distylous

Distylous refers to a specific breeding system found in some flowering plants. It is characterized by the presence of two distinct flower morphs within a population. These morphs differ in the relative lengths of their stamens (male reproductive parts) and styles (female reproductive parts). In one morph, called the pin flower, the style is long and extends above the stamens. Conversely, in the thrum flower, the stamens are long and extend above the style.

This dimorphism is genetically controlled and typically involves a single gene locus with two alleles. The inheritance pattern ensures that pin flowers are pollinated by thrum pollen, and thrum flowers are pollinated by pin pollen. This cross-pollination mechanism, known as reciprocal herkogamy, is thought to promote outbreeding and increase genetic diversity within the species.

Distyly is a form of heterostyly, which is a broader phenomenon of having different flower forms with varying lengths of pistils and stamens. Examples of distylous plants include species of *Primula* (primroses) and *Linum* (flax). The pollination strategy associated with distyly is effective in preventing self-pollination and encouraging gene flow between individuals.