101 



THE SIMPLE HOLOCARPIC BIFLAGELLATE PHYCOMYCETES 



contracted and encysted content of an irregular, 

 lobed thallus. In Aphanomycopsis bacillariacearum 

 one or more asexual resting spores are formed in a 

 thallus or a segment thereof, and Scherffel accepted 

 this as an indication of eYen closer relation to the 

 Saprolegniaceae. Obviously much more study on the 

 occurrence of sexuality in this family is essential 

 before this character can be used as a basis of com- 

 paring relationships. Nevertheless, Scherffel's be- 

 lief that there is a close affinity between the Ectro- 

 gellaceae and the Saprolegniaceae has been rather 

 widely accepted, although subsequent workers have 

 not been certain about whether the former family 

 represents an ascending or descending series in the 

 evolution of the Phycomyeetes. Gaumann ('26), 

 Gaumaim and Dodge ('28) included Eurychasma 

 and Ectrogella in the Ancylistaceae (Lagenidia- 

 ceae) and emphasized their close relationship to 

 Lagenidium, Myzocytium, and the Saprolegniaceae. 

 Sparrow ('33. '36) included Ectrogella and Apha- 

 nomycopsis in the Saprolegniales without commit- 

 ting them to any particular family, but in 1912 he 

 placed them in the Ectrogellaceae and made this 

 family the first and most primitive of the Sapro- 

 legniales. Coker and Matthews ('37) also included 

 the Ectrogellaceae in this order next to the Sapro- 

 legniaceae. 



Very little can be said about the origin and rela- 

 tionships of the Sirolpidiaceae at present, because 

 this family is even less known than the Ectrogel- 

 laceae. It has no particularly outstanding family 

 characteristics which relate it distinctly to any of 

 the other groups. The genera Sirolpidium and 

 Pontisma were formerly included by Petersen ('05) 

 ,in the Holochytriaceae of the Mycochytridiales, 

 although he had discovered that the zoospores of 

 the type species, S. Bryopsidis, are biflagellate and 

 not chytrid-like. His findings were confirmed by 

 Sparrow ('34. '36) who later ('12) proposed the 

 family Sirolpidiaceae for these genera and placed 

 it in the Lagenidiales between the Olpidiopsidaceae 

 and Lagenidiaceae. Whether or not present-day 

 knowledge warrants this position is obviously open 

 to question, but the nature of the vegetative thallus 

 of the Sirolpidiaceae nevertheless suggests such a 

 relationship. The thallus has a tendency to become 

 elongate, filamentous, and somewhat mycelioid and 

 may fragment into sections like those of some species 

 of Lagenidium and Myzocytium. On the other hand, 

 the thallus may sometimes be unicellular and olpi- 

 dioid like those of the Olpidiopsidaceae. These like- 

 nesses are also correlated with similarities in the 

 general appearance of the protoplasm, the presence 

 of a large central vacuole bounded by a parietal layer 

 of protoplasm in the mature zoosporangium before 

 cleavage, and the method of cleavage. Size, shape. 

 and structure of the thallus. however, are vegetative 

 characters which vary markedly and by themselves 

 are not always significant phylogenetically, so that 

 too much emphasis must not be placed on them. 



So far as is now known the zoospores are not 

 diplanetic. and in this respect do not show affinity 

 with the Ectrogellaceae or the diplanetic members 

 of the Lagenidiaceae. According to Sparrow ('31), 

 the zoospores of S. lagenidioides are strikingly simi- 

 lar in behavior and appearance to those of Rozellop- 

 sis inflata, while in Petersenia lobata the late cleav- 

 age and early zoospore stages resemble those of 

 Pythium. Thus, aside from their arched, pyriform 

 or slightly reinform shape and the presence of two 

 flagella the zoospores offer few clews to the rela- 

 tionship of the Sirolpidiaceae. Comparisons on the 

 basis of type of sexual reproduction cannot be made 

 because nothing is known about sexuality in this 

 family. Resting spores are unknown in most species, 

 and in those for which they have been reported they 

 appear to be nothing more than vegetative tlialli 

 which have encysted and become thick-walled. 



Most species of the family Olpidiopsidaceae are 

 fully known as to life cycles and development, and 

 the indications of relationship are accordingly more 

 clearly defined. In thallus structure and appearance 

 all species show a striking parallelism to the olpi- 

 diaceous chytrids, and for this reason they were 

 first included in the Olpidiaceae and later in the 

 Pseudolpidiaceae and Woroninaceae by most my- 

 cologists and designated as biflagellate chytrids. 

 This close resemblance in vegetative structure is 

 probably due to convergent evolution and may not 

 be indicative of affinity. Sharply defined diplanetism 

 does not occur in this family except in Olpidiopsis 

 Oedogoniorum and Pythiella vernalis — two species 

 which possibly do not belong in the Olpidiopsida- 

 ceae. In most species of Olpidiopsis, however, the 

 zoospores may come to rest, retract their flagella, 

 become amoeboid, and then remain quiescent for a 

 while, but they do not encyst. After a short while 

 flagella are formed again, and the zoospores resume 

 their motility. The insertion and position of the 

 flagella appear to be the same during both motile 

 periods. Butler ('07) compared this interruption of 

 motility to diplanetism in the Saprolegniaceae. and 

 later Scherffel ('25) described it as diplanetism 

 without encystment. Whether or not the behavior of 

 these zoospores is to be regarded as evidence of 

 primitive and rudimentary diplanetism which fore- 

 shadows the development of true diplanetism in the 

 Ectrogellaceae, Lagenidiaceae, and Saprolegniaceae 

 is, of course, a debatable question. In O. Oedogonio- 

 rum, as noted before, true diplanetism has been re- 

 ported by Scherffel, but the primary swarming pe- 

 riod may be reduced to nothing more than the emer- 

 gence of the zoospores and a slight beating of the 

 flagella. Occasionally the entire content of the spo- 

 rangium may emerge as a protoplasmic mass and 

 then undergo cleavage into zoospores on the outside 

 as in Lagenidium and Pythium. A similar behavior 

 was occasionally noted by Coker ('23) in 0. Sapro- 

 legniae. In P. vernalis the primary swarmers are 

 aflagellate and merely glide out of the exit tube, near 

 the mouth of which they encyst. The behavior of the 

 zoospores in these three species ranges from that of 



