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Name

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).

Formal description

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.

Distribution

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.

Additional info

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.

Flowering time

November-January.

Representative specimens

ECUADOR:

Galápagos Islands:

Baltra (South Seymour Island):

San Salvador (James Island):

San Cristóbal (Chatham Island):

Santa Cruz (Indefatigable Island):

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