Date May 2 , i960 ii ABSTRACT A cytological examination of the life-history of Nereocystis luetkeana has shown that an alternating chromosome number corresponds to the morphological alternation of generations. The first division sequence of the zoosporangLal nucleus is meiotic and is followed by three mitotic divisions. The result is a mature sporangium containing 32 nuclei. Thirty-two zoospores are liberated from each sporangium and their germination gives rise to male and female gametophytes.

more »

... Genotypic determination of the sexes is believed to take place in Nereocystis. Mitosis in the gametophytes is regular and cytokinesis follows each nuclear division, producing few cells in the female and many cells in the male gametophytes. Thirty-one chromosomes can be counted at the mitotic prophase. Oogamy exists in Nereocystis and fertilization takes place after the egg is extruded from the oogonium. The sporophyte develops initially into a uniseriate filament of 5 -8 cells before divisions in a second plane give rise to a flat, monostromatic thallus. Nuclear division in the sporophyte appears to be preceded by division of the nucleolus. Colorless and non-septate rhizoids develop as elongations of the basal cells of the sporophyte. Some of the unfertilized eggs develop parthenogenetically and give rise to stunted, deformed plants with multinucleate cells. Temperature is an important factor in the development of various stages of the life cycle of Nereocystis grown in culture. This is particularly evident in the gametophytic stage where sexual structures are produced only at temperatures less than-10° C, and vegetative growth is most prolific at 14 -18° C.