SAPROLEGNIALES 1 4 7 



abruptly to pointed ends, and external filaments which arise from 

 the former, float in the water, and have in mass a whitish cottony 

 aspect. The hyphal walls give the test for true cellulose. 



Asexual spores are borne in specialized sporangia, and sexual 

 reproduction when present takes place by the formation of 

 antheridia and oogonia which differ from each other strikingly in 

 size and shape. The oogonium is usually more or less globose, and 

 at maturity contains one or more naked protoplasmic oospheres. 

 Later these assume a cellulose wall and are then termed oospores. 

 The antheridium is clavate and smaller than the oogonium. In 

 fertilization a tube put out by the antheridium enters the oogo- 

 nium and discharges its contents directly into the oosphere. 

 Some species mature their oospores without fertilization and are 

 termed parthenogenetic or apogamous. Some lack antheridia 

 entirely and are termed apandrous. Ciliate sexual cells are 

 wholly absent. Swarmspores are commonly formed and are 

 always biciliate. 



In the development of abundant mycelium and in the forma- 

 tion of morphologically distinct oogonia and antheridia the group 

 shows a marked advance from the condition in the Chytridiales 

 and Ancylistales. In the absence of cihate sexual cells it is 

 sharply separated from the Monoblepharidales and Blasto- 

 cladiales. It is clearly most closely related to the Peronosporales, 

 these two orders indeed being so closely connected by inter- 

 mediate forms that no single absolute point of distinction between 

 them exists as the basis of a taxonomic separation. The Sap- 

 rolegniales are distinguished in general by their aquatic habit 

 and persistent sporangia, while the Peronosporales for the most 

 part are characterized by a terrestrial habit and deciduous 

 sporangia, the latter often being termed conidia. The members 

 of the family Pythiaceae of the latter order possess characters 

 more or less intermediate between those of the Saprolegniales 

 and Peronosporales, and in some classifications are included in 

 the Saprolegniales. 



The order Saprolegniales, as here constituted, contains two 

 families, Saprolegniaceae and Leptomitaceae, embracing a total 

 of nearly one hundred species in about twenty genera. 



Key to Families of Saprolegniales 



I. Thallus composed of cylindrical branching hyphae lacking definite 

 constrictions. 



1. Saprolegniaceae, p. 148 



