Primroses are coming into flower, most so far are in the West Country. Sweet-smelling delicate yellow blooms with crinkled, pale green leaves, the flowers have two different forms.
In one, the anthers with their pollen are at the top of the flower tube, while the stigma the female part of the flower that receives the pollen, is deep down inside the tube. In the other form, the reverse is the case: the pollen lies deep down while the stigma is high up. So when a brimstone butterfly comes to the flowers for nectar, and picks up the low-down pollen on its proboscis from one kind, it wipes it off on the low-down stigma on a flower of the other kind. Similarly, if it picks up on its proboscis the high pollen, it will find a high stigma for it on another flower. In this way the primroses are cross-pollinated, with no risk of self-pollination. These differences are easily seen if one examines the flowers closely.