The stomata found on surfaces of leaves control the plant’s gas exchange and water loss by evaporation. Management of stomatal apertures feeds in to management of the plant’s hydration status and photosynthetic metabolism. Brodribb and McAdam now show that stomata of seed-bearing plants function differently from those of spore-bearing plants. Stomata of the angiosperm Helianthus annuus and the conifer Callitris rhomboidea open and close in response to the hormone abscisic acid (ABA), whereas those of the lycophyte Lycopodium deuterodensum and the fern Pteridium esculentum instead seem to be controlled by a passive hydraulic reaction. Thus, the acquisition of sophisticated metabolic controls of stomatal function occurred after lineages for modern ferns and angiosperms diverged many millions of years ago.