Vachellia insulae-iacobi (L. Riley) Seigler & Ebinger, Phytologia 87: 160. 2005.syn. Acacia insulae-iacobi L. Riley, Bull. Misc. Inform. Kew. 1925: 220. 1925.
Synonymy and types
Basionym: Acacia insulae-iacobi L. Riley, Bull. Misc. Inform. Kew 1925: 220. 1925. - TYPE: ECUADOR. GALÁPAGOS: San Salvador (James Island), James Bay, in ravine near seashore, 27 Jul 1924, C. Penny & L. A. M. Riley 391 (holotype: K).
Tree to 10 m tall. Bark not seen. Twigs dark purplish brown, slightly flexuous, glabrous. Short shoots commonly present above the stipular spines, to 7 mm long, covered with acuminate stipules and old leaf bases. Leaves alternate, also commonly clustered on the short shoots, 3-25 mm long. Stipular spines dark purple to dark brown, becoming dark gray with age, symmetrical, terete, straight, stout, to 30 x 1.5 mm near the base, glabrous. Petiole adaxially grooved, most less than 6 mm long, glabrous or nearly so; petiolar gland solitary, located on the upper half of the petiole, stalked, circular to elongated, 0.3-1.5 mm long, apex bulbous, glabrous, rarely absent. Rachis adaxially grooved, to 20 mm long, glabrous or nearly so; a stalked, circular to slightly elongated gland 0.3-1.0 mm long present between to just below each pinna pair. Pinnae 1 to 3(5) pairs per leaf, 14-40 mm long, 6-11 mm between pinna pairs. Petiolules 1.0-2.5 mm long. Leaflets 4 to 10 pairs per pinna, opposite, 2.5-4.0 mm between leaflets, oblong, (6)7-11(14) x 0.7-2.4(3.0) mm, glabrous, lateral veins obvious, only one vein from the base, base oblique, margins slightly ciliate, apex acute to acuminate. Inflorescence a densely flowered globose head, 7-10 mm across, solitary from the short shoots. Peduncles 20-40 x 0.5-0.8 mm, glabrous. Involucre 4- to 6- lobed, located at the base of the globose head, glabrous, persistent. Floral bracts spatulate, 1.7-2.1 mm long, ciliate, deciduous. Flowers sessile, bright orange yellow; calyx 5-lobed, 1.5-2.2 mm long, glabrous; corolla 5-lobed, 2.3-3.2 mm long, glabrous; stamen filaments 3.0-4.5 mm long distinct to weakly fused at the base; ovary glabrous, on a stipe to 0.2 mm long. Legumes dark brown to black, straight to slightly curved, elliptic in cross section, not constricted between the seeds, 90-170 x 10-15 mm, coriaceous, lightly reticulately striate, glabrous, eglandular, indehiscent; stipe to 4 mm long; apex acute. Seeds uniseriate, imbedded in a light red to white pulpy material, dark brown, ovoid, slightly flattened, 7.5-8.8 x 6.5-7.8 mm, smooth; pleurogram U-shaped to oval, 4-5 mm across. Flowers in November to January. Chromosome number: Not determined.
Thorn-scrub vegetation in sand and cracks in lava rock from sea level to 100 m on the Galápagos Islands. Rudd (1971) reports this species from the islands of Baltra, San Salvador, and Santa Cruz.
Vachellia insulae-iacobi is restricted to the Galápagos Islands, where it is relatively common (Rudd 1971), being particularly abundant on Santa Cruz. This taxon is highly variable in size, ranging from a low shrub to small tree depending upon growing conditions. It also varies extensively in leaf and leaflet size. Generally, the leaves that develop from the short shoots have 1 (rarely 2) pinna pairs and short and narrow leaflets; in contrast, the primary leaves that develop from between the stipular spines have 2-3 (5) pinna pairs and longer and wider leaflets.
This taxon has been considered conspecific with Vachellia albicorticata (Rudd 1971) as both have short spur branches, bipinnately compound leaves with 1-4 pairs of pinnae, relatively large leaflets (usually exceeding 7 mm in length) with obvious secondary veins, and similar rachis glands. Numerous traits, however, can be used for consistent separation, the most obvious being the papery, peeling bark of V. albicorticata. Also, in V. insulae-iacobi, petioles are less than 6 mm long and have a medial gland, the inflorescence is solitary, and the calyx is 1.6-2.2 mm long. In V. albicorticata, in contrast, the petiole is more than 7 mm long, the petiolar gland is located between the lower pinna pair, the inflorescences are clustered, and the calyx is less than 1.5 mm long.
A filament-tube occurs in some members of Vachellia insulae-iacobi, a condition mentioned by Riley (1925) in the original description. During the present study, eight flowering specimens, mostly from San Salvador, had filament-tubes, sometimes to 1.5 mm long. The tube was very delicate, and separated easily into the individual filaments. The floral remains at the base of young fruits of some of these specimens had the filaments separate. The specimens with a filament-tube generally were more robust, had slightly larger, and more leaves, and were possibly associated with more mesic conditions.
Adsersen et al. (1988) found living material of this species to be strongly cyanogenic. Of the 37 herbarium specimens tested for the presence of cyanogenic compound during the present study, none gave a positive test even after adding emulsin. It is possible that the cyanogenic glycoside or the enzyme involved with cyanogenesis in this species was destroyed when the specimens were preserved.
Baltra (South Seymour Island):
- May 1899, R.E.Snodgrass & E.Heller 586 (CAS, DS, GH).
San Salvador (James Island):
- James Bay, Aug 1891, G.Baur 64 (GH);
- Orchilla Bay. Aug 1891, G.Baur 65 (GH);
- James Bay, 4 Jun 1932, J.T.Howell 9660 (CAS, US);
- Sullivan Bay, 13 Jun 1932, J.T.Howell 10043 (CAS, GH);
- James Bay, Apr 1899, R.E.Snodgrass & E.Heller 366 (DS, GH);
- Jun 1899, R.E.Snodgrass & E.Heller 398 (DS, GH, NY);
- N side, 28 Jul 1905, A.Stewart 1617 (CAS);
- James Bay, 26 Dec 1905, A.Stewart 1618 (CAS, GH).
San Cristóbal (Chatham Island):
- roadside between Puerto Bacqueriso (Wreck Bay) and El Progreso, 8 Feb 1967, I.L.Wiggins & D.M.Porter 391 (US).
Santa Cruz (Indefatigable Island):
- Charles Darwin Research Station Area, 31 Oct 1982, P.S.Bently 19 (US);
- alt. 50 ft., 7 Dec 1952, R.I.Bowman 48 (CAS, UC);
- Academy Bay, rare, on lava ridge in scrub on lava rock, alt. 1-3 m, 24 Jan 1964, F.R.Fosberg 44706 (MO, UC, US);
- Darwin Research Station, Academy Bay, alt. 8 m, 5 Feb 1964, L.A.Fournier 146 (CAS, DS, US);
- sea level, 400 m from coast, sand-lava stone, 2 Nov 1935, W.von Hagan 95 (NY, US);
- Academy Bay, alt. 10 m, 6 Oct 1932, H.J.F.Schimpff 56 (A, M);
- North Indefatigable Island, May 1899, R.E.Snodgrass & E.Heller 677 (GH);
- along "old trail" from Academy Bay toward Bella Vista, 0.6 km inland from "Cabo Colorado", 13 Aug 1963, D.Snow 413 (CAS, DS);
- N side, small tree at 250 ft., 24 Nov 1905, A.Stewart 1541 (CAS, GH);
- NE side, low bush near shore, 25 Oct 1905, A.Stewart 1615, 1616 (CAS, GH);
- NE side, small stunted tree near the shore, 1905-06, A.Stewart 1676 (CAS);
- Seymour Bay, 19 Apr 1923, W.M.Wheeler, R.Rose & W. Beebe 74 (GH, NY);
- Nelson's Landing, on bare lava rock, 24 Jan 1964, I.L.Wiggins 18323 (CAS, DS, GH, US).