Mentha pulegium

''Mentha pulegium'', commonly (European) pennyroyal, or pennyrile, also called mosquito plant and pudding grass, is a species of flowering plant in the mint family, Lamiaceae, native to Europe, North Africa, and the Middle East. Crushed pennyroyal leaves emit a very strong fragrance similar to spearmint. Pennyroyal is a traditional folk remedy, emmenagogue, abortifacient, and culinary herb, but is toxic to the liver and has caused some deaths. European pennyroyal is related to an American species, Hedeoma pulegioides. Though they differ in genera, they share similar chemical properties.

Description

An annual to perennial plant with creeping or erect branched stems to about 40 cm in height. The stems are square in cross-section and can vary from hairless on some plants to densely hairy on others, with a green to sometimes red or purplish colour. The leaves, which grow in opposite pairs, are narrowly oval, 2–3 cm long x 1 cm wide, downy, sparsely toothed towards the tip, and taper into a short stalk. All parts of the plant are strongly scented when crushed but it does not have noticeable glands on its surface. The small (6 mm) flowers are densely packed in whorls at the nodes, widely separated, above pairs of leaf-like bracts. The calyx is a ribbed tube about 3 mm long, with 5 triangular teeth, the lower 2 being narrower and slightly longer than the upper 3; it is hairy both on the inside and the outside. The corolla has 4 mauve lobes or "petals" and is hairy only on the outside. The flowers are bisexual and have 4 long stamens, 2 (or all 4) of which project well beyond the corolla lobes. There is one long style which is forked to produce 2 stigmas, which also project from the flower. The fruits consist of a cluster of four brown, 1-seeded nutlets, each about 0.7 mm long.

The flowering period starts in June and continues into mid-summer, although in northern countries it flowers rather later, sometimes as late as September, when it can fail to set seed.

Its chromosome number is 2n = 20.

Distribution

The native range of pennyroyal is thought to be around the eastern Mediterranean, where it grows in damp meadows, around pools and in stream margins. It is, however, very widely established around the world, including North and South America, throughout Africa, Asia, Australia and New Zealand. In many places it is considered a troublesome weed of agriculture. Towards the northern edge of its range, as in Britain, it is considered to be rare and declining, except where introduced.

Habitat and ecology

The habitat of pennyroyal is seasonally damp pasture, where standing water over the winter leaves bare ground in the summer, and where livestock preferentially graze other plants. An analogue of this habitat is found on roadsides, where trampling or disturbance of the ground produces similarly bare soil, particularly where there is waterlogging in winter. It is also found along watercourses, in wet woodland and in abandoned fields.

In California, where it is considered an invasive species, it occupies a similar niche, in seeps, streamsides, vernal pools and swales, marshes, and ditches. There is some speculation that it may displace native species in these areas.

Few animals eat pennyroyal. In Britain, the only insect known to feed on it is the bug ''Heterogaster artemisiae'' Schilling, which is a seed bug that normally feeds on wild thyme.

It is considered an axiophyte in many British counties, because low-intensity pasture is a rare habitat, although it has been spreading in recent decades. Its Ellenberg values in Britain are L = 8, F = 7, R = 5, N = 7, and S = 0.

History

Ancient to modern use

Documented use of pennyroyal dates back to ancient Greek, Roman, and Medieval cultures. Its name – although of uncertain etymology – is associated with Latin ''pulex'' (flea), alluding to the manner it was used to drive away fleas when smeared on the body. Pennyroyal was commonly incorporated as a cooking herb by the Greeks and Romans. A large number of the recipes in the Roman cookbook of Apicius called for the use of pennyroyal, often along with such herbs as lovage, oregano and coriander. Although it was commonly used for cooking also in the Middle Ages, it gradually fell out of use as a culinary herb and is seldom used as such today.

Records from Greek and Roman physicians and scholars contain information pertaining to pennyroyal's medicinal properties, as well as recipes used to prepared it. Pliny the Elder, in his encyclopedia ''Naturalis Historia'' (''Natural History''), described the plant as an emmenagogue, and that it also expelled a dead fetus