C'hapUr 1 



Introduction 



The Plasmodiophoralks iiicliulo one family of or- 

 ganisms wliicli are often referred to as parasitie 

 slime molds heeause tiiey arc i-liaraeterized by a 

 multiniuleate ulasmodial stauv as in the trne slime 

 molds and ))arasitize tilamentous fiingi, alu'ae. eryp- 

 togams. and liiirher plants. While this eomnion name 

 mav lie deseri|)tive. its use is unfortunate, since it 

 suggests a relationsliip witli the Myxomyeetes which 

 has not been definitely established. Most genera of 

 this order have rather complex life cycles which in- 

 clude zoosj)ores, amoebae, sjiorangiosori, zoosjio- 

 rangia. secondary zoospores, plasmodia, cystosori, 

 resting s))orcs, and probably isoniorjihic gametes. 

 Sporangiosori and tliin-walled evanescent zoospo- 

 rangia were first observed by Borzi in Rhizovii/jca 

 hypoi/ea as early as 188 t. and later by Neniec ('11. 

 '13) in Sorolpidiiim and Anis07ni/jra, but at that time 

 the relationship of these genera to the Plasmodi- 

 ojihoraccae was not clearly understood. Zoospo- 

 rangia were subsetjuently rediscovered by Cook 

 ('26). Cook and Schwartz ('30). I.edingham ('33. 

 '3i. '3.5. '39). Fedorintscliik ('3.")), Coueli. et at. 

 ('39) in Lignii-ra, Plasmodiophora, Poli/mi/.ra, 

 Sponr/ospora, and ()ctomi/xa and are now generally 

 believed to be a characteristic developmental phase 

 of the order as a whole. The zoosporangia are re- 

 garded by some workers as gametangia in which 

 meiosis precedes gametogenesis, but this has not been 

 conclusively ])roven. 



The s])orangial phase is followed by the develop- 

 ment of a conijiaratively large multinucleate sporo- 

 genous Plasmodium in which meiosis is reported to 

 occur before or during cleavage into resting spores. 

 The latter may remain loose and free of each other 

 or unite in more or less compact cystosori. Upon 

 germination, tlie resting spores ))roduee uninucleate 

 amoebae or motile flagellate zoos|)ores. These cells 

 are regarded by many workers as isomorphic gam- 

 etes which fuse in pairs and thus initiate the diploid 

 generation, but so little is known about sexuality in 

 this order that nothing conclusive can be said as yet 

 about the sexual nature of these cells. Some my- 

 cologists contend that a true ])lasmodinm does not 

 exist in the Plasmodiophorales on the grounds that 

 the naked multinucleate tliallus is not formed by the 

 coalescence of numerous mutually attracted amoebae 

 in the manner described by Cienkowski ('63) for the 

 Myxomyeetes. In so doing, these mycologists disre- 

 gard the reports of Woronin ('77), Halsted ('93), 

 Nawaschin ('99), Evcleshvmer ('01). Massee ('08), 

 Osborn (']!), Kunkel ('15). Terby ('2t), Jones 

 ('28). Home ('30). Cook and .Schwartz ('30). Milo- 

 vidov ('31 ). I.edingham ('39). and others that amoe- 

 bae as well as small plasmodia coalesce in Plasmodi- 

 ophora, Sponf/ospora, Pol i/nii/.ra, etc. \\ hether or not 



these re])orts are accurate may be oi)en to question, 

 because they are not all based on observations of liv- 

 ing material. These data nevertheless exist in the 

 liter.-iturc and must be given serious <'onsideration. 

 I'urtherniore, the above-mentioned reasons for ex- 

 cluding the term ))lasmodiuin from the Plasmodi- 

 ophoraceae would also ])reclude its use in relation to 

 the .Myxomyeetes according to recent data on this 

 group. .lahn (11. '36), Skupienski ('28), Wilson and 

 Cadnian (28), Cadman ('31), and others have 

 shown that the ])lasmodium is initiated by fusion in 

 pairs (if isomorphic gametes and that the zygotes 

 may subsequently ingest unfused lia])loid amoebae 

 as food material. Thus, the conception of a Plas- 

 modium as Cienkowski interjjreted it has undergone 

 considerable modification and is now used jirinci- 

 l)ally as a deseri))tive term for the naked, multinu- 

 cleate, assimilative phase of the slime molds. In this 

 sense it may be equally well employed for the naked 

 multinucleate thallus of the Plasniodioiihorales. 

 Cook's use of the term myxamoeba for this stage is 

 unfortunate, misleading, and obviously unwar- 

 ranted. According to standard dictionaries .md glos- 

 saries, the term myxamoeba relates to the naked, 

 amoeboid, and usually uninucleate protoplasts 

 formed by the germinating resting spores of the 

 Myxomyeetes, and its introduction as a deseri])tive 

 name for the naked multinucleate plasmodial stage 

 of the Plasmodiophorales will lead to nothing but 

 confusion. Likewise, his use of the term "swarm 

 cells ' for the products of spore germination as a dis- 

 tinctive contrast to the name "zoos])ores " for the 

 flagellate cells formed in zoosporangia is not war- 

 ranted at present and should be avoided. I.edingham 

 and Barrett have clearly shown that the zoospores 

 are biflagellate and hcterocont regardless of whether 

 they are formed in zoosporangia or from resting 

 s])ores and that there are no structur.il distinctions 

 between the so-called swarm cells and zoosjiores. If 

 in the future it is found tliat the resting spores form 

 gametes and the s))orangia zoospores, or vice versa, 

 the two products may then be distinguished and 

 designated as gametes and zoospores, respectively. 



.Mtliough most s|)ecies of this order, except P. 

 lirax.iicae and S. suhti-rratwa, a])pear to be compara- 

 tively rare in occurrence, they are nevertheless 

 world wide in distribution and have been re))orted 

 from North and .South .\merica. .\frica, Kurope, 

 Asia. Australia and several Atlantic and Pacific 

 islands. Three s])ecies occur in fungi, algae, and 

 cry))togams. while the remainder parasitize higher 

 |)lants. All s))ecies, exce)>t members of the genus JAfj- 

 n'tera, cause distortion of the host and marked 

 changes in its cells. These changes involve enlarge- 

 ment and divison of infected as well as of adjacent 



