Interpolation hypothesis for origin of the vegetative sporophyte of land plants

This paper reviews interpretations of the morphology and evolution of the bryophyte sporophyte, and proposes a scenario for the early evolution of the land plant sporophyte. Paleobotanical and neobotanical evidence implies that the earliest land plants were bryophyte‐like and diverged to bryophytes and pteridophytes. The current hypothesis for the sporophyte evolution is that the seta, i.e., an axis bearing the sporangium of bryophytes, elaborated to the ramified polysporangiophyte stem. From comparison of reported developmental data with a special focus on the apical cell and other meristems, we propose that the entire bryophyte sporophyte is a footed sporangium (sporogonium) with or without a seta. The seta is interpreted as a sporangial stalk and is not homologous to the stem of polysporangiate plants. Having the persistent apical meristem, the shoot of polysporangiate plants seems to be a novel vegetative organ interpolated in the life cycle, while the sporogonium evolved to the sporangium separated from the foot and later produced on the stem or leaf.