ON THE LIFE HISTORY OF MONOSTROMA FUSCUM (POSTELS ET RUPRECHT) WITTROCK(1)

Results of the study of Monostroma fuscum from Son Juan Island, Washington, fully support the separation of the species M. fuscum from Monostroma sensu Wittrock into a taxon including M. obscurum. It would appear from this and previous studies that the M. obscunim-M. fuscum complex is clearly Ulvacean. The characteristics of the complex include (1) alternation of morphologically similar haploid and diploid generations; (2) production of biflagellate gametes and quadriflagellate zoospores; (3) a uniseriate form in early ontogeny; (4) thallus development progressing through a vesicular stage that, as a result of localized degenerative processes, opens at a size of less than I cm into a monostromatic blade; (5) cells elongated perpendicular to the plane of the thallus and with rotund-truncate ends: (6) a single elongate plastid, thickened at the opposite poles of the cell and thinner in the equatorial region of the cell; (7) a thallus composed of cells with relatively inelastic walls; and (8) discrete discharge pores. The chromosome number of Monostroma fuscum is n =9. The first division of the zoosporangium nucleus is meiosis I.