Annual herb c. 50-155 mm tall, stem simple to sparingly branched from the base or higher, moderately leafy, glandular-puberulous (hairs c. 0.1 mm) on peduncles and uppermost nodes, lower down replaced by tiny eglandular retrorse hairs, lowermost 1 or 2 nodes with patent glandular hairs up to c. 0.5 mm long. Leaves opposite below, alternate above, lowermost 1 or 2 pairs c. 5-16 x 2.54 mm, 1/3 to 1/2 the length petiolar, blade narrowly to broadly elliptic, tapering at the base, diminishing rapidly in size upwards, uppermost linear, margins entire to few-toothed, whole leaf glandular-puberulous when young, soon glabrescent except for hairs up to c. 0.5 mm long on margins and midrib below. Flowers up to c. 20 in small heads soon elongating into short (up to c. 30 mm) racemes, lowermost flowers somewhat distant, otherwise crowded. Bracts c. 3.5-5.5 x 0.4-1 mm, oblong, adnate to pedicel and calyx tube nearly or quite to apex, glabrous except for patent marginal hairs up to 0.5 mm long. Pedicels up to 0.5-1 mm long. Calyx obscurely bilabiate, tube 1.3-1.5 mm long, puberulous with tiny eglandular hairs, gland-dotted as well, anticous lobes 1.25-1.3 x 0.75-1 mm, small patent hairs on margins. Corolla tube 2.75-3.5 x 0.75-1 mm, widening slightly in mouth, limb c. 5 mm across the lateral lobes, posticous lobes c. 1-1.5 x 1 mm, anticous lobe 2-2.75 x 1.25-1.5 mm, all lobes elliptic, white, possibly with a yellow patch at base of posticous lip, bearded there. Stamens 4, anthers 0.5-0.75 mm long, dark-coloured. Stigma well exserted. Capsules c. 3 x 2 mm Seeds c. 0.75 x 0.5 mm, colliculate and irregularly wrinkled, amber-coloured.
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Like P. micranthum but flowers larger, with prominent dark anthers.