752 THE AMERICAN NATURALIST. [Vol. XXXVIII. 



cells of the more complicated coenogametes (oogonia and anthe- 

 ridia) are also terminal cells. All of these sexual organs are 

 multinucleate. In the Siphonales (Vaucheria excepted) all of 

 the nuclei are functional gamete nuclei. This is also true of 

 simplest types of coenogametes, but in the more complicated 

 forms (Albugo, Pyronema, etc.) large numbers of the nuclei 

 degenerate or fail to function sexually in sterile accessory re- 

 gions of the protoplasm. The same conditions of sexual degen- 

 eration are also found in the oogonia of Vaucheria (Davis, : 04) 

 and Saprolegnia (Davis, :03). The agreement of all of the 

 structures mentioned above in structure and protoplasmic behav- 

 ior seems to establish beyond question their common homology. 

 The problems of the origin of the coenogametes are very 

 difficult with the meager evidence at hand. The author believes 

 that the simplest types have probably been derived from struc- 

 tures like the sexual organs of the isogamous Siphonales, which 

 structures gave up the habits of forming uninucleate gametes 

 and acting as coenocytic units became multinucleate sexual ele- 

 ments. A physiological development very similar to such a 

 change must have taken place in Peronospora and some species 

 of Pythium when their conidia ceased forming zoospores and 

 took the habit of germinating directly by a tube. This view 

 regards the coenogamete as a coenocyte derived from a proto- 

 plasmic structure that at one time produced a large number of 

 independent sexual elements, represented in the coenocyte by the 

 numerous nuclei. Whether the higher types of coenogametes 

 (Albugo, Pyronema, etc.) have developed directly from the sim- 

 pler forms or from levels of the heterogamous algas, such as are 

 illustrated by Vaucheria, are very complicated problems that can- 

 not be treated here. They with other topics, mentioned above, 

 have been considered in recent papers of the author (Davis, : 03, 

 ;o4«, -.OA^b). Coenogametes are proving to be among the most 

 interesting types of sexual cells in plants and research in this 

 field is likely to prove very fruitful of results. 



