PAPENFUSS: CLASSIFICATION OF THE ALGAE 125 



nation divide into a number of cells, each oi which later gives rise to a zoospore. 

 Pringsheim and others regarded this structure as the sporophyte of Coleochaete. 

 Allen established, however, that meiosis occurs during the first two divisions of 

 the germinating zygote and that the cellular structure produced by the zygote 

 is haploid. Thus Allen eliminated the only green alga that till then was re- 

 garded as exhibiting an alternation of generations. Cytological studies by vari- 

 ous later workers on diverse green algae confirmed the observations of Allen 

 and until 1925 it was generally held that green algae are haploid, with meiosis 

 always occurring during the germination of the zygote. 



Since 1925 when Miss AVilliams showed that Codiuni is a diploid alga, our 

 concept of the life histories of the green algae and the associated nuclear cycles 

 has undergone a profound change. It is now known that the Siphonocladales 

 (Schechner-Fries, 1934; Schussnig, 1938), Siphonales (Williams, 1925; Schuss- 

 nig, 1930, 1932, 1939, 1950; Zinnecker, 1935), and Dasycladales (Schulze, 1939) 

 are diploid algae, at least those that have been studied cytologically, and that it 

 is the diploid soma that in them, as in the Fucales, diatoms, and animals, func- 

 tions as the gamete producing generation. (See, however, the statements below 

 on Derhesia and Halicystis.) 



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

 was first demonstrated by Hartmann (1929) and F0yn (1929, 1934a, 1934b) in 

 members of the orders Cladophorales {Cludophora, Chaetomorphn) and Ulo- 

 trichales {Enteromorpha, Viva). Singh (1945, 1947) has reported the occur- 

 rence of a similar cycle in Draparnaldiopsis and FritscJiiella (both Ulotrichales), 

 and Iyengar and Ramanathan (1940, 1941) have established its occurrence in 

 Anadyomene and Microdictyon (both here placed in the Cladophorales). 



Juller (1937) has shown that Stigeodonium subspinosum (Ulotrichales) pos- 

 sesses an alternation of heteromorphic generations, with the diploid asexual gen- 

 eration smaller than the haploid one. Jorde (1933) has brought forth fairly 

 convincing evidence indicating that certain species of the unicellular Codiolum, 

 a genus of the family Chloroeoccaceae in the order Chlorococcales, represent the 

 diploid asexual generation of species of the filamentous Urospora, a member of 

 the family Cladophoraceae in the Cladophorales. These observations still re- 

 quire corroboration, but if they should prove to be correct, we would have here 

 an alternation of heteromorphic generations, representing two kinds of green 

 algae which for a long time have been regarded as phylogenetically very far 

 apart. 



Kornmann (1938) and Feldmann (1950) have made observations suggesting 

 that Halicystis and Derhesia also constitute phases in the life history of one 

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

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

 representatives of two distinct families, Ilalicystidaceae and Derbesiaceae, of 

 the order Siphonales. In view of the fact that Derhesia is the only genus of 

 green algae outside the order Oedogoniales that is known to possess swarmers 

 with a subterminal collar of flagella arid that Halicystis forms terminall}^ bi- 

 flagellate gametes, the apparent existence of an intimate relationship between 

 these two kinds of plants is a matter of far-reaching signifleanco. 



Although the majority of Chlorococcales appear to be haploid, there is some 

 indication that CMorocliytrium Lemnae shows an alternation of generations 



