Abstract
Stomata are key innovations for the diversification of land plants. They consist of two differentiated epidermal cells or guard cells and a pore between that leads to an internal cavity. Mosses and hornworts are the earliest among extant land plants to have stomata, but unlike those in all other plants, bryophyte stomata are located exclusively on the sporangium of the sporophyte. Liverworts are the only group of plants that are entirely devoid of stomata. Stomata on leaves and stems of tracheophytes are involved in gas exchange and water transport. The function of stomata in bryophytes is highly debated and differs from that in tracheophytes in that they have been implicated in drying and dehiscence of the sporangium. Over the past decade, anatomical, physiological, developmental, and molecular studies have provided new insights on the function of stomata in bryophytes. In this review, we synthesize the contributions of these studies and provide new data on bryophyte stomata. We evaluate the potential role of stomata in moss and hornwort life histories and we identify areas that will provide valuable data in ascertaining the evolutionary history and function of stomata across land plants.