The rapid respiration rate of cotton (Gossypium hirsutum) fruits produces a microenvironment with intercellular CO2 concentrations of 500–1300 μmol mol–1, and Hu et al. hypothesize that the adaptations that have evolved in response to this can provide insights relevant to future increases in atmospheric CO2. They compare morphological and physiological traits of bracts and leaves of cotton, and find that bracts show significantly lower stomatal conductance, which results in a significantly higher water use efficiency. Both gas exchange and protein content show a significantly greater RuBP regeneration/RuBP carboxylation capacity ratio (Jmax/Vcmax) in bracts than in leaves, which agrees with theoretical predictions regarding adaptation to elevated CO2. They conclude that cotton bracts provide readily available material for studying adaptations to elevated CO2.