156 THE LOWER FUNGI— PIIYCOMYCETES 



The single nucleus of the mature oospore undergoes repeated 

 divisions to furnish the nuclei for the sporangiospores, these 

 divisions taking place, in some species at least, before germina- 

 tion occurs. 



The most recent monographs on the family are those of Coker 

 (1923) and von Minden (1912), but those of Schroter (1893), 

 Fischer (1892), and Humphrey (1893) will also be found valuable. 

 Since a number of papers have appeared recently containing 

 descriptions of new genera and species no one of the larger 

 monographs is wholly complete. As a result of an investigation 

 of the soil for "water moulds" Coker and his co-workers have 

 described a number of forms which in the Hght of earlier work 

 are extremely atypical. Several new genera {Calyptralegnia, 

 Geolegnia, Brevilegnia, Protoachlya, and Isoachlya) have been 

 described. In some cases these are based on forms intermediate 

 in type between members of older established genera. While 

 the writer is unprepared in the light of present knowledge to 

 accept all of these genera as good, it seems best to incorporate 

 them in the key, and give them adequate discussion in order 

 that the student may orient himself clearly regarding the present 

 status of the group. 



Key to Genera of Saprolegniaceae 



I. Sporangiospores normally encysting within the sporangium. 

 A. Oogonium containing more than one oosphere. 



1. Sporangia extremely rare; behavior of sporangiospores very 



imperfectly known; their germination in situ by germ tube 

 occurring at least at times; oogonium provided with an 

 exceptionally thick, prominently pitted wall. 



1. Aplanes, p. 158 



2. Sporangia abundant; encysted sporangiospores germinating 



after their escape from the sporangium; wall of oogonium 



not of unusual thickness. 



a. Dehiscence of sporangium not occurring at a definite 

 point; the wall either deliquescent throughovit, or 

 ruptured by internal pressure so that a wide opening 

 of indeterminate position results; sporangiospores 

 germinating by swarmspores or by germ tubes; 

 oosphere eccentric. 



2. Thraustotheca, p. 100 



6. Dehiscence of sporangium accomplished by breaking 

 off of an apical cap, leaving a wide opening through 

 which the swelling spores escape intermittently in 



