46 PHYCOMTCETEAE 



reports the haploid chromosome number in the Chytridiales to be usually 

 4 or 5 but 10 to 12 in Polyphagus euglenae Now. 



A plant body which consists of but a single rounded cell without any 

 rhizoids or haustoria is said to be holocarpic, but if such structures are 

 present it is eucarpic. 



In the majority of the eucarpic genera (the monocentric series) the 

 zoospores produce but one enlargement from which, directly or indirectly, 

 the /oosporangium arises. In several genera (the polycentric series) the 

 germinating zoospore produces an enlarged cell from which arise rhizoids 

 which here and there in their course produce other enlargements into 

 which enter nuclei derived from the original cell. These secondary en- 

 largements may become zoosporangia themselves or may give rise to 

 zoosporangia as well as to other rhizoids with further swellings. 



Until recently it has been customary (Fitzpatrick, 1930) to include in 

 the Order Chytridiales all those organisms with structures and life 

 histories similar to those outlined above. The differences in the types of 

 flagellation of the motile cells and of the composition of the cell wall 

 have led mycologists recently to exclude from this order those genera 

 with a single anterior fiagellum of the tinsel type (Order Hyphochytriales) 

 and those with two anterior or lateral flagella, one of the tinsel and one of 

 the whiplash type (Family Olpidiopsidaceae). 



Sparrow (1942) makes the primary division of this order, on the basis 

 of mode of escape of the zoospores, into two series: Inoperculatae and 

 Operculatae. Miss Whiff en (1944) on the contrary considers the mono- 

 centric or polycentric structure to be of prime importance, using the 

 method of zoospore escape as a subordinate character. 



In the treatment of this order the author follows mainly the arrange- 

 ment of Miss Whiffen, with constant reference to the work of Sparrow. 

 First, the holocarpic forms are considered. Whether these are more 

 primitive than the eucarpic monocentric forms, or have arisen from some 

 of these by loss of the rhizoidal or haustorial apparatus in connection 

 with their habitat entirely within the host cell, must remain undecided 

 at present. Sparrow recognizes three families, the relationship of the 

 second of which is somewhat doubtful. They are distinguished as follows: 



Olpidiaceae: vegetative cell enlarging to form a single sporangium. 

 Achlyogetonaceae: vegetative cell elongated and by septation forming a linear 



'series of sporangia. 

 Synchytriaceae : vegetative cell dividing internally into numerous sporangia, or 



the latter formed within an outgrowth from the vegetative cell. 



Family Olpidiaceae. This family contains six or more genera, 

 totaling forty or more well-substantiated species and many more de- 

 scribed species whose status, according to Sparrow, is uncertain. They 

 are entirely endobiotic, i.e., living entirely within the host cells or tissues. 



