PROGRESS AND ACHIEVEMENTS IN PHYCOLOGY 555 



that meiosis occurs during the first two divisions of the nucleus of the ger- 

 minating zygospore and that the multicellular structure produced by the zygote 

 is haploid. Thus Allen eliminated the only green alga that up to that time 

 was regarded as showing an alternation of generations. Cytological studies by 

 a number of later investigators on diverse green algae confirmed the observa- 

 tion of Allen, and until 1925 it was generally believed that all green algae 

 were haploid, with meiosis always occurring during the germination of the 

 zygote. 



In 1925 Williams showed that C odium is a diploid alga, and since that 

 time our concepts of the life histories of the green algae and of their associated 

 nuclear cycles have undergone a profound change. The evidence now at hand 

 indicates that the Siphonocladales (Schechner-Fries, 1934; Schussnig, 1938), 

 most of the Siphonales (Williams, 1925; Schussnig, 1930, 1932, 1939, 1950; 

 Zinnecker, 1935), and the Dasycladales (Schulze, 1939) are diploid algae 

 (at least those that have been studied cytologically) and that it is the diploid 

 soma that in them functions as the gamete-producing generation. 



The occurrence of an alternation of generations in the green algae was 

 first observed by Hartmann (1929) and F0yn (1929, 1934a, 1934b), who 

 established that members of the orders Cladophorales {Cladophora, Chacto- 

 morpha) and Ulotrichales {Entcromorpha, Ulva) show an alternation of 

 isomorphic generations. A similar cycle has since been reported by Singh 

 (1945, 1947) for Draparnaldiopsis and Fr'itschicUa (both Ulotrichales) and by 

 Iyengar and Ramanathan (1940, 1941) for Anadyomcne and Microdktyon 

 (both Cladophorales). 



Juller (1937) has demonstrated that Stigeoclonium subspinosum (Ulotri- 

 chales) possesses an alternation of heteromorphic generations, with the diploid 

 sporophytic generation smaller than the haploid one, and Jorde (1933) has 

 obtained fairly convincing evidence indicating that certain species of the 

 unicellular Codiolum (a genus usually regarded as belonging to the Chloro- 

 coccales) represent the diploid sporophytic generation of species of the fila- 

 mentous, septate Urospora (a genus usually placed in the Cladophorales). 

 The observations of Jorde still require confirmation, but if they should be 

 correct, we would have here an instance of two kinds of green algae, which 

 for a long time have been regarded as phylogenetically very far apart, 

 representing phases in the same life cycle. 



Another apparent instance of this kind has been brought to light by Korn- 

 mann (1938) and Feldmann (1950). These authors have made observations 

 which suggest with reasonable certainty that the vesicular Halicystis and the 

 filamentous, nonseptate Derbesia constitute phases in the life history of one 

 and the same alga, with Halicystis representing the gametophytic and Derbesia 

 the sporophytic generation. These two genera have been regarded as the type 

 representatives of two distinct families, Halicystidaceae and Derbesiaceae, in 

 the order Siphonales. Inasmuch as Derbesia is the only genus of green algae 



