Acacia melanoceras Beurl,

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: 132

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Acacia melanoceras Beurl


11. Acacia melanoceras Beurl  ., Kongl. Svenska Vetensk. Acad. Handl. 1854: 123. 1856. Myrmecodendron melanoceras (Beurl.)  Britton & Rose, N. Amer. FI. 23: 93. 1928. type: Panama. Colon: Portobello , sea level, Apr. 1826, J. G. Billberg 289 (lectotype, designated here, S,  photo, F)  .

Acacia multiglandulosa Schenck  , Repert. Spec. Nov. Regni Veg. 12: 362. 1913. TYPE: Panama. Colon: Portobello, J. G. Billberg 1825 (lectotype, designated here, US, photo [ B destroyed]).

Tree to 15 m tall; young twigs dark brown to dark reddish brown, puberulent. Stipular spines shiny, dark reddish brown to black, grooved and with low, rounded longitudinal ridges, rarely with two narrow, bladelike longitudinal flanges, glabrous to lightly puberulent, terete in cross section, symmetrical, broadly V-shaped with an angle of 80-120°, 20-55 mm long, 5-9 mm wide near the base, abruptly tapering to a narrow, sharp-pointed tip, many spines not enlarged, usually less than 5 mm long and 0.4 mm wide. Leaves 150-290 mm long; pinnae 12-28 pairs per leaf, 20-50 mm long, 6-11 mm between pinna pairs; rachis grooved, puberulent; rachis glands puberulent, striate, columnar to narrowly volcano-shaped, one located at the node of each pinna pair, its apex 0.2-0.6 mm across; petiole grooved to flattened on the adaxial surface, densely puberulent, (5) 10-28 mm long. Petiolar glands columnar to narrowly volcano-shaped, usually numerous (6-30) on the flattened adaxial surface of the petiole, reduced to 0-5 on petioles of smaller leaves, puberulent, striate, apex 0.3-1.1 mm across, base 0.4-1.5 mm across. Leaflets 12-29 pairs per pinna, glabrous, linear, 4.5-7.6 mm long, 1.0- 1.7 mm wide, lateral veins not obvious, only one vein from the base, apex acute. Inflorescence a densely flowered globose head 5-6.5 mm across, solitary or in clusters of 2-6 in the axil of small spines on lateral, usually leafless, axillary branches to 350 mm long; peduncles densely puberulent, 5-10 mm long, 0.5-0.8 mm thick, nearly the same thickness throughout; involucre located near the base of the peduncle, puberulent, 5-lobed. Floral bracts peltate, apex circular, ciliate, stalk 0.8-1.2 mm long. Flowers sessile; calyx 5-lobed, glabrous, 0.9-1.1 mm long; corolla 5-lobed, glabrous, pale yellow, 1.5-1.8 mm long. Legumes curved, particularly at the narrowing apex, inflated to slightly flattened, 65-110 mm long, 10-20 mm wide, glabrous, longitudinally striate, dark brown to black, indehiscent, stipe absent, the apex narrowing and usually acuminate. Flowering April-July.

Distribution. Wet, relatively undisturbed forests on the Atlantic lowlands of central Panama (Colon Province) and the Canal Zone.

Representative specimens. PANAMA. Colon: Rio Boqueron , about 6-8 km upstream from Peluca Hydrographic Station , Dressier 4657 ( F,  GH,  MO)  ; 500 m upstream from mouth of Rio Piedras , Janzen 1641, 1912, 1913, 1915, 1916 ( F,  GH,  MEX,  MO)  . Canal Zone: Barro Colorado Island , Aviles 20b ( F,  MO)  ; along Rio Mendosa near Pipeline Road bridge, 8 km NW of Gamboa , 95 m, Nee 7739 ( MO)  .

Acacia melanoceras  has the most restricted range of all ant-acacias and is relatively rare throughout this range. According to Janzen (1974), rarely more than two individuals are found per acre in forest communities. Also, this species, which is usually restricted to lowland wet forests, disappears from any habitat subjected to disturbances that are any more catastrophic than infrequent cutting.

This species can easily be distinguished from all other ant-acacias by the extremely high number (6-30) of volcano-shaped petiolar glands on the flattened adaxial surface of the petiole. Also, the large leaves (to 300 mm) and the rachis glands at the node of each pinna pair help to separate it from most other ant-acacias.

Beltian body production is relatively low in Acacia melanoceras  , rarely more than half of the leaflets on mature plants producing these structures, which vary from 1 to 1.6 mm long. This characteristic, which is typical of most wet forest ant-acacias, is probably due to the high cost to the plant of Beltian bodies in a habitat with low solar energy input and perhaps somewhat reduced herbivore loads (Janzen, 1974). None of the individuals of this species tested positive for cyanide production.