The atmospheric CO.sub.2 concentration CO.sub.2 has been increasing markedly since the industrial revolution and is predicted to reach 500-1,000 µmol mol.sup.-1 by the end of this century. Although the short-term and acclimatory responses to elevated CO.sub.2 have been well studied, much less is understood about evolutionary responses to high CO.sub.2. We studied phenotypic and genetic differences in Plantago asiatica populations around a natural CO.sub.2 spring, where CO.sub.2 has been consistently high over an evolutionary time scale. Our common-garden experiment revealed that plants transferred from habitats with higher CO.sub.2 had higher relative growth rates, greater leaf to root ratios, lower photosynthetic rates, and lower stomatal conductance. The habitat-dependent differences were partly heritable because a similar trend of leaf to root ratio was found among their offsprings. Genetic analyses indicated that selfing or biparental inbreeding might promote local adaptation in areas with high CO.sub.2 despite substantial gene flow across the CO.sub.2 gradient. These results indicate that phenotypic and genetic differences have occurred between high and normal CO.sub.2 populations. Keywords Adaptation to elevated CO.sub.2 concentration * CO.sub.2 spring * Evolutionary response * Plant function * Plantago