SPORANGIAL STRUCTURE AND ZOOSPOROGENESIS IN CHORDA TOMENTOSA (LAMINARIALES)1,2,3

Members of the genus Chorda represent the simplest form of sporophyte in the order Laminariales. The present study deals with reproduction in Chorda tomentosa, involving the initiation, growth, and structure of the sporangium and the process of zoosporogenesis. The simple tube-like sporophyte of Chorda tomentosa grows in diameter by means of repeated anticlinal divisions in a superficial meristematic layer known as the meristoderm. The onset of reproduction is marked by the conversion of the meristoderm from contributing cells to the vegetative plant body to producing 2 new cell types: paraphyses and sporangial mother cells. At the time of initiation, sporangial mother cells are crescent shaped and possess a densely staining cytoplasm. Sporangial mother cells increase in size, become ellipsoid, decrease in staining density, and undergo meiosis. After meiosis, sporangia increase in size while their nuclei undergo successive cycles of synchronous mitotic divisions. Sporangia increase to a maximum length of 120 μ;m at which time they possess the characteristic “cap” found in all members of the order studied thus far. At this stage the protoplast of the sporangium is organized into a peripheral layer of nucleus-chloroplast pairs and a central region of vacuoles. Cleavage furrows begin to form at the cell membrane and are met by furrows developing in the interior of the cytoplasm resulting in the division of the entire protoplast into separate units. Each unit is an individual zoospore. The biflagellate zoospores contain a nucleus, one chloroplast with eyespot, perinuclear Golgi, and several bodies of presumed storage carbohydrate. The occurrence of a small population of early developing sporangia is described. In essential details, the origin, development, and structure of sporangia in Chorda tomentosa are identical to all earlier observations in the Laminariales.