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Name

Vachellia rorudiana (Christophersen) Seigler & Ebinger, Phytologia  87:  166.  2005.
syn.  Acacia rorudiana Christophersen, Nyt. Mag. Naturvidensk  70:  76.  1931.

Synonymy and types

Basionym:  Acacia rorudiana Christophersen, Nyt Mag. Naturvidensk.  70: 76.  1931. - TYPE:  ECUADOR.  GALÁPAGOS:  Santa Cruz Island (Indefatigable), Academy Bay, the dry zone, Dec 1926, B. Rorud 12 (holotype:  O).

Acacia tortuosa (L.) Willd.   glabrior Hook. f., Trans. Linn. Soc. London  20: 229.  1847. - TYPE:  ECUADOR.  GALÁPAGOS:  San Salvador (James Island), C. Darwin s.n. (holotype:  K).

Formal description

Prostrate shrub or small tree to 8 m tall.  Bark not seen.  Twigs dark purplish brown, slightly flexuous, lightly puberulent to pubescent, rarely glabrous.  Short shoots commonly present above the stipular spines, to 5 mm long, covered with acuminate stipules and old leaf bases.  Leaves alternate, the ones clustered on the short shoots commonly 5-45 mm long and with 2 to 12 pinna pairs per leaf, the ones on rapidly growing branches to 170 mm long and with up to 35 pinna pairs.  Stipular spines purplish to yellowish brown, becoming lightgray with age, symmetrical, terete, straight, stout, to 45(55) x 2.5 mm near the base, pubescent at least toward the base.  Petiole adaxially grooved, 2-8 mm long, lightly pubescent; petiolar gland solitary, usually located at or just below the lower pinna pair, stalked, circular to slightly elongated, 0.4-1.1 mm long, apex bulbous, glabrous.  Rachis adaxially grooved, 5-170 mm long, usually lightly pubescent, a stalked, circular to slightly elongated gland, 0.2-0.4 mm across between the upper 1 to 4(6) pinna pairs.  Pinnae 2 to 35 pairs per leaf, 7-25 mm long, 1.0-5.2 mm between pinna pairs.  Petiolules 0.2-1.0 mm long.  Leaflets 11 to 35 pairs per pinna, opposite, 0.2-1.1 mm between leaflets, linear, 0.6-2.8 x 0.3-0.8 mm, mostly glabrous, lateral veins not obvious, only one vein from the base, base oblique, margins ciliate, apex acute.  Inflorescence a densely flowered globose head, 6-10 mm across, solitary or in clusters of 2 to 3 on the short shoots.  Peduncle 14-35 x 0.4-0.7 mm, pubescent.  Involucre 4- to 5- lobed, located at the base of the globose head, glabrous to lightly puberulent, persistent.  Floral bracts spatulate, 1.1-1.6 mm long, ciliate, deciduous.  Flowers sessile, yellow orange: calyx 5-lobed, 0.9-1.7 mm long, the lobes puberulent; corolla 5-lobed, 2.0-2.7 mm long, the lobes puberulent; stamen filaments 2.5-4.0 mm long, distinct; ovary glabrous to lightly pubescent, on a stipe to 0.2 mm long.  Legumes light brown, becoming dark purplish brown with age, straight, flattened to slightly elliptical in cross section, not constricted between the seeds, oblong, 70-175 x 10-17 mm, coriaceous, reticulately striate, glabrous to lightly puberulent, usually glandular with minute purple glands, indehiscent; stipe to 5 mm long, apex acute.  Seeds uniseriate, no pulp, light to dark brown, ovoid, slightly flattened, 5.7-7.5 x 4.5-6.0 mm, smooth; pleurogram U-shaped to oval, 2.7-4.0 mm across. Flowers in October to March, or sporadically when moisture is available. Chromosome number:  Not determined.

Distribution

Disturbed sites and open scrub forests on scanty soil and bare lava rock in the Arid and Transition Zones of the Galápagos Islands.  Rudd (1971) lists this species from the islands of Española, Isabela, Pinzón, San Cristóbal, San Salvador, Santa Cruz, and Santa María.

Additional info

Vachellia rorudiana is restricted to Ecuador, and mostly to the Galápagos Islands, although Rudd (1971) indicated that it may occur in Chile.  Confusion exists concerning this taxon, and it has been variously treated as a distinct species (Robinson 1902, Rudd (1971) and as a synonym of V. macracantha (Svenson 1946).  These two taxa are distinct, though closely related. The stalked bulbous glands on the petiole and rachis of V. rorudiana are much different from the stalked, flattened glands of V. macracantha.  Also in V. rorudiana the small (usually less than 2.5 mm long, and often less than 1.5 mm long), narrow (0.3-0.8 mm wide) leaflets, the short petiole (2-8 mm long), the short shoots above each pair of stipular spines, the extreme variation in the number of pinna pairs per leaf (2-35), and the wide fruits (10-17 mm) separate this taxon from V. macracantha.  During the present study no specimens referable to V. macracantha were found on the Galápagos Islands, all specimens previously annotated by various authors as Acacia macracantha being referred to V. rorudiana.

Vachellia rorudiana is a common species of the arid and transition vegetation zones of the Galápagos Islands (Rudd 1971) where it occurs as a prostrate shrub or small tree to 8 m tall, depending upon disturbance and available moisture.  Collecting labels sometimes mention browsing by goats.  This taxon is highly variable morphologically, with leaves and leaflets varying greatly in size.  In particular, leaves that usually are clustered on the short shoots commonly are very small (5-45 mm long) with 2-12 pairs of pinnae, and the leaflets are mostly less than 1.2 mm long.  Leaves on rapidly growing branches, in contrast, are larger (to 170 mm long) with up to 35 pinna pairs, and the leaflets may be 2.8 mm long.  These differences are probably due to available moisture; specimens collected from wetter areas have larger leaves and leaflets.

Adsersen et al. (1988) found that both fresh and preserved materials of V. rorudiana were strongly cyanogenic.  During the present study more than 75% of the 63 herbarium specimens tested were strongly cyanogenic.  Usually, the more recently collected material gave a stronger positive test than material that was collected 30 to 50 years ago.

Flowering time

October-March, or sporadically when moisture is available.

Representative specimens

ECUADOR:

Galápagos Islands:

Española (Hood Island):

Isabela (Albemarle Island):

Pinzón (Duncan Island):

San Cristóbal (Chatham Island):

San Salvador (James Island):

Santa Cruz (Indefatigable Island):

Santa Maria (Charles Island or Floreana Island):

Manabí:

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