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.
This plant is indigenous, and is found in wet places, ditches, by streamlets, and bogs. The habit is prostrate or erect. The whole plant is bluish-green, more or less devoid of hairs. The stem is 4-angled, spreading, much-branched. The leaves are ovate, lance-shaped, oblong, acute, narrowed both ends, the tip hard, fringed with hairs below, soft. The flowers are few, in cymes. The bracts have a smooth, membranous border. The calyx has a funnel-shaped tube, with narrow, long-pointed, 3-veined sepals. The petals are not so long as the latter, 2-partite. The capsule is ovoid, about as long as the calyx. The seeds are very small. The plant is 3-18 in. in height. It flowers from May to July, and is a herbaceous perennial.