/■?'- 



Chapter I 

 Introduction 



LIBRARY 



The ruNGi presented herewith as tin- simple, holo- 

 carpic, biflagellate Phycomycetes comprise a hetero- 

 geneous collection of approximately eighty species 

 which .ire characterised by relatively small or re- 

 duced, holocarpic thalli and biflagellate zoospores. 

 Although tlu>e characters are common to all mem- 

 bers of the group, present day evidence does not 



fully warrant the inclusion (if these species in one 



coherent family or order. The simple species were 

 formerly included by most mycologists in the family 

 Woroninaceae and placed among the Chytridiales, 



while the elongate, more mycelioid members were 

 incorporated in the Lagenidiaceae and regarded as 



closely related to the Saprohgniaeeae and Py- 



thiaceae. Inasmuch as the Chytridiales are char- 

 acterised by posteriorly uniflagellate zoospores, the 

 members of the Woroninaceae can no longer be in- 

 eluded in this order as it is now reeognized. For this 

 reason these biflagellate species as well as the 

 I agenidiaceae are described apart from the cby- 

 trids. As will become evident helow, they do not 

 constitute a well-defined order or family and it is 

 accordingly impossible to give the group as a whole 

 a distinctive name. They arc thus presented as sim- 

 ple, holocarpic, hiflagellate Phycomycetes. The ma- 

 jority of them arc Oomycete-like and appear to he 

 either simple and primitive or reduced and degen- 

 erate Oomycetes, hut since some species are re- 

 ported to he isogamous. like the Zygomycetes, they 

 must for the time being he listed under the more 

 general term Phycomycetes. Further study of known 

 species and the discovery of new ones will doubtless 

 invalidate many of the present day concepts con- 

 cerning their phylogeny and relationships, and it 

 is not improbable that they may eventually be in- 

 eluded in existing families and orders of the Oomy- 

 cetes and Zygomycetes. 



Whether or not the known genera and species 

 comprise several distinct and clearly defined fami- 

 lies is not at all certain at the present time. The 

 critical developmental stages of many species are 



unknown, and the limits of the genera are not 



sharply defined. Nevertheless as an aid iii classifica- 

 tion and an expedient of reference, they have been 



grouped into rive families on the hasis of thallus 

 structure and method of sexual reproduction. The 



structure of the vegetative thallus alone is obviously 

 of questionable diagnostic value, hut in species 



where resting spores and sexual reproduction are 

 unknown, it is the only hasis of distinction for the 

 present. The order in which the families are pre 



seated does not always indicate degree of com- 

 plexity, phylogenetic relationship, and evolution. 

 The provisional family Woroninaceae is presented 

 first because of its plasmodium-like thallus and 



other structural similarities it has in common with 



the Plasmodiophorales, hut the second family, 

 Bctrogellaceae, s h o w s distinct saprolegniaceous 



characteristics by its diplanctic zoospores. How 

 closely this family is related to the Saprolgcnialcs 

 cannot he ascertained at present, because so little is 

 known about its resting spores or the occurrence of 

 sexuality. The Olpidiopsidaceae should perhaps be 

 placed next to the Lagenidiaceae because of its pre- 

 dominantly heterogamous type of sexual reproduc- 

 tion. The Sirolpidiaceae, however, is placed in this 

 relative position because its thallus has a tendency 

 to become elongate, filamentous, mycelioid and frag- 

 mented like that of some species of Lagenidium and 

 Mysocytium. On the other hand, its thallus may 

 sometimes be reduced and distinctly olpidioid. The 

 occurrence of such thalli and the fact that nothing 

 is known about the type of sexual reproduction 

 makes the position of this family next to the Lageni- 

 diaceae very problematical. The Lagenidiaceae ap- 

 pears to be the most complex as far as thallus struc- 

 ture and method of sexual reproduction are con- 

 cerned and is accordingly presented as the highest 

 family in the evolutionary series. In most species 

 tin thallus is elongate and often distinctly mvee- 

 lioid, but it may also be reduced, unicellular, and 

 olpidioid. However, the resting spores or oospores 

 are usually developed in partially or fairly well 

 differentiated oogonia as in the higher Oomycetes. 



The simpler, olpidioid species were discovered 

 shortly before the middle of the last century. Al- 

 though they had probably been observed earlier. 

 Nageli ('44) was among the first to describe and il- 

 lustrate them as globular bodies in the mycelium of 

 water molds, but he mistook them for a part of the 

 normal life cycle of the host. Cienkowski ('.55) like- 

 wise misinterpreted the parasites which he found in 

 Achlfia prolifera as a third type of sporangia de- 

 veloped by this host, hut in the same year the para- 

 sitic nature of these intrainatrical bodies was clearly 

 recognized by Alexander Braun. The species which 

 he described in Saprolegnia fcra.r is now believed to 

 relate to 0/ pidiopsis Saprolegniae. He named this 

 parasite Chytridium Saprolegniae and placed it in 

 his sub-genus Olpidium of the Chytridiaceae. Thus. 



at an early stage these (Jlpiiliupxix parasites became 

 associated with the ehytrids in myeologieal litera- 

 ture, and this fact together with their striking simi- 

 larity of thallus structure .and type of development 

 to that of the olpidioid ehytrids arc probably the 

 chief reasons why most mycologists up to the pres- 

 ent time have included them in the Chytridiales. 

 Despite Braun's excellent study Pringsheim ('60) 

 was still doubtful about the nature oi these parasites 



and believed that they might possibly be the anthcri 



