Floral organ number is often fixed within families, and the basic floral ground plan of Brassicaceae is well conserved. Cardamine hirsuta L. (Brassicaceae) shows variation in lateral stamen number, that is, zero to two lateral stamens. The aim of this study was to examine whether temperature conditions alter stamen number. Temporal changes in the frequency of flowers with zero, one, and two lateral stamens were assessed during the flowering periods of a natural population and a garden-transplanted population in Japan. We conducted an experiment to evaluate how temperature regimes during flowering (15°/15°C and 15°/5°C) alter the number of stamens. The proportion of flowers with zero lateral stamens increased in both the natural and the garden-transplanted population as the flowering season progressed. In the growth experiment, lateral stamen numbers fluctuated even within individual inflorescences, but the frequency of flowers with zero lateral stamens was higher in the high-temperature condition than in ...