The Spermacoce of the Australia

Harwood, Aust. Syst. Bot. 18: 339, 2005.

Type: Philchowskis Creek, 25 km E of Kununurra, Western Australia, 21.iii.2003, G.M. Wightman 8108; holo: DNA; iso: AD, BR, BRI, CANB, K, L, MEL, NSW, PERTH, SI.

Annual herb, erect or more rarely spreading to 80 cm tall; stems ribbed, glabrous or sparsely scabrous to shortly hairy particularly on the ribs. Leaf blades linear, 7–120 x 0.5-3 mm, acute at the apex, hardly narrowed at the base, glabrous at both sides or sparsely scabrous or hairy; mid-vein slightly prominent below; secondary veins invisible; margin not callused. Stipules fimbriate; stipular base 1.5-4 mm long, glabrous or sparsely hairy; fimbriae 3-9, to 10 mm long, glabrous. Flowers subsessile in many-flowered capitula; capitula pseudoaxillary, to 20 mm wide in flowering stage, 10 mm wide in fruiting stage, usually subtended by one pair of leaves; subtending leaves similar to vegetative leaves; bracts stipuliform with numerous setae to 2 mm long. Calyx tube to 1.1 mm; calyx lobes 4, green often intermingled with red, triangular to narrowly triangular, to 2.5 mm long but usually much shorter, hairy at margins and sometimes also at mid-rib outside. Corolla white, mauve or blue; tube broadly funnel-shaped, 1-3.5 mm long, glabrous outside, some trichomes inside; lobes triangular sometimes with a somewhat oblong base, ascending with the upper part sometimes spreading, 2-4 mm long, glabrous or with a few trichomes outside, several longer moniliform hairs inside; appendages absent. Stamens exserted, always exceeding corolla lobes, inserted at corolla tube apex; anthers elliptic to narrowly oblong, 0.5-1 mm long; filaments 2.5-5 mm long. Style exserted, 3-9.1 mm long; stigma appearing capitate or shortly bilobed; nectary disc entire, c. 0.1 mm high, minutely hairy. Fruit irregularly obovate, 1.7-3.1 x 1.3-1.8, shortly hairy, indehiscent and only one locule developed. Seeds brown usually covered with a pale grey bloom, obovate-triangular and  trigonous, lengthwise somewhat curling up at the apex, 1.7-2.6 x 0.8-1.3 mm; ventral side without distinct groove; testa surface faintly reticulate; obturator not persistent.

Distribution
WA and NT: common throughout a large area of WA with two collections just crossing the NT border.

Habitat and phenology
Always on sand, in open areas of sandstone woodland or beside rivers or the coast. Flowering and fruiting February to June.

Notes
A rather variable species, particularly in growth form and size of reproductive organs. The species is well characterized, though, by its one seeded, indehiscent fruits. It is closely related to Spermacoce stenophylla from which it is hardly to distinguish in flowering state. The seeds of the two species are very different, however. Spermacoce gibba has obovate-triangular and  trigonous seeds that are curling-up lengthwise towards the apex, and a distinct ventral groove is absent. Seeds of Spermacoce stenophylla, on the contrary, are obovate-elliptic with a distinct ventral groove.

Etymology
Latin gibbus (humped, with a swelling), referring to the raised upper part of the seed.

Selected specimens (26 examined)
WESTERN AUSTRALIA: The Grotto, 2 km S of the Wyndham to Kununurra road, 22 km from Wyndham, 29.v.1986, J.R. Clarkson 6541 (BRI, DNA, K, MBA, MEL, PERTH, QRS). NORTHERN TERRITORY: Keep River NP, 2.iii.1981, C.R. Dunlop 5825 (DNA).