Tectonic processes drive megacycles of atmospheric carbon dioxide (CO ₂ ) concentration, c _a , that force large fluctuations in global climate. With a period of several hundred million years, these megacycles have been linked to the evolution of vascular plants, but adaptation at the subcellular scale has been difficult to determine because fossils typically do not preserve this information. Here we show, after accounting for evolutionary relatedness using phylogenetic comparative methods, that plant nuclear genome size (measured as the haploid DNA amount) and the size of stomatal guard cells are correlated across a broad taxonomic range of extant species. This phylogenetic regression was used to estimate the mean genome size of fossil plants from the size of fossil stomata. For the last 400 Myr, spanning almost the full evolutionary history of vascular plants, we found a significant correlation between fossil plant genome size and c _a , modelled independently using geochemical data. The correlation is consistent with selection for stomatal size and genome size by c _a as plants adapted towards optimal leaf gas exchange under a changing CO ₂ regime. Our findings point to the possibility that major episodes of change in c _a throughout Earth history might have selected for changes in genome size, influencing plant diversification.