Acacia janzenii Ebinger & Seigler

9. Acacia janzenii Ebinger & Seigler, Southw. Naturalist. 32: 245. 1987. type: Mexico. Tabasco: 9.8 mi. W of Lázaro Cárdenas on hwy. 180, 17 June 1966, D. H. Janzen 515 (holotype, MO; isotypes, BM, CAS, F, GH, MICH, MO, UC, US).

Tree to 12 m tall, young twigs dark reddish brown, lightly puberulent. Stipular spines dark brown to black, smooth, terete, densely pubescent with straight, yellowish hairs to 0.5 mm long, asymmetrical, U-shaped with an angle of 30-70°, usually curved around the stem, 30-60 (rarely 80) mm long, 6-9 mm thick near the base. Leaves 150-370 mm long; pinnae 30-60 pairs per leaf, 6-9 mm between pinna pairs; rachis grooved, densely pubescent with yellowish hairs, a columnar to narrowly volcano-shaped gland located at the node of each pinna pair, its apex 0.5-0.7 mm across; petiole densely pubescent, 5-8 mm long. Petiolar glands absent. Leaflets 40-70 pairs per pinna, glabrous, linear, 3-5.5 mm long, 0.6-0.9 mm wide, lateral veins not obvious, only one vein from the base, apex acute, margins ciliate. Inflorescence a densely flowered globose head 4-6 mm across, in clusters of 16-42 in the axil of slightly reduced leaves on normal branches, also located on fertile branchlets to 250 mm long with 16-42 inflorescences in each of 6-12 fascicles that may not be subtended by leaves; peduncles glabrous to lightly puberulent, 7-16 mm long, 0.3-0.7 mm thick, the same thickness throughout; involucre located near the middle of the peduncle, usually 4-lobed. Floral bracts peltate, apex circular, stalk 0.4-0.7 mm long. Flowers sessile, calyx 5-lobed, glabrous, 0.6-1.1 mm long; corolla 5-lobed, glabrous, 1.2-2.1 mm long, about twice as long as the calyx. Legumes not seen, but probably very similar to those of Acacia cookii. Flowering May-July.

Distribution. Disturbed habitats, roadsides, pastures, and moist disturbed forests, to 400 m elevation, in Chiapas, Tabasco, and Veracruz, Mexico.

Representative specimens. MEXICO. Chiapas: stream bank, lower montane rainforest ridge above Ixtacomitan, 300 m, Breedlove 35053 (MEX, MICH, MO). Tabasco: 9.8 mi. W of Cardenas on hwy. 180, Janzen 515 (MEX). Veracruz: Las Cruces, 250 m, Nevling & Gómez-Pompa 1555 (F).

Acacia janzenii is easily distinguished from all other ant-acacias by its asymmetrical stipular spines, which usually curve around the stem and are covered with straight, yellowish hairs to 0.5 mm long. The only other species that commonly has asymmetrical thorns is A. cookii. These two closely related species differ in the petiolar and rachis glands, the type of pubescence on the spines, the leaflet length and width, and the flowering branchlets (see discussion under A. cookii).

Janzen (1974) considered this taxon a part of Acacia cookii. However, he noted that specimens from Tabasco and Chiapas, Mexico (herein referred to A. janzenii) differ from typical A. cookii. In particular, he noted that progressing from north (Mexico) to south (Belize, Guatemala, Honduras), there is a change in spine length, a shortening of the fertile branchlets, an increase in the number of inflorescences per axil, an increase in the number of Beltian bodies per leaf, and a higher percentage of plants occupied by ants. Although he suggested that the extremes are clinally connected, our principal component analysis (PCA) suggests that these two species are specifically distinct. No intermediate plants were found, and the characters used for separation (petiolar and rachis glands, spine size, pubescence, leaflet size, fertile branchlets) are as significant as those characters used to distinguish other taxa of ant-acacias.

Originally, Acacia janzenii probably was a species of disturbed sites in open forests, particularly landslide scars and stream banks. Most recent collections seen are from roadsides and pastures.

Beltian body production in this species is relatively low; usually less than 1/4 of the leaflets contain these bodies. Also, these bodies are usually less than 0.7 mm long. None of the specimens tested positive for cyanide production.