ON THE LIFE HISTORY OF MONOSTROMA FUSCUM (POSTELS ET RUPRECHT) WITTROCK1

Abstract

Results of the study of Monostroma fuscum from San 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. obscurum-M. fuscum complex is clearly Ulvacean. The characteristics of the complex include (1) alternation of morphologically similar haploid and diploid generations; (2) production of blflagellate gametes and quadriflagellate zoo-spores; (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 1 cm into a mono-stromatlc blade; (5) cells elongated perpendicular to the plane of the thallus and with rotund-truncate ends; (6) a single elongate plastld, 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 in-elastic walls; and (8) discrete discharge pores. The chromosome number of Monostroma fuscum is n = 9. The first division of the zoo-sporangium nucleus is meiosis I.