Mercurialis perennis (female)
Mercurialis perennis has separate male and female plants. For some reason the male plants appear to dominate, in the areas in the UK that I've visited at least.
In 2008 an article written by R. G. Jefferson was published in the Journal of Ecology. Biological Flora of the British Isles: Mercurialis perennis L.
It mentions an interesting relationship between sex and light conditions:
"Mukerji (1936b) was the first to show niche differentiation
of the sexes in relation to light flux. Female plants were more
abundant under low light and male plants more abundant
in high light. Males were absent at light fluxes below 10%,
whereas females did not occur below 1% or above 60%. The
highest dry mass for male shoots occurred between 50% and
70% light flux and for females between 20% and 40%. He
suggested that sex is not fixed at the seedling stage but is
determined subsequently by light levels. The basis for sex
determination would repay further investigation."
This corroborates with the ecotope where I saw these female plants: heavy shaded woodland.