This section is from the book "British Wild Flowers - In Their Natural Haunts Vol5-6", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
The habitat of this plant is wet heathy places, the margin of ponds, damp places. The habit is creeping-. The stem is long, creeping. The plant is devoid of hairs, branched, tufted. The stems are 4-angled, creeping, fragile. The leaves are opposite, shortly-stalked, inversely ovate. The flowers are small, solitary, purple, axillary. The petals are often absent. The tube of the calyx is short, shortly bell-shaped, not so long as the capsule, 12-ribbed. The teeth are triangular. There are 6 or 12 stamens, The capsule is rounded. The plant is 3-8 in. in height, flowering from July to August, and is a herbaceous perennial.