LAUSR

Stomata are the valves controlling the exchange of carbon dioxide and water vapor between plants and the atmosphere, and their evolution played a crucial role in enabling the colonization of terrestrial habitats and formation of terrestrial ecosystems 1,2 . Stomata are made up of pairs of guard cells, which are kidney-shaped in most plants, and dumbbell-shaped in grasses, and fl anked by specialized subsidiary cells. Several studies among small groups of diverse species showed that opening and closing processes of these stomatal types differ. The elongated dumbbell shaped guard cells of grasses results in a rectangular pore that can change in area with minimal changes in stomatal width 3 . In general, dumbbell shaped guard cells have lower volume than kidney shaped guard cells, and thus higher membrane surface area relative to volume, which would contribute to greater rates of ion transport between subsidiary and guard cells and less requirement for solutes and water for changes in turgor, resulting in more rapid changes in volume for opening and closing 4,5 . The specialized subsidiary cells of grasses have also been implicated in rapid changes in guard cell turgor and stomatal apertures 3,6 – 8 . The more rapid stomatal responses of dumbbell shaped guard cells may also contribute to greater water-use ef fi ciency under fl uctuating light 5 . Yet, the importance of guard and subsidiary cell anatomy on stomatal responses and their relevance to whole plant function are not completely understood. Studies of more diverse species as well as greater numbers of species within the