872 AQUATIC PHYCOMYCETES 



the thallus into pseudocells by pseudosepta, the pedicellate reproductive 

 organs, and the formation of a single (except in Apodachlyella) egg in 

 the oogonium. 



KEY TO THE FAMILIES OF THE LEPTOMITALES 



Thallus filamentous throughout, not differentiated into a basal cell and 

 hyphal branches; zoospores diplanetic; oogonial contents not 

 differentiated into ooplasm and periplasm Leptomitaceae, p. 872 



Thallus more or less well differentiated into holdfasts, basal cell, and 

 hyphal branches ; zoospores only of the secondary type ; oogonial 

 contents differentiated into ooplasm and periplasm 



Rhipidiaceae, p. 882 



LEPTOMITACEAE 



Thallus filamentous throughout, conspicuously jointed and pseudo- 

 septate, of unlimited growth, contents bearing refractive granules of 

 cellulin ; zoosporangia either undifferentiated hyphal segments or spe- 

 cialized pedicellate smooth-walled structures; zoospores diplanetic, the 

 primary swarmers apically biflagellate, the secondary laterally biflagel- 

 late; oogonium forming one or several oospheres the contents of which 

 are homogeneous and not differentiated into ooplasm and periplasm, 

 oospore smooth-walled, at germination forming several germ tubes 

 which reestablish the thallus; antheridium a relatively unspecialized 

 segment of the thallus which subtends the oogonium, not forming a 

 fertilization tube. 



Saprophytic on various types of submerged debris, frequently fruits 

 and twigs, in fresh water. 



KEY TO THE GENERA OF THE LEPTOMITACEAE 



Sex organs never formed; plant coarse, the zoosporangia being un- 

 differentiated segments of the mycelium Leptomitus, p. 873 



Sex organs formed under ordinary conditions of culture; plant more 

 delicate, the zoosporangia (if formed) being well-defined pedi- 

 cellate structures 



Oogonium forming a single oospore Apodachlya, p. 874 



Oogonium forming more than one oospore .... Apodachlyella, p. 881 



