108 PHYCOMYCETEAE 



dition. Of the more than 20 genera included in this order the egg is single 

 in all but 6 or 8 genera and also in some of the species of each of these 

 genera that are normally pluriovulate. The species of this order have 

 been monographed by Coker (1923) and the North American species by 

 Coker and Matthews (1937). 



The author recognizes three families in this order: Saprolegniaceae, 

 Leptomitaceae and Rhipidiaceae. The two latter are considered by Miss 

 Kanouse (1927) and by Sparrow (1943) to deserve segregation into a 

 separate order, the Leptomitales, with which the author does not agree. 

 Until the various forms included in this book in the Lagenidiales are 

 better known, it is uncertain whether some of them should be more 

 closely associated with the Saprolegniales. There is undeniably a close 

 relationship between these two orders and also with the next order, the 

 Peronosporales. 



The three families of Saprolegniales may be distinguished as follows: 



Saprolegniaceae: mycelium not definitely constricted at intervals nor with 

 cellulin plugs, mostly cylindrical or gradually narrowing toward the extrem- 

 ities but not consisting of an enlarged basal portion and slender branches. 

 Zoospores mostly dimorphic or with modifications of dimorphism, only pri- 

 mary zoospores produced in one genus. Oogone with one or more eggs and 

 lacking periplasm. 



Leptomitaceae: mycelium definitely constricted at more or less regular inter- 

 vals, frequently with cellulin plugs. Zoospores dimorphic. Oogones usually 

 with one egg, in one species with more, with no periplasm. 



Rhipidiaceae: mycelium more or less well differentiated into an enlarged basal 

 portion with slender, mostly constricted, branches which bear the zoospo- 

 rangia and sexual organs. (In Mindeniella these arise directly on short stalks 

 from the basal segment.) Oogones with single eggs and with periplasm. 



Family Saprolegniaceae. The members of this family are in some 

 cases strictly aquatic, but a considerable number of species are soil in- 

 habitants. Contrary to the belief prevailing earlier they are mostly 

 saprophytic on vegetable matter, less often on animal matter. Only a few 

 species of Achlya and Saprolegnia are sometimes destructive to young fish 

 and fish eggs in fish hatcheries. Several species of Aphanomyces and one or 

 two other genera are parasitic in algae, in the roots of higher plants, or in 

 aquatic animals. 



In about half of the genera the oogone contains but one egg, but the 

 pluriovulate species are far in the majority. The number of eggs per 

 oogone in these may vary from 2 or 3 up to 50. Fertilization of the eggs is 

 accomplished by the passage of sperm nuclei, one to each egg, usually 

 through conjugation tubes that penetrate the oogone wall from the 

 adhering antherids. In some cases although antherids are present there 

 seems to be no opening for the passage of the sperm nucleus so that the 

 egg develops parthenogenetically. The oospore may germinate by a germ 



