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 species is marshes and ditches, bogs and wet places. The plant is erect or ascending in habit. The stem is rough, prickly. When dry it does not turn black, but remains green. The stem is slender, brittle, and weak, with the bristles turned-back. The leaves are 6-8 in a whorl, or rarely four, narrower than in G. palustre, more rigid, with a blunt point, linear to lance-shaped, with a bristle point, the margins rough. The flowers are white, few, in a small axillary cyme, on erect flower-stalks. The branches of the cyme are 3-fid, spreading. The fruit is granular, dark-brown, on erect stalks. The plant is 1-2 ft. high, flowering in July and August, and is a herbaceous perennial.