PAPENFUSS: CLASSIFICATION OF THE ALGAE 123 



of operculate cysts and certain other features. It is now known ( Ilammerling, 

 1931) that the members of this order are actually uninucleate during their vege- 

 tative phase and become multinucleate only when they become fertile. 



The status of the monogeneric family Sphacropleaceae remains to be consid- 

 ered. Owing to the multinucleate condition of the septate thallus, Sphaeroplea 

 was for a long time associated either with the Siphonocladales or the Cladopho- 

 rales. As has been emphasized by Fritsch (1929; 1935, pp. 224-225; 1947, pp. 

 43-48), however, SphacropJca possesses a number of characters that appear to 

 ally it with the Ulotrichales rather than the complex of siphonous orders con- 

 sidered above, which Egerod (1952) would derive from the Chlorococcales. 

 Fritsch (1935) considers Sphaeroplea as constituting a suborder in the Ulotri- 

 chales. Prescott (1951) has elevated this suborder to the rank of order. I con- 

 cur with the view of Prescott. 



In addition to the orders which have so far been considered, namely, the Vol- 

 vocales (including the Tetrasporales), Chlorococcales, Cladophorales, Siphono- 

 cladales, Siphonales, Dasycladales, and Sphaeropleales, the Chlorophycophyta 

 have been credited in recent times with the following orders: Zygnematales 

 (=the order Zygophyeeae of Stizenberger), Ulotrichales, Chaetophorales, Ul- 

 vales, Microsporales, Cylindrocapsales, Oedongoniales, and Schizogoniales. These 

 orders, with the exception of the Zygnematales, constitute the bulk of the Nema- 

 tophyceae of Stizenberger. 



The Zygnematales (Conjugales of some authors) embrace a well-marked 

 group of algae characterized by the conjugation of nonflagellated gametes. The 

 relationship of the essentially unicellular desmids with the filamentous Zygne- 

 mataceae (which family includes the classical Spirogyra) has been generally 

 recognized since the time that De Bary (1858) pointed to their alliance. The 

 Zygnematales occupy an isolated position in the Chlorophycophyta. At one 

 time the order was regarded by some authors (e.g., Engler and Gilg, 1924) as 

 sufficiently distinct from the other green algae to merit recognition as an inde- 

 pendent phylum. There seems to be little justification, however, for considering 

 the Zygnematales as distinct from the Chlorophycophyta, and in recent systems 

 of classification the assemblage has usually been treated as an order of the 

 Chlorophycophji;a. It is not inconceivable that the Zygnematales evolved, in the 

 distant past, from the Volvocales. Monographic treatments of the order or of 

 some of its families have in recent times been published by Czurda (1932, 1937), 

 Krieger (1933-1937, 1939), Kolkwitz and Krieger (1941-1944), and Transeau 

 (1951). The classification of the Zygnematales adopted in the arrangement at 

 the end of this section is essentially that of Fritsch {in West, 1927). 



The Ulotrichales as here accepted include the Chaetophorales, Ulvales, Micro- 

 sporales, and Cylindrocapsales. The order Chaetophorales was established by 

 Wille (1901) to receive essentially the same algae, including Ulothrix, that Borzi 

 (1895) assigned to his order Ulotrichales. In current systems of classification 

 the Chaetophorales are either accepted as an autonomous order (e.g., by Fritsch, 

 1935) or they are merged in the Ulotrichales (e.g., by Smith, 1950). Fritsch 

 (1935; 1939; 1944, p. 245) especially, has argued for the retention of the order, 

 primarily because of the heterotrichous habit of the majority of the forms. How- 

 ever, in some genera of the Chaetophorales either the prostrate or the erect 

 system may be absent or poorly developed (cf. Papenfuss, 1951b). Heterotrichy 



