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Acacia intorta (foreground) and A. xiphophylla (background). Photographer: B.R. Maslin
Photographer: B.R. Maslin
Photographer: B.R. Maslin
Photographer: B.R. Maslin
Seed from one herbarium voucher. Scale in mm. Photographer: F. McCallum.
Acacia intorta Maslin, Nuytsia 4: 398, figs 9, 10 & 14 (1983)
Narrow phyllode Snakewood
Rather gnarled, rigid shrubs 1.5-3 (-4) m tall, with 2-4 main stems from ground level, the stems and main branches rather contorted or twisted (branches on plants outside Pilbara often spread horizontally but this has not been observed in the few Pilbara plants seen, perhaps because these have been heavily grazed by cattle), crown rather dense. Bark grey, fibrous, longitudinally fissured. Branchlets with silvery white, closely appressed, straight hair at extremities, soon glabrous. New shoots with appressed silvery hairs. Phyllodes linear, (4-) 5-10 (-12.5) cm long, (1.5-) 2-4 mm wide, flat (but normally terete to sub-terete outside the Pilbara), rigid, erect, straight to shallowly incurved, glabrous or (youngest phyllodes) inconspicuously appressed-hairy, green to sub-glaucous, very glaucous on juvenile phyllodes of young plants; parallel longitudinal nerves numerous, fine, close together and obscure, the central nerve slightly more evident than the rest; drawn out at apex and ending in a straight, slender, brown, needle-like point. Inflorescences simple or extremely reduced racemes with axes about 0.2 mm long, comprising 1 or 2 pedunculate spikes; peduncles (3-) 5-15 (-20) mm long, glabrous or appressed -hairy; spikes 1-5 cm long, golden, the flowers not close together. Flowers 5-merous; calyx ¼ or less length of corolla, very shallowly dissected into broad-triangular lobes. Pods narrowly oblong, flattened but rounded over seeds, not or only shallowly constricted between seeds, size variable (3.5-9 cm long and 5-9 mm wide), firmly chartaceous, straight or shallowly curved, glabrous, dark brown over seeds and pale brown or dull cream between them. Seeds longitudinal in pods, obloid to ellipsoid or almost circular, flattened, 7-9.5 mm long and 6-8 mm wide (sometimes smaller outside the Pilbara), dull, uniformly mid-brown to dark brown; funicle ±linear and expanded into a small, capitate, dull yellow aril.
'Snakewood' growth form. Phyllodes flat, narrow (mostly 2-4 mm), rigid, erect, with many fine, parallel, longitudinal nerves (observe at x10 magnification), the apices drawn out into needle-sharp brown points. Flowers loosely arranged in golden, pedunculate spikes. Pods thin-textured, flattened but rounded over seeds. Seeds large (7-9.5 x 6-8 mm) and flattened.
Occurs in northwest Western Australia where the main distribution is south of the Pilbara in the upper reaches of the Ashburton River where it is found along drainage systems between Bulloo Downs Station and Mt Vernon Station; there is also an outlier about 300 km to the southeast on Wongawol Station near Lake Carnegie. The Pilbara occurrence of A. intorta also represents an outlier, occurring on Mt Florance Station (north of Wittenoom) about 200 km north of Mt Vernon Station. At Mt Florance A. intorta is apparently uncommon and it grows with its closest relative, A. xiphophylla (Snakewood) on alkaline clay soil in 'crabhole' country adjacent to the Fortescue River. In the Ashburton region A. intorta also grows on alkaline clays where it occurs on calcrete slopes, shale slopes and saline drainage floors; it apparently does not extend to higher plains where the soils are acidic.
Flowers from May to around early July although occasional flowers may be found on fruiting plants in October. Pods with mature seeds occur in September and October.
On most plants of A. intorta the phyllodes are very narrow and terete or almost so, but broader, flat phyllode forms do occur. For example, on Tangadee Station in the Ashburton district south of the Pilbara, individuals with flat phyllodes (to 4 mm wide) occur at a low frequency within a population dominated by individuals with ±terete phyllodes. All of the few Pilbara specimens of A. intorta that have been seen possess flat phyllodes (3 mm wide) and further study is needed to determine whether or not plants with terete phyllodes also occur in this region. Similarly, the pods and seeds of A. intorta display considerable variation in size. The one Pilbara fruiting specimen seen had broad pods (about 9 mm wide) and large seeds (7-9.5 mm long and 6-8 mm wide). Plants with similar carpological characters have been collected from the Ashburton but there are also specimens from that region with much smaller pods (5-6 mm wide) and seeds (3 mm wide). It is not known what factors are responsible for this unusually wide range of variation. The growth form displayed by the very few Pilbara plants of A. intorta that have been seen is slightly different from that found on plants from the Ashburton district (see description above). If additional Pilbara populations of A. intorta can be found and examined, then perhaps at least some of the variation discussed here will be resolved.
Acacia intorta is most closely related to Snakewood (A. xiphophylla) which is very common in central and western areas of the Pilbara; the two species co-occur on Mt Florance Station. These species have a somewhat similar growth form (although note comment in description above), finely multi-nerved phyllodes, flowers rather loosely arranged in long, cylindrical spikes, large, thin-textured pods and large, flattened seeds. Acacia xiphophylla is most readily recognized by its phyllodes, which are wider (mostly 6-13 mm) and have tips that are not drawn out into long, needle-like points.
Although in the Pilbara A. intorta is known from only Mt Florance Station it is not uncommon in parts of the Ashburton region and is therefore not considered rare or endangered.
The botanical name is derived from the Latin intortus (twisted or bent on itself) in reference to the branches that give this species it's characteristic, spreading Snakewood growth form. However, plants seen on Mt Florance Stn were heavily grazed by cattle and as a consequence developed a more compact, less spreading habit than normal. The qualifier 'Narrow phyllode' in the common name serves to distinguish the species from true Snakewood (A. xiphophylla).