What are pin and thrum flowers in common primroses?

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A honeybee collects pollen from the yellow stamens of a pink and cream honeysuckle flower against a blurred green background. — Pollination Common carder bumblebee (*Bombus pascuorum*) pollinating a honeysuckle (*Lonicera* species) flower.

In the common primrose (Primula vulgaris), pin and thrum flowers are a structural mechanism known as heterostyly, which discourages self-pollination and promotes cross-pollination.1

The two types are distinguished by the relative lengths of their style and stamens:

Pin flowers: These possess a long style, which positions the stigma at the mouth of the flower, and short stamens.1
Thrum flowers: These have a short style and long stamens that form a "thrumhead" at the flower's opening.1

This arrangement ensures that when bees visit these flowers, they efficiently transfer pollen from the stamens of one type to the stigmas of the other. A genetic system regulates this structure, maintaining an equal 50:50 ratio of pin and thrum individuals within populations.1