Is there a tapetum in the hornwort capsule? Evidence from the sporogenesis of Phaeoceros.

This article investigates the presence and nature of the tapetum—a specialized nutritive tissue supporting spore development—in the hornwort Phaeoceros laevis, addressing longstanding contradictions about its existence in bryophytes. Through detailed ultrastructural and developmental analyses using light, fluorescence, scanning, and transmission electron microscopy, the study finds that neither pseudoelaters (sterile cells in the capsule), the lining layer of the capsule wall, nor the outer layer of the columella exhibit the characteristic secretory ultrastructure of a true tapetum as defined in mosses and tracheophytes. Instead, sporocytes accumulate nutrients early and develop largely autonomously within special walls, while the mucilaginous matrix in the capsule locule facilitates nutrient transfer from the gametophyte via the capsule wall and columella, which show structural specialization for upward transport but not secretion. The authors conclude that a true tapetum is absent in hornworts, suggesting its evolutionary origin postdates the divergence of hornworts and that the tapetum likely evolved as an accessory tissue in embryophytes to assume functions previously performed by spores themselves.