Acacia chiapensis Safford, G. N.

2. Acacia chiapensis Safford, J. Wash. Acad. Sei. 5: 356. 1915. TYPE: Mexico. Chiapas: near San Fernando, between Tuxtla and Chicoasén, 12 Jan. 1907, G. N. Collins 164 (holotype, US; photo, F).

Shrub or small tree to 10 m tall, young twigs brown to reddish brown, usually densely puberulent. Stipular spines (Fig. 1 A) usually dark brown to black, smooth, terete, usually densely puberulent (rarely glabrous), symmetrical, V-shaped with an angle of 70-100°, straight to slightly reflexed near the apex, 30-80 mm long, 4-7 mm thick near the base. Leaves 100-250 mm long; pinnae 12-30 pairs per leaf, 30-70 mm long, 5-12 mm between pinna pairs; rachis grooved, densely puberulent, a columnar gland commonly present between each pinna pair (sometimes absent); petiole grooved, densely puberulent, 7-17 mm long. Petiolar glands (Fig. 1 E) columnar, densely puberulent, striate, apex 0.5-0.8 mm across, base 0.6-1.2 mm across, usually 6-13 scattered along the petiole. Leaflets 22-50 pairs per pinna, glabrous, linear, 3-8 mm long, 0.7-1.6 mm wide, lateral veins not obvious, only one vein from the base, apex acute to obtuse. Inflorescence a densely flowered globose head 5-9 mm across, in clusters of 10-40 in the axil of slightly reduced leaves, or in clusters of 10-40 in the axil of much reduced leaves on axillary branches; peduncles glabrous to lightly puberulent, 10-35 mm long, 0.6-1.1 mm thick, nearly the same thickness throughout; involucre located near the middle of the peduncle, glabrous to puberulent, 4-lobed. Floral bracts peltate, apex circular, stalk 0.8-1.5 mm long. Flowers sessile; calyx 5-lobed, glabrous, 1.0- 1.3 mm long; corolla 5-lobed, glabrous, pale yellow, 1.6-1.9 mm long. Legume nearly straight, flattened, 65-90 mm long, 8-10 mm wide, glabrous, longitudinally striate, black to dark brown, tardily dehiscent, stipe less than 5 mm long, the apex narrowing to a very short spinelike beak, margins ridged. Flowering January-May.

Distribution. Usually rare in dry lowland sites, and along and near watercourses in heavily disturbed vegetation in extreme southern Mexico.

Representative specimens. MEXICO. Chiapas: steep slope with montane rainforest, 18-20 km N of Ocozocoautla along road to Mal Paso, 800 m, Breedlove 23862 (MEX, MICH, MO, NY, TEX). Oaxaca: gravel pit, second growth vegetation, 1 mi. W of Temascal, Janzen 185 (F, MEX, MO). Veracruz: Dos Ríos, Mell 506 (NY).

Acacia chiapensis appears to be most closely related to A. globulifera in its morphology and habit. These two species are easily separated, because in A. chiapensis there are usually 6-13 columnar petiolar glands scattered along the petiole, the globose inflorescences are in axillary clusters of 10-40, and the legume has a distinct marginal ridge. Acacia globulifera, in contrast, usually has only 3-6 narrow, volcano-shaped petiolar glands on a petiole, the globose inflorescences rarely exceed 8 in a cluster, and the legume lacks marginal ridges. The only other species with which A. chiapensis could be sometimes confused is A. collinsii, but this species has elongated inflorescences, fewer petiolar glands, and its leaflets have obvious secondary veins.

Acacia chiapensis is usually shrubby, but can reach a height of 10 m (Janzen, 1967b). When repeatedly cut it commonly forms a dense bush, but rarely produces root sprouts. In many respects this species has a life form similar to that of non-ant-acacias such as A. farnesiana and A. macracantha. It is similar to these species in seed dispersal, seedling ecology, low Beltian body production, and the fact that many leaves on elongated lateral branches are not subtended by swollen spines (Janzen, 1974).

It appears that A. chiapensis is a marginal host for obligate acacia-ants. Janzen (1974) indicated that this species can sometimes survive in the absence of ants. Most specimens of this species retain cyanogenic glycosides in the leaves, which probably limits herbivory (Seigler & Ebinger, 1987). Low Beltian body production in this species may also indicate that it is a marginal host for acacia-ants. Beltian bodies of this species are generally less than 0.6 mm long, commonly are not well developed, and usually occur only on the lower 3-9 pairs of leaflets on some of the pinnae.

According to Janzen (1974), Acacia chiapensis occasionally hybridizes with A. cornigera and with non-ant-acacias such as A. macracantha. During the present study, no specimens were found that would indicate hybridization involving these species. Hybrids involving A. pennatula and A. chiapensis, however, have been observed.