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.
Ludwigia palustris, Ell.(= Isnardia palustris, L. =L. apetala, Walt.).- The habitat of this plant is pools, and peaty streams and pools. The habit is prostrate then ascending, or floating. The stems root at the nodes, and are 4-angled, branched, hairless, often reddish. The leaves are opposite, elliptic, egg-shaped, acute, narrowed to the leafstalk, shining. The flowers are small, green, often apetalous, stalkless, in the axils, the parts in fours, the lobes triangular, acute. The bracts are awllike. The style is short, the stigma large, pin-headed. The capsule is egg-shaped, blunt, 4-sided, with the persistent calyx spreading horizontally. The seeds are angular. The plant is 6-10 in. in height, flowering in June and July, and is a herbaceous perennial.
The habitat of this plant is in waste places. The plant is erect, downy or hairy. The stem is leafy. The leaves are ovate to lance-shaped, flat, with distant teeth. The leaf-stalk is short, with a stout, white midrib. The flowers are bright yellow, in a sort of spike, stalkless, large, numerous. The petals are inversely heart-shaped, and exceed the stamens, which are as long as the style. The calyx-lobes are longer than the ovary. The capsules are oblong, more or less cylindrical, or roundly 4-angled, and taper upwards. The plant is 2-3 ft. in height, flowering from July to September, and is a herbaceous biennial.