Confused by girl's mercury

I don’t know whether it’s because I’m not yet running at full steam, but I found today’s word somewhat confusing. “Girl’s mercury” is ‘The male of either of two plants of the genus Mercurialis, M. tomentosa and M. annua, formerly supposed to have the property of inducing the generation of female children.’ (OED). ‘The’ generation? Whose children? And the citations didn’t really help, advising that boy’s mercury was the name applied to the female plant. All in all, this cross-gendering was too much to cope with along with the pouring rain, disruption on the Central line and some confusion as to what these mercurial plants actually look like. I settled, therefore, for this olive tree, which does resemble the genus mercurialis insofar as it is a plant, it is green and it has leaves.
Day #147 of a 365 project, where the daily pic is informed by the OED word of the day.