Pin and Thrum on Primroses

Primula vulgaris (Primroses) are a common rosette forming herbaceous perennial, with solitary flowers which rise in clusters during springtime. They’re often an annual addition to borders, meadows, and wildlife gardens. Interestingly, and unbeknown to most gardeners is the heterostylous dimorphism first described by Charles Darwin, known as ‘pin or thrum’ formation.

Pin or thrum involves the stamen and stigma being differently positioned in individual plants to avoid self-pollination. Thus, the flowers are known as pin or thrum depending on the arrangement of the floral organs. In the pin formation the stigma dominates, occupying the mouth of the flower, and the stamen are hidden away at the base of the flower. In the thrum formation the stigma is shorter, and the stamen occupy the flower mouth.

Similar cases of heterostyly have also been observed in Pulmonarias (Lung wart)

Image: Handbook of botanical diagrams, Blodwen Lloyd, University of London Press 1949

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