary branches 40-150 mm long; peduncles lightly puberulent, 10-15 mm long, 0.6-0.9 mm thick, nearly the same thickness throughout; involucre located on the lower Vz of the peduncle, puberulent, 5-lobed. Floral bracts peltate, apex circular, ciliate, stalk 0.8-1.0 mm long. Flowers sessile; calyx 5-lobed, puberulent on the lobes, 0.9-1.2 mm long; corolla 5-lobed, glabrous, pale yellow, 1.2-1.5 mm long.. Legumes (only one specimen, Janzen 11051, with fruits seen) slightly curved, flattened, 65-110 mm long, 15-20 mm wide, glabrous, longitudinally striate, dark brown to black, dehiscent along two sutures, base not stipitate, the apex acute. Flowering April-July.

Distribution. Heavily shaded understory at lower elevations (to 160 m) in wet forests, in moist disturbed stream banks, at forest edge, and on roadsides, in Puntarenas Province, Costa Rica.

Representative specimens. COSTA RICA. Puntarenas: forested hills near Golfito de Golfo Dulce , Allen 5997 ( F, GH, MO, US) ; 5 mi. SW of Rincen , Osa Peninsula , Janzen 791 ( F, GH, MEX, MO) .

The narrow geographic range of Acacia allenii is typical of many of the ant-acacia species of Central America. Its normal habitat appears to be at lower elevations in heavily shaded understory of wet tropical forests. In general, it reacts fairly well to disturbances, and most present-day collections are from roadsides, recent landslides, disturbed stream banks, and secondary successional areas after logging. It is rarely found in open pastures, and according to Janzen (1974) will not survive fire.

As is typical of many wet forest ant-acacias, Beltian body production in Acacia allenii is not particularly extensive. However, the Beltian bodies are much larger (to 3 mm long) than those found in dry area ant-acacias. In A. allenii , these relatively large Beltian bodies are formed only on the lower 3-6 leaflets of each pinna, and occasionally the lower leaflet of a pinna has been replaced by a large Beltian body. None of the individuals tested positive for cyanide production.

Acacia allenii is most closely related to A. melanoceras and possibly A. collinsii . The globose inflorescences and the presence of rachis glands easily separate this species from A. collinsii , which has elongated inflorescences and usually lacks rachis glands. It is easily separated from A. melanoceras , since its leaflets are nearly twice as large and have obvious secondary veins. Also, well-developed leaves of A. allenii have only 4-10 widely scattered petiolar glands, while A. melanoceras commonly has 6-30 closely spaced petiolar glands on the flattened adaxial surface of the petiole. Furthermore, the fertile branches of A. allenii are commonly less than 150 mm long, are scattered along major branches throughout the crown, and have 5-15 inflorescences in the axis of reduced leaves. In A. melanoceras , the fertile branches are to 350 mm long, tend to be bunched near the ends of the sterile branches, and have 2-6 inflorescences at each leafless node.