A small, short-lived, herbaceous plant with creeping to semi-upright stems. Its slender stems are often reddish, contain a milky sap, and are softly hairy. Its paired leaves (3-12 mm long) are borne on very short stalks and have entire or finely toothed margins. These leaves are green and sometimes bear a purplish or reddish-brown spot at their centre. Its tiny greenish or reddish 'flowers' are borne singly in the leaf forks. Its tiny hairy capsules (1.3-2 mm long) have three compartments, each containing a single pale brown, grey or greyish-brown seed.