94 THE PHYCOMYCETES 



As an important exception to the saprophytic mode of life of 

 these fungi may be mentioned Saprolegnia parasitica, \vhich causes 

 a serious disease of fishes and fish fry [Tiffney (1939)]. Like- 

 \\'ise, yarious species of Aphanomyces are known as parasites on 

 the roots of such higher plants as peas, radish, and beets [Ken- 

 drick (1927), Drechsler (1929)] and on Spirogyra and other 

 aquatic fungi. 



Asexual reproduction. A number of species reproduce 

 asexually by means of gemmae of various size and shape, usually 

 formed in terminal chains. After a dormant period the gemma 

 may form a germ tube, produce sporangia, and give rise to 

 oospores. 



The regular means of asexual reproduction, however, is by 

 sporangia and zoospores. The young sporangium is multinu- 

 cleate and cut off from its parent hypha by a septum. Cleavage 

 planes, which appear in the protoplasm, gradually branch and 

 divide the sporangial content into a number of uninucleate masses, 

 each of which will eventually form a zoospore. It is principally 

 by means of variations in the structure and behavior of the zoo- 

 spores that the various genera are delimited. Other character- 

 istics of importance in this connection include the method of 

 renewal of the sporangia, whether by proliferation or by cymose 

 branching from below, the shape of the sporangium, and the 

 number of eggs within the oogonium [Coker and Mat^iews 

 (1937)]. 



In the rather uncommon Genus Pythiopsis the sporangia are 

 spherical to pyriform in shape, and the zoospores are mono- 

 planetic; that is, there is but a single period of motilit\'. The 

 zoospore itself is pyriform and has two apical cilia. 



All the remaining genera have sporangia \\hich are cyhndrical, 

 fusiform, or clavate in shape. In Aplanes sporangia are rarely 

 produced; but when they are developed, the spores germinate in 

 situ within the sporangium, and the germ tubes protrude through 

 the sporangial wall. 



In Saprolegnia, Isoachlya, and Leptolegnia the spores are di- 

 planetic, that is, have two motile phases. On discharge from the 

 sporangium the pyriform "primary" zoospores are motile by 

 means of two apical cilia. After a brief motile period they encyst 

 and then give rise to "secondary" zoospores that are reniform 

 and possess two lateral cilia. 



