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.
This common hedgerow plant is found in Interglacial beds in Sussex, and Neolithic beds in Essex and Edinburgh. To-day it is found in the N. Temperate Zone in Europe and N. Africa. In Great Britain it is absent in Hunts, Cardigan, S. Lincs, Mid Lancs, Isle of Man, E. Sutherland, Hebrides, Shetlands, but elsewhere general northwards to the Orkneys, up to 1700 ft. in the Highlands. It is native in Ireland and the Channel Islands.
What exactly are the requirements of this plant are somewhat puzzling, for it is absent in the same district from large areas which possess the same characteristics of shade which it requires; but it, is apparently not fond of some sandy districts, but rather of a humus subsoil, which it obtains in the dry woods and hedge-banks, which are its natural habitat. In some districts such surface may be leached out, causing it to disappear.
The root-stock is creeping, and from it the stems issue more or less in an erect manner, being simple, with many leaves, but leafless below, rounded, with wings. The leaves vary and may be rough, smooth, or hairy, oval, acute, stalked, with saw-like teeth, in pairs, with white glands on the margin. At the base of the leaf-stalks are 2 small acute stipules or leaflike organs. They form a cup to catch rain, and a rounded ridge in it with a row of hairs occurs and absorbs moisture.
The flowers are in loose spikes in the axils of the upper leaves, greenish, with no corolla. The female flowers are hidden among the leaves, more or less stalkless, the male on long flower-stalks very slender, with acute sepals. Male flowers may occur on the female rarely. The capsule is rounded, double, with 2 cavities with white cuticle, and there are 2 carpels.
Dog's Mercury is about 1 ft. in height. It flowers in April and May, and is perennial, as the second name implies, and reproduced by root-division.
The plant is dioecious, the stamens and carpels being on different plants, the males in axillary spikes, and the females clustered in a short raceme of 3 flowers. The styles are long and bent back, stigmatic in front. There is no corolla, and 2 carpels. The flowers are pollinated by the wind. The pollen is dust-like. The stigmas are said to be ripe at least two days before the anthers are ripe. On some female plants there may be a few male flowers capable of pollination.
When ripe the seeds fall out of the capsule around the parent plant.
Dog's Mercury is more or less a humus plant, requiring a humus soil.
The fungus Cceoma mercurialis attacks it.
Several beetles are found on Dog's Mercury, Hermceophaga mercurialis, Apion germari, A. pallipes, Trophiphorus mercurialis, Meli-gethes kunzei, and a moth, Phlogophora meticulosa.
Mercurialis, Pliny, was so called after the god Mercury, who is said to have discovered its virtues, and the second Latin name indicates its perennial character.
This plant is called Adder's-meat, Boggard-flower, Bristol-weed, Cheadle, Dog's Mercury, Dog's Cole, Kentish Balsam, Maiden Mercury, Wild Mercury, Leaf Mercury, Sapwort, Snake's Bit, Snake Weed, Town-weed. Dog's Mercury is so called to distinguish it not from the so-called English Mercury, or Goosefoot, but from the French Mercury (M. annua), formerly used in medicine. It is called Kentish Balsam, "from the similarity of the leaf to that of the Garden Balsam", and Town-weed from the growth of the plant in towns and town gardens, though this name may refer to M. annua.
The plant is poisonous, and not eaten by animals. When dry it turns blue, and steeped in water yields a deep blue dye, which is not permanent. It is acrid. The plant has been eaten as a spinach. It is laxative in effect.
Photo. Flatters & Garnett - Dog's Mercury (mercurialis Perennis, L.)
The male and female plants are not usually found in the same district, and therefore Dog's Mercury does not always produce perfect seed, being largely increased by the root-stock.
Essential Specific Characters: 276. Mercurialis perennis, L. - Dioecious, stem erect, simple, leafless below, leaves petioled, lanceolate, hispid, male flowers in axillary spikes, female in clusters hidden by leaves.