THE MORPHOLOGY OF PLATOMA IZUNOSIMENSE (SCHIZYMENIACEAE, RHODOPHYTA)

The little-known red alga Platoma izunosimense Segawa was collected in abundance from deep water offshore from Cape Shionomisaki, Wakayama P refecture, at the southernmost Pacific end of the Island of Honshu, Ja pan. Its morphology has been examined in detail and compared with that of other members of the genus Platoma. Both sexual reproductive phase s, production of connecting filaments, the development of the carpospo rophyte, as well as some details of vegetative features, are newly pub lished for this species. Gametophytic thalli are foliose, reddish-brow n to pinkish-red in color, and reach lengths of up to 16 cm from a sin gle crustose holdfast that either gives rise to the blade directly or subtends a short stipe. The stipe has proliferations. Mature gland cel ls are pyriform, intercalary in cortical filaments, and occur singly o r rarely in adjacent pairs. There are X-shaped cells in the medulla. P lants are nomoecious. Spermatangia are in small sori or widely scatter ed over the thallus surfaces. Outermost cortical cells produce 1 or 2 spermatangial mother cells, each cutting off 1-4 spherical to subspher ical spermatangia. Carpogonial branches are 3-celled and directed outw ardly from an inner-cortical supporting cell. The carpogonium bears a long straight trichogyne. After presumed fertilization, the carpogoniu m connects with a nutritive auxiliary cell to form a fusion cell which thereafter produces a connecting-filament-initial that develops monop odially prior to the production of connecting filaments. A second cont iguous nutritive auxiliary cell may also connect to the carpogonial pa rt of the fusion cell by a secondary pit-connection prior to its produ ction of a connecting-filament-initial branch. The connecting filament s branch sparingly dichotomously, fuse with generative auxiliary cells which are intercalary in separate cortical filaments, and may or may not grows onwards to diploidize further generative auxiliary cells. Th e generative auxiliary cell produces a gonimoblast initial outwardly a fter fusion with the connecting filament. Usually 4 gonimolobes develo p sequentially from the gonimoblast initial. Mature carposporophytes a re 60-140 mu m in diameter, the carpospores being discharged through a n irregularly spreading opening in the overlying cortex.