Practice question

During early land plant evolution, stomata composed of paired guard cells controlling gas exchange and transpiration appeared first in sporophytes of hornworts and mosses and evolved further in vascular plants with complex patterning. Liverworts uniquely among extant embryophytes completely lack true stomata on either gametophyte or sporophyte generation. They do possess simple air pores surrounded by ring of cells that cannot actively open and close, reflecting alternative ventilation strategy for photosynthetic thallus with internal air chambers. Presence of functional stomata correlates with need for regulated transpiration and homoiohydry.