Stomata and atmospheric CO₂ in bryophytes

Stomatal abundance has been widely used to reconstruct palaeo-atmospheres, but there is a view that early-diverging clades of vascular land plants may differ in their responsiveness to atmospheric CO₂. Field et al. grow a number of hornworts and moss sporophytes with contrasting stomatal morphologies under different atmospheric CO₂ concentrations representing both current and ancient atmospheres, and find that densities and dimensions are mostly unaffected by changes in CO₂. The results strengthen the hypothesis of incremental rather than early acquisition of stomatal regulatory processes through land plant evolution, and suggest that caution is required in using stomatal densities as proxies for paleo-atmospheric CO₂ concentrations.