Stomata: the holey grail of plant evolution

The greatest cost associated with terrestrial photosynthesis is maintaining hydration in the presence of phenomenal evaporative forces from the atmosphere. Without the capacity to maintain internal water reserves, vascular plants (tracheophytes) would never have escaped the soil boundary layer. Two key adaptations enable homoiohydry in vascular land plants: (1) a means to rapidly conduct water over long distances via xylem and (2) the ability to regulate water use by stomata. Xylem alone has long been credited for the evolutionary success of tracheophytes. Trees are only found in this clade, with most “nonvascular” land plants (bryophytes) confined to the soil boundary layer and relying on vegetative desiccation tolerance to survive drought. In contrast, stomata, which predate xylem in the fossil record and are found in most extant land plant clades, are often relegated to a level of lesser importance for driving the evolution of homoiohydric land plants. We would argue that physiological data, particularly from bryophytes, challenge this conventional wisdom rooted in morphological observation and suggest that the evolution of stomatal function was a critical innovation for the evolution of large plants.