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

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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.

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