PHYSICAL SEPARATIONS. “In some plants and most animals, species are driven to separate the sexes. This can be achieved in several ways. One partial solution is the spatial separation of male and female gonads in the same individual, as in monoecious plants with separate male and female flowers (e.g., maize) and in most hermaphroditic animals.”
http://journals.plos.org/plosbiology/article?id=10.1371/journal.pbio.1001899