STUDIES ON THE SPOROGENOUS LINEAGE IN THE MOSS TIMMIELLA-BARBULOIDES .9. DEVELOPMENT OF THE TAPETUM

The ultrastructure of tapetal cells in Timmiela barbuloides was investigated in relation to events of sporogenesis. After their establishment both internally and externally to the sporogonial initials, tapetal cells enlarge and assume a permanently polarized organization after completion of meiosis. A large vacuole is formed in the cell region distal to the spore sac, the nucleus becomes centrally located, and amyloplasts lie in the cytoplasm adjacent to the spore sac. An extensive endomembrane system develops in tapetal cells during the stage of exine deposition in spore tetrads. Sheets of rough endoplasmic reticulum develop first around the nucleus then also in close proximity to the plasma membrane abutting the spore sac. Concomitantly, intervening dictyosomes produce a variety of vesicles. Unusual structures with vesicle-like profiles also occur in the inner tapetum cell walls abutting the spore sac. At the same time most of the starch is lost from the plastids in which grana-fretwork systems develop. A massive secretion of extremely electron-opaque material is associated with perine deposition onto the free spore surfaces. Degeneration of the tapetal cells during the terminal stages of spore maturation is marked by distortion of the organelles, increase in vacuolation and the appearance of electron-opaque material between the sheets of endoplasmic reticulum.