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Primrose flowers come in two types, 'PIn-eye' and 'Thrum-eye'. This is pin-eye (whatever a 'thrum' is!), with the stigma prominent, and the anthers arranged in a ring deep inside the opening. The stigma therefore protrudes, waiting to receive pollen from a (you guessed it) thrum-eyed visiting insect, which has collected pollen from the upper ring of anthers of the thrum-eye. See thrum-eye in this set.  And vice versa...the pollen from this pin-eye sticks to the tip of the pollinating insect's mouthparts, or head, and will be positioned just right to pollinate the next door thrum-eye flower, with the stigma buried deep. So self-pollination of nearby flowers from the same clump, or clone, is avoided.  At least that is what the books used to say, but it always seemed to me rather crude. What is to stop thrum-eye pollen falling down off the insect into the thrum-eye stigma, for example? And the pin-eye pollen could easily hit the 'pin' as the insect starts to feed.  The answer is that the two types of pollen are chemically prevented from germinating on the same type of stigma. So why the pin/thrum thing which seems unnecessary? Well, this is in fact a device to stop the stigma being clogged up, or blocked, by too much of its own type of pollen, as the majority of the pollen will come from the opposite type, due to its position on the pollinating insect.  Neat, eh?