The annual Mercury ou Mercuriale ou la Foirolle (in Latin Mercurialis annua)
There are two common species of Mercury. The name descends from antiquity and is the name of the Roman God who is supposed to have discovered its medicinal virtues. The more well known species is coarse leaved, perennial and evergreen. That grows in woods and is called the dog's mercury. The prefix 'dog' indicates its inferior nature in herbal use. The other is the true mercury. That is an annual plant with soft leaves. It is common in vegetable plots though probably unrecognised. It was used as a purgative and is supposed to be edible if cooked like spinach. I would not advise it! ( la foire is also a French word for diarrhoea - thus foirolle .) My interest here is solely botanical not medicinal.
Both species normally have separate male and female plants. My photo shows those of the annual mercury. The male flowers are carried on long tassles and the pollen is plentiful. This pollen can cause hay fever. The plants will flower until the winter frosts kill them, so the plant can be responsible for allergies late in the season. The female flowers are small and tucked into the axils of the branches. In the photo at the bottom right - in grey- is an enlargement of a seed pod. These tiny fruits are dispersed in several ways. They are easily caught on fur or clothing by means of their tiny stiff hairs. But the pods also explode and the seeds scattered to a metre away. Each seed has on the outside a small swelling, a caruncle, attractive to ants. These insects carry the seeds to their nests and there eat the caruncle, leaving the seed to germinate.
But the annual mercury also has a form where the male and female flowers grow on the same plant. Otherwise the plants look the same. This form is genetically distinct and is better described as a separate species ( M. ambigua ); but the two interbreed. M. ambigua is predominantly found in Portugal, Spain and in the Midi of France. M. annua exists throughout Europe and as far north as central and south-east England. Recent research at Oxford has shown that where the populations meet, the form M. ambigua dies out. In northern Spain areas of up to 200 km wide have seen the replacement of the southern species with the northern wihin the last four decades. It seems that the copious pollen of the form annua swamps and pollinates the female flowers of the ambigua but the resultant seeds are infertile. In consequence the population is weakened.
The story promotes interesting hypotheses on the evolution of species. It would be supposed that long ago the distribution of the double species 'annua/ambigua' became split by some feature, perhaps something as obvious as the Pyrenees. The southern population developed, probably by quite a sudden genetic change, into the hermaphrodite M. ambigua species; and the northern evolved into M. annua . Now by some quirk of ecological change (climate?) the two meet up more than they were used to, and the southern species is in effect destroyed by the northern by screwing up its genetics.