Stomata are integral to water transport in plants because evaporation of water throught the stomatal pore draws water through the water conducting tissue. This pore also enables gas exchange, especially carbon dioxide entrance. Among bryophytes, stomata are restricted to sporophytes of mosses and three hornwort genera. This study of the anatomy and ultrastructure of the stomata in Phaeoceros and Folioceros was undertaken to evaluate homology of stomata among basal embryophytes. Light microscope studies indicate that the reniform guard cells of hornworts contain a large vacuole, one to two starch-filled plastids and a peripheral nucleus. Compared to other epidermal cells, the guard cells walls are thickened, especially adjacent to the pore. Transmission electron microscopy revealed that the pronounced cell wall is of three layers, each containing parallel microfibrils oriented in different directions. Intercellular spaces are found internal to the stomata in the assimilative cells. Such evidence, combined with the fact that other epidermal cells lack chloroplasts, implies that hornwort stomata function in gas exchange. To evaluate whether or not these stomata open and close as a result of osmotic changes, we have performed histochemical studies on potassium and organic ion sequestration in guard cells. The role of stomata in transpiration will be evaluated in regards to water transport through the sporophyte. Comparisions will be made with stomata of lycophytes and mosses.

Key words: Anthocerotaceae, homology, stomata