Plants go to enormous lengths to avoid self-pollination. The reason for this is that in-breeding is a bad thing and most organisms try to avoid it. Out-crossing, encouraging the greatest genetic diversity, discourages genetic defects and allows the greatest possibility for adapting to changing conditions. There are some exceptions of course: dandelions are apomictic meaning that the flowers produce seeds without pollination and do not have genetic diversity. It allows them to colonise their particular niche quickly and effectively but the plan does have its risks.
Some plants go to such extreme lengths to avoid self pollination that the male and female flowers are on different plants – such as holly (ilex) and skimmia. But one of the most fascinating, that demonstrates an amazing cooperation with an insect, is the foxglove (digitalis). The common and humble D. purpurea, and its hybrids are pollinated by bumble bees. The large tubular flowers with their clearly marked spots and big landing strip are perfectly suited to these large insects. Bumble bees, along with other bees, are methodical pollinators and will stick to a group of flowers when they find them rich in nectar. Bumble bees are even more methodical and when they get to a stand of foxgloves they visit the lower flowers first and then work their way up the stem until they reach the blooms that are not fully open and then fly off to visit the next stem, starting again at the base. Together with the way that foxgloves open their flowers ensures the flowers are cross pollinated.
When the flowers first open the stamens are closed but the soon split to reveal the pollen. After a few days the stamens wither and then the style withe receptive stigma at the end elongates slightly so it protrudes beyond the stamens and the stigma splits open to reveal the receptive surface.
On the flower stem the youngest flowers will be at the top and these are functionally male. The older flowers will be at the base and these are functionally female.
When a bee flies to the base of the stem, visiting the lowest flowers, these are female and, hopefully, the bee will be carrying pollen on its back from a flower on a different plant. As the bee pushes up the tube of the flower to get to the nectar it will deposit pollen on the stigma. This continues as the bee visits each flower up the stem until it reaches the younger flowers that are ‘male’ and it will pick up more pollen. The bee continues until it cannot get into the partly open blooms and then flies off to another plant, now loaded with pollen to repeat the process.