This section is from the book "British Wild Flowers - In Their Natural Haunts Vol2-4", by A. R. Horwood. Also available from Amazon: A British Wild Flowers In Their Natural Haunts.
Common in damp places, and known as Creeping Jenny in the garden, Moneywort is a plant of the Northern Temperate Zone, found in Europe generally, and a garden escape only in the Northern United States. It is unknown earlier than in the present-day flora. In Great Britain it is absent from Cornwall in the Peninsular province, occurring in the Channel province; in the Thames, Anglia, Severn provinces generally, but in S. Wales not in Radnor, and in N. Wales in Merioneth, Montgomery, Carnarvon, and Denbigh only; and throughout the Trent and Mersey provinces, except in Mid Lancs in the Humber; Tyne provinces, except in Cheviotland; in the Lakes province, except the Isle of Man; in the W. Lowlands, except in Kirkcudbright, and it "is found in Stirling. Watson considers that it is not native in Scotland, or beyond York and Durham in England, and it is introduced in E. Lowlands. It is not native in Ireland, and rare.
Moneywort is a more or less common plant in damp places, such as marshy tracts bordering a river or lake, where there is some shade and shelter from trees, and it is also characteristic of moist woodlands, carpeting the banks of ditches and banks where natural glades or artificial rides allow daylight to enter unhindered.
The habit of this species is very different to that of the Common Loosestrife, being trailing or creeping. It has numerous simple stems, branched, jointed, and channelled each side, or square. The leaves are roundish, opposite, heart-shaped, smooth, shortly-stalked, and decurrent.
The flowers are large, yellow, solitary, axillary, and wheel-shaped. The sepals are egg-shaped to heart-shaped, bent back, and keeled underneath. The corolla is deeply divided. The anther-stalks are united at the base. Fruit is rarely produced. Being prostrate it is scarcely more than 3 in. in height. Flowers are found in June and July. It is a herbaceous perennial, reproduced by division.
The flowers are conspicuous but prostrate, and though homogamous they do not set good seed, because the flowers of the same neighbourhood are usually from the same stock. Otherwise the flowers are as in Yellow Loosestrife. Pollen may fall on the stigma and self-pollination may result. The capsule splits up into five valves, and the seeds are thrown out from the fruit by the wind.
Photo. Flatters & Garnett - Moneywort (Lysimachia Nummularia, L.)