The Sexual Cycle of Netrium Digitus

The life cycle of 2 varieties of Netrium digitus, a saccoderm desmid, was studied in pure culture on mineral media. Netrium digitus var. digitus from Indiana is homothallic, and Netrium digitus var. lamellosum from Minnesota is heterothallic. Previous observations by other investigators on the events of the sexual cycle are confirmed and extended. Zygospores of both varieties are formed in the conjugation tube. Fusion of gametic nuclei occurs after the dormant period of zygospores, and meiosis occurs at the time of germination. Conjugation is induced in a reliable manner in both varieties by omitting nitrogen from the medium and by exposing the cultures to the proper conditions of light and temperature. The relationship between photoperiod, nitrogen depletion, and induction of conjugation is discussed.