Spore wall ultrastructure and development in a basal euphyllophyte: Psilophyton dawsonii from the Lower Devonian of Quebec (Canada).

PREMISE OF THE STUDY: Euphyllophytes, a clade including living ferns, horsetails, and seed plants, have a rich fossil record going back to the Early Devonian. The euphyllophyte spore wall has a complex structure, the evolutionary origins of which are incompletely understood. Psilophyton is the best-characterized basal euphyllophyte genus; thus, data on this genus can inform current hypotheses on spore wall structure and development, which propose a bilayered spore wall organization of combined spore and sporangial origin for the ancestral euphyllophyte. METHODS: We employed cellulose acetate peel sectioning of permineralized Lower Devonian (Emsian) Psilophyton dawsonii sporangia, combined with electron microscopy, to document spore wall structure and development. KEY RESULTS: The Psilophyton dawsonii spore wall is bilayered. The inner spore wall is homogeneous, probably of lamellar construction. The outer spore wall, loosely attached to the inner wall, covers distal and equatorial spore areas, and has a foveolate base layer upon which stacks of sporopollenin lumps accrete centrifugally, forming the scaffolding for the final apiculate ornamentation. CONCLUSIONS: This is the most complete account on spore wall structure, allowing developmental interpretations, in a basal euphyllophyte. The bipartite organization of the Psilophyton dawsonii spore wall reflects development as a result of two processes: an inner layer laid down by the spore cell and an outer layer of tapetal origin. Providing direct evidence on the spore wall of a basal euphyllophyte, these data confirm previous hypotheses and mark an empirically supported starting point for discussions of the evolution of spore wall development in euphyllophytes.