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Why do white clover and cabbage reject some of their own pollen?

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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.

White clover and cabbage reject their own pollen through a mechanism known as chemical self-incompatibility. This process serves as a barrier to self-fertilization, ensuring that the plant favors cross-pollination over self-pollination.

In this phenomenon, the plant utilizes internal chemical substances to prevent its own pollen from successfully fertilizing its ovules. Specifically, the pollen may fail to germinate on the stigma of the same flower, or the pollen tube may be unable to grow normally down the style to reach the ovary. This mechanism is genetically controlled and can vary in intensity throughout the flowering season. By limiting self-fertilization, these plants promote genetic variability, which is often advantageous for the survival and adaptation of the species in changing environments.12