ORDER CHTTRIDIALES 59 



of the original cell pass. Then the nucleus of the other cell with which it 

 is in contact enters through a small opening and the wall thickens to form 

 a thick-walled resting spore. After several months a zoosporangial sac is 

 formed in which the two nuclei fuse and then divide to form the nuclei 

 of the zoospores. Three species of Polyphagus are recognized by Sparrow. 



The genus Harpochytrium is sometimes placed in this family. The 

 plant consists of a slender or stout tubular, straight or curved, and sessile 

 or stalked zoosporangium with blunt or pointed apex. It adheres to the 

 surface of the host (mostly fresh-water filamentous algae) by a foot, 

 rarely penetrating the algal wall. When young it is uninucleate but be- 

 comes multinucleate as it grows. The upper half to three-quarters of the 

 protoplasm becomes divided transversely into four or five up to many 

 posteriorly uniflagellate zoospores which escape out of an inoperculate 

 apical opening, attaching themselves to a host cell by the tip of the 

 flagellum. The protoplasm, containing one or more nuclei, which remains 

 in the zoosporangium grows and produces a new sporangium by prolifera- 

 tion. Very much resembling this genus is the alga Chytridiochloris, based 

 on H. viride Scherf., which has a chloroplast and reproduces in the same 

 manner. For this reason the correctness of the assignment of Harpo- 

 chytrium to this family is doubted by some authors, especially by Jane 

 (1946) who has made a monographic revision of the genus. 



Because of its operculate manner of dehiscence Sparrow (1943) places 

 the genus Chytridium in a separate family, Chytridiaceae, while Miss 

 Whiffen includes it and other operculate forms of the same general struc- 

 ture in the Rhizidiaceae. The original encysted zoospore becomes the 

 sporangium. At its base is the endobiotic haustorium or rhizoidal system 

 which may produce an apophysis in some species. This haustorial system 

 may be a simple unbranched peg or a typical branched system of rhizoids 

 may arise from the tip of the peg, from the base of the sporangium, or 

 from the apophysis. The operculum is apical. Resting spores are endo- 

 biotic, apparently asexually produced. They act as prosporangia, giving 

 rise to extramatrical operculate sporangia. Sparrow recognizes twenty- 

 seven species besides a number of doubtful ones. They are mostly para- 

 sitic in fresh-water algae but a few occur in marine algae. They are re- 

 ported from Europe, Asia, and North America. 



Family Entophlyctaceae. In this family the zoospore cyst becomes 

 emptied and usually soon disappears or remains as an empty cap. The 

 fungus body enlarges within the host and sends out rhizoids which in 

 some species are very extensive, over 0.5 mm. long and up to 10 m thick. 

 Some forms are strictly parasitic, mostly upon algae, but many are 

 saprophytes on various types of material. In Entophlyctis the germ tube 

 enlarges to become the sporangium within the cell of the algal host. The 

 usually single exit papilla is inoperculate. Resting spores are known in 



