Scarlet pimpernel (Anagallis arvensis; also known as red pimpernel, red chickweed, poorman's barometer, poor man's weather-glass, shepherd's weather glass or shepherd's clock) is a low-growing annual plant. The native range of the species is Europe and Western Asia and North Africa. The species has been distributed widely by humans, either deliberately as an ornamental flower or accidentally. A. arvensis is now naturalised almost worldwide, with a range that encompasses the Americas, Central and East Asia, the Indian Subcontinent, Malesia, the Pacific Islands, Australasia and Southern Africa.
This common European plant is generally considered a weed and is an indicator of light soils. The origin of the pimpernel name comes from pympernele [1400–50]; late Middle English, derived from Middle French pimprenelle, Old French piprenelle; Vulgar Latin *piperīnella= Latin piper pepper + -īn- -ine + -ella diminutive suffix.
Scarlet pimpernel has weak sprawling stems growing to about 50 cm (20 in) long, which bear bright green ovate sessile leaves in opposite pairs. The small orange, red or blue flowers are produced in the leaf axils from spring to autumn. The petal margins are somewhat crenate and have small glandular hairs. Blue-flowered plants (A. arvensis Forma azurea) are common in some areas, such as the Mediterranean region, and should not be confused with the related Blue pimpernel, Anagallis foemina, sometimes ssp. foemina. In 2007, a molecular phylogenetic study showed that Anagallis foemina is more closely related to Anagallis monelli than to Anagallis arvensis, and should be treated as a separate species. Scarlet pimpernel flowers are open only when the sun shines.
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