Dog's mercury
Mercurialis perennis is a familiar native, ground-layer species of a wide range of types of British woodlands and hedgerows, where it is frequently regarded as an indicator of ancient, semi-natural woodland. The species has a high tolerance of low light levels, and an underground stem (rhizome) that stores carbohydrate food reserves once the above ground parts have died back during the autumn. The new shoots, of this apparently unremarkable plant, usually appear between January and March. Temperature-dependent shoot emergence happens when mean air temperatures have been above about five degrees Celsius for at least four consecutive days and but not below one degree Celsius.
Mercurialis perennis is dioecious, that is it has separate male and female plants. Plants will flower from late January to May, with male plants producing their long spikes of tiny, greenish-white flowers slightly earlier than the females. Female plants have their flowers tucked away in the axils of leaves, only becoming obvious following wind-pollination when they develop into hairy, two-seed, spherical fruits. When mature the fruits break apart, releasing the seeds that bear a whitish, oil-rich fleshy appendage (elaiosome) close to where they were attached to the fruit wall. The elaiosome is a food reward for Mercurialis perennis's main dispersal agent, ants. Seeds rarely establish though; the main way Mercurialis perennis propagates itself is via its rhizome. Consequently, Mercurialis perennis populations may comprise large areas of plants with a single gender of limited genetic variation.
Before the experimental demonstration that plants reproduced sexually at the end of the seventeenth century, botanists referred to the two different sorts of Mercurialis perennis as female and male ('Mercurialis foemina' and 'Mercurialis mas'). However, these names did not refer to the plant's genders; 'Mercurialis foemina' was the male plant and 'Mercurialis mas' the female. This misalignment of name and gender was not restricted to Mercurialis, it was also found in other dioecious plants such as cannabis and hops.
Mercurialis perennis is found throughout Britain, except the extreme north and west of Scotland and parts of East Anglian fenland; in the latter case, probably because it is intolerant of waterlogging. It occurs throughout Europe from Norway to the Mediterranean and North Africa through Central Europe to the Caucuses and eastern Russia, in habitats extending to c. 2000 m above sea level. As might be expected, large amounts of variation have been reported, some of which has been recognised by formal names.
Further reading
Kirby KJ and Thomas RC 2000. Changes in the ground flora in Wytham Woods, southern England from 1974 to 1991 - implications for nature conservation. Journal of Vegetation Science 11: 871-880.
Jefferson RG 2008. Biological Flora of the British Isles: Mercurialis perennis L. Journal of Ecology 96: 386-412.
Stephen Harris