Acacia

Taxonomy

Acacia P. Miller Gard. Dict. Abr. ed. 4. 28 Jan 1754.

Subfamily: Mimosoideae.
Phylogenetic Number: 2.4.01.
Tribe: Acacieae.
Species Studied - Species in Genus: 70 studied; ca. 1200 in genus.

Description

Fruit: A legume; unilocular; 2–30 cm long; 0.4–5 cm wide; 0.2–2 cm thick; 2–9 times longer than wide; with deciduous androecial sheath; with deciduous corolla; with deciduous calyx; without orifice formed by curving of fruit or fruit segments; straight to curved, or 0.5-coiled to 1-coiled to 1.5-coiled to 2-coiled to 3-coiled to 4-coiled to 5- to 10-coiled, or contorted; not plicate; not twisted to twisted; asymmetrical, or symmetrical; oblong to linear, or ovate to moniliform, or C-shaped, or falcate; with both sutures parallelly curved; not inflated; flattened to terete; without beak; rounded at apex to tapered at apex (rarely beaked); aligned with longitudinal axis of fruit; short tapered at base to tapered at base; aligned with longitudinal axis of fruit, or oblique with longitudinal axis of fruit (rarely); with the apex and base uniform in texture; membranous to ligneous; seed chambers externally visible, or invisible; margin not constricted to constricted; margin constricted along both margins; margin without sulcus; margin plain, or embellished (rarely); margin with prickles; wing(s) absent, or present (rarely); stipitate, or substipitate to nonstipitate; with the stipe up to 12 mm long; with all layers dehiscing (to tardily so), or indehiscent; splitting along suture(s). Dehiscence of valves along 1 suture, or both sutures (remaining attached to sutures); medial and up and down; passive, or active; with valves twisting. Replum invisible. Epicarp dull to glossy; monochrome; various shades or in combination with other colors brown to red, or black; with surface texture uniform; glabrous to pubescent and indurate (composed of various types of hairs); with hairs erect; with simple hairs; eglandular, or glandular (occasionally); with glandular dots; without spines; not smooth; with elevated features; faintly to strongly veined; reticulately veined; not tuberculate; occasionally longitudinally rugose, or glandular dotted; not exfoliating; without cracks; without embedded tissue, much thicker than epicarp, running from base to apex. Mesocarp absent to present; surface not veined; 1-layered; without balsamic vesicles; without reniform canals; when fresh fleshy, or spongy to fibrous throughout (on drying); ligneous (when dry). Endocarp present; visible; dull; opaque; monochrome, or streaked; black to tan; with purple overlay; without adhering pieces of testa; nonseptate to septate; pulpy (packed between seeds), or chartaceous (occasionally and enclosing individual seeds); not exfoliating; remaining fused to mesocarp and epicarp; entire. Seed(s) 1–30 (high value estimated); length transverse to fruit length to parallel with fruit length; neither overlapping nor touching; in dehiscent fruits 1 series, or 2 or more series (some indehiscent fruits, in 2 or 3 series). Funiculus 0.5–20 mm long; of 1 length only, or 2 different lengths, or 3 different lengths; filiform to thick (occasionally adnate to mature seeds); curved to plicate, or convoluted. Aril absent, or present (indurate and expanded funiculi best developed in Australian species are labeled arils); dry; when dry rim-aril, or tongue-aril, or caplike (clavate to foliaceous or elogate to 1–5 pliacte to encircling seed 1 or more times); entire; orange to red, or black (upon drying), or yellow.

Seed: 3–17 mm long; 1.5–12 mm wide; 1–10 mm thick; not overgrown; not angular; symmetrical; circular to elliptic, or ovate to oblong; flattened to terete; with surface smooth; without visible radicle and cotyledon lobes; without hilar sinus; without umbo on seed faces, or with umbo on seed faces (occasionally); without medial ridge on each face. Cuticle not exfoliating; not inflated; not wrinkled. Testa present; without pieces of adhering epicarp; not adhering to endocarp; free from endocarp; glossy; not modified by a bloom; colored; monochrome to mottled, or streaked; brown (various shades or in combination with other colors), or black, or green (with areola same shade or different shade than area outside of pleurogram or area immediately adjacent to pleurogram lighter or darker shades); with brown overlay; glabrous; smooth; osseous to coriaceous. Pleurogram present; 75–100 %. Pseudopleurogram absent. Fracture lines present, or absent. Rim absent. Wing(s) absent. Raphe not visible. Hilum present; visible, or fully concealed; concealed by funicular remnant; without faboid split; punctiform; subapical to radicle tip to apical at apex of radicle tip; flush; within halo, or not within corona, halo, or rim. Lens discernible; 0.1–1.1 mm long; with margins straight, or curved; triangular, or linear; elliptic; not in groove of raphe; mounded to recessed (pit); similar color as testa, or dissimilar color from testa; lighter than testa, or darker than testa; tan, or brown, or black, or green; within corona, or not within corona, halo, or rim; corona color lighter than testa to darker than testa (The lens of A. erioloba mimics a faboid hilum that has white lips along hilar groove). Endosperm present, or absent; thin to thick; not pluglike and not resembling tip of radicle; adnate to testa. Cotyledons smooth; both outer faces flat; both the same thickness; both more or less of equal length; not folded; margin entire 180 degrees from base of radicle; similar at apex; partially concealing radicle; with lobes; with lobes touching (auriculate); without basal groin formed by lobes; with the interface division terminating at base of radicle; without margins recessed; inner face flat; glabrous on inner face. Embryonic axis straight; parallel to length of seed, or oblique to length of seed (rarely); without a joint evident between the radicle and the cotyledons. Radicle differentiated from cotyledon; straight with embryonic axis; centered between cotyledons. Plumule well developed; glabrous.

Distribution

Generic Notes

In the Mimosoideae, Acacia is one of two (the other is Pithecellobium) genera whose seeds may have arils. Arils in Acacia are highly developed, especially the Australian species, and when present are a factor in seed dispersal (Glyphis et al. 1981). Bravato (1974) related the presence or absence of endosperm to three previously recognized segregte genera: Poponax C.S. Rafinesque-Schmaltz with "abundant and encircling endosperm" and Acacia s.s. and Senegalia C.S. Rafinesque-Schmaltz with "endosperm absent or scanty." The West African fruits and seeds of Acacia were studied and illustrated by Nongonierma (1978, 1979) and the South African seeds by Iksanova and Kaden (1971). For other recent reports, see Vassal (1972), Maslin(1975), Pettigrew and Watson (1975), Guinet and Vassal (1978), Pedley (1978–1979), and Ross (1979). Ross reported that the African species had seeds that are "exendospermous." Scott and Smith (1998) studied cotyledon architure and anatomy in this genus. Maslin (2001a, 2001b) taxonomically treated the 955 species of Australian Acacia. The phylogeny of Acacieae and Acacia has been studied using molecular and morphological data (Maslin et al., 2003; Miller and Bayer, 2000, 2003; Murphy et al., 2003). It was concluded that: 1) both are not monophyletic; 2) they are intertwined with tribes Ingeae and Mimoseae; 3) there are three major clades within Acacia that should be recognized as separate genera; and 4) extensive further studies are needed to resolve the situation.