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 ponds and ditches. The stems are densely leafy. The leaves are in whorls, several times forked, dark-green, with distant coarse teeth. The segments are awl-like, spreading, those of the upper leaves broader, more rigid. The spines are wanting. Flowers are not often to be found, and are very small. The calyx segments are notched above. The fruit has a slender, curved, awl-like style, and is armed with spines, without wings. The plant is 1-3 ft. in height, and flowers in June and July, being a herbaceous perennial.
The habitat of this plant is as in the last. From this it differs mainly in the fruit, which bears no spines, but cylindrical tubercles. The calyx-segments are all entire. The leaves are light-green. The leaf-segments are awl-like. When young the fruit is smooth. The plant flowers in June and July.