Botanical name

Acacia thoma Maslin, Nuytsia 18: 176, fig. 10 (2008)

Common name

Thoma's Wattle

Description

Multi-stemmed, rounded or obconic shrubs (1.5-) 2-3.5 m tall and 3-4 m across, crowns sparse to sub-dense, main stems and branches often sub-contorted. Bark grey, smooth but becoming rough and longitudinally fissured at base of mature stems. Branchlets terete, very finely (and obscurely) ribbed with silvery, appressed, straight hairs between the ribs at the extremities, soon glabrous. New shoots light green (the youngest phyllodes often brownish or tinged yellow but drying dark-coloured), resinous but not sticky. Stipules rudimentary, caducous, present only on very young new shoots. Phyllodes narrowly linear to very narrowly elliptic, 5-8 (-10) cm long, 2-3 (-4) mm wide, flat, coriaceous, sub-rigid, erect, straight to shallowly incurved, with short, straight, closely-appressed, silvery white hairs over the entire surface on uppermost phyllodes but indumentum becoming confined to between the nerves with age (the hairs difficult to see without magnification), sub-glaucous, dull green (with a slight bluish tinge) to silvery light greyish green; parallel longitudinal nerves numerous, very fine, ą resinous and of uniform prominence; apices straight to curved or occasionally uncinate, narrowed to a small, blunt, brown callosity; pulvinus indistinct, 1-2 mm long, finely transversely wrinkled when dry. Gland minute and indistinct, situated on upper margin of phyllode 0-2 mm above the pulvinus. Inflorescences (few seen) simple, 1 or 2 within axil of phyllodes; peduncles 3-6 mm long, appressed silvery-white hairy with the hairs sometimes confined to lower half of peduncle and often obscured by resin, sometimes glabrous when in fruit; spikes 10-25 mm long, light golden, the flowers not especially densely arranged within the spikes (best observed in mature bud), the buds slightly resinous but not sticky; receptacle glabrous. Bracteoles sub-peltate, very small (about same length as calyx). Flowers 5-merous; calyx minute, 1/5-ź length of corolla, irregularly divided for ˝-ž its length into ąoblong or broadly triangular, minutely and sparsely ciliolate lobes; petals 1.3-1.4 mm long, joined for ˝-2/3 their length (the free portion recurved following dehiscence), glabrous, nerveless. Pods sub-moniliform (i.e. terete to sub-terete, gently raised over seeds and shallowly to moderately constricted between them), 2-6 cm long, 2-3 mm wide, thinly coriaceous-crustaceous, straight to very shallowly curved, resinous but not sticky, brown, minutely silvery sericeous between the obscure, longitudinal nerves; marginal nerve discrete (but not thickened) and yellowish. Seeds longitudinal in the pods, narrowly ellipsoid to narrowly obloid-ellipsoid, 4-5 mm long, 1.5-2 mm wide, shiny, dark brown; pleurogram commonly bordered by a diffuse band of dull yellow tissue; areole 'u-' or 'v-'shaped, open towards the hilum, very small (0.2-0.5 mm long); funicle folded beneath the small, white aril.

Characteristic features

Multi-stemmed shrubs. Branchlets silver sericeous towards extremities. Phyllodes narrowly linear to very narrowly elliptic, mostly 5-8 cm long and 2-3 mm wide, parallel longitudinal nerves fine and of uniform prominence, with silvery appressed hairs that become confined to between the nerves with age (hairs difficult to see without magnification and often obscured by resin). Peduncles short (3-6 mm) and appressed -hairy; spikes golden with flowers not especially densely arranged. Flowers small (petals 1.3-1.4 mm long); calyx irregularly dissected for ˝-ž its length, minute (1/5-ź length of corolla). Pods sub-moniliform (i.e. terete to sub-terete, gently raised over seeds and shallowly to moderately constricted between them), short and very narrow (2-6 cm x 2-3 mm), obscurely longitudinally nerved, minutely silvery sericeous between nerves. Seeds narrowly ellipsoid to narrowly obloid-ellipsoid, 4-5 mm long.

Distribution and ecology

Occurs in Western Australia where it has a scattered distribution from the Pilbara region south to Meekatharra and Leinster and extending east to Mt Nossiter in the Little Sandy Desert (A. subcontorta shows a somewhat similar distribution pattern). In the Pilbara A. thoma is confined to the Mt Channar region (Eastern Range), east of Paraburdoo where it occurs in two areas (about 15 km apart), both of which contain a few small, discontinuous populations that extend over a distance of about 1 km. In the Pilbara A. thoma grows in skeletal soil on ridges and rocky slopes towards the base of banded ironstone ranges, in open shrubland with common associates including Acacia aneura, A. sibirica, A. rhodophloia, A. tetragonophylla, Eremophila jucunda and E. fraseri. At its more southerly localities A. thoma is also normally associated with rocky habitats, for example, at Leinster it is reported to occur in soil over ironstone, ultramafic rocks in association with Mulga shrubland, while at Mt Nossiter it is reported to grow in a 'rocky habitat' but the rock type was not identified. The Meekatharra collection was gathered from a roadverge situation in 'dry red soil', and the underlying rock type was not identified.

Flowering and fruiting period

This species appears to flower in response to summer rainfall events. Plants in flower have been collected in May and June and pods with mature seed have been collected from August to October.

Affinities

Acacia thoma appears to be most closely related to the more southerly distributed A. coolgardiensis subsp. effusa; indeed, Cowan and Maslin (1995) treated the Pilbara occurrence of A. thoma as a northern outlier of subsp. effusa. Subspecies effusa comprises two informal variants which are distinguished on the basis of their peduncle length and it is the variant with pedunculate spikes that most closely resembles A. thoma but which is most readily distinguished in the following ways: main stems fluted with at least some vertical furrows and folds (stems not fluted in A. thoma), peduncles invested with red-brown resin hairs additional to the silvery-white appressed orthodox hairs (resin hairs not present in A. thoma), flowers larger (petals about 1.8 mm long) and more densely arranged in the spikes, calyx much longer (˝ -2/3 length of corolla) with the sepals normally free (or sometimes shortly united at the base), pods not or scarcely constricted between the seeds and seeds slightly shorter (3-4 mm long) and differently shaped (i.e. obloid). Additionally, there are subtle differences in the phyllode nerves (broader in subsp. effusa than in A. thoma) which suggests underlying anatomical differences between the two. Although there is overlap in the range of variation for phyllode size between the two taxa, subsp. effusa often has longer phyllodes (to 14 cm). In the Pilbara A. thoma is most similar to A. sibirica as the two species are often similar in habit and phyllode shape and size, and both have pedunculate spicate inflorescences. Acacia thoma and A. sibirica sometimes grow together and special care is needed not to confuse them, at least when pods are not present on the plants. Fruiting specimens are easy to distinguish because A. sibirica has narrowly oblong, flat pods which are 3-5 (-7) mm wide, whereas those of A. thoma are sub-moniliform, ąterete and narrower (2-3 mm wide). Indumentum is another character that enables these two species to be distinguished; in A. sibirica the branchlets, phyllodes and peduncles are glabrous whereas in A. thoma these structures are invested with silvery-white appressed hairs. However, the hairs in A. thoma are not especially obvious and specimens must be carefully examined at x10 magnification or above to see them. The calyx of A. sibirica is also different from that of A. thoma in being longer, broader at its base and less deeply dissected into broadly triangular lobes. Some flat phyllode forms of A. aneura (Mulga) may also superficially resemble A. thoma (these two species grow together in some areas) but are readily distinguished by their broader, flat pods and linear, free sepals; also, the phyllode nerves of these Mulgas are resinous and normally more obvious than those of A. thoma.

Conservation status

Not considered rare or endangered.

Origin of name

This species is named for Emil Thoma, Botanical Advisor with Rio Tinto Iron Ore. Emil has worked in the Pilbara region for 30 years during which time he has provided the authors with much assistance (particularly in the form of specimens and information) concerning the Acacia flora of that region. Apart from Acacia Emil has been instrumental in documenting the distribution of numerous Declared Rare and Priority Flora species and has discovered many new plant records for the region.

References

Cowan, R.S. and Maslin, B.R. (1995). Acacia miscellany 15. Five groups of microneurous species of Acacia (Leguminosae: Mimosoideae: section Plurinerves), mostly from Western Australia. Nuytsia 10(2): 205-254.