22 



PL ASMODIOP MORALES 



Sorosphaern. Although mycologists and protozoolo- 

 gists have clearly recognized the inadequacy of these 

 criteria, they have nevertlieless continued to use tliem 

 as the basis of classification. More recently. Cook 

 ('33) has used the presence of zoosporangia and zoo- 

 spores as another basic distinction. However, since 

 zoosporangia have been subsequently found in nu- 

 merous genera, the mere presence of sucli structures 

 is no longer generically distinctive. Likewise, liis em- 

 pliasis on tlie presence or absence of a membrane 

 around the cystosori as a diagnostic character is open 

 to question, since there is considerable doubt about 

 the occurrence of soral membranes in any of the 

 genera. Palm and Burke, in particular, Iiave severely 

 criticized tlie present-day system of classification 

 and characterized it as artificial. From tlieir obser- 

 vations on the wide variations exiiibited by cystosori 

 of S. 1 eronicae , tliey concluded that Spongoxpora, 

 Lif/niera, Sorodiscus, Ostenfeldiella, Clathrosoriis, 

 and Memhranosorus should be regarded as synonyms 

 of Sorosphaera. On the basis of similarity of life 

 cycles and general structure, they further advised 

 the merging of all known genera except C ystospora 

 into one large genus, presumably Plasmodiophora. 

 The author is in complete agreement with these 

 workers on the low taxonomic value and inadequacy 

 of present-day generic distinctions. However, Palm 

 and Burke's suggestion of reducing the number of 

 genera or merging them does not solve the difficulties 

 of classification in this group. As Ledingham ('39) 

 pointed out, it merely shifts the generic indistinc- 

 tions to tlie species. 



Further taxonomic distinctions ajjpear to be 

 emerging from the discovery of zoosporangia in old 

 and new genera. When these developmental stages 

 have been fully investigated, the relationship of the 

 various genera will doubtless become clearer, and it 

 may then be possible to separate or merge them with 

 greater accuracy. In the meantime, Schroeter's sys- 

 tem of classification serves as a working basis, and 

 although an unsatisfactory expedient, it may be used 

 to advantage. In the key wliich follows, size, number, 

 and sha])e of zoosporangia are used to some extent in 

 diagnosis, but these characters are of doubtful gen- 

 eric value. Some of the genera — i.e., Memhranosorus 

 and Lif/niera — listed here are obviously questionable 

 and sliould ])erhaps be merged with Sorodiscus and 

 Sorosphaera, but until more is known about the fam- 

 ily as a whole, it may be worth while to treat tliem 

 separately. 



PLASMODIOPHORACEAE 



Zopf, 188-i. Die Pilzthiere oder Schleimpilze 



Thallus a naked. ])lasmodial, multinucleate proto- 

 plast capable of amoeboid movement and undergoing 

 sciiizogony into uni- or multinucleate meronts, wliicli 

 in turn may function as schizonts. Sporogenous thal- 

 lus cleaving into uninucleate spores at maturity. 

 Resting spores loose and free or united in small clus- 

 ters and cystosori ; usually producing one zoospore 



or amoeba in germination. Zoospores anteriorlj' bi- 

 flagellate and heterocont. Zoosporangia formed di- 

 rectly from zoospores or cleavage segments of young 

 Plasmodia ; free or united in sporangiosori ; produc- 

 ing a few to numerous zoospores which are similar to 

 those formed from resting spores. 



Key to Genera 



I. Resting spores not united, free and loose. Zoosporangia 



few or numerous, small, and producing few zoo- 

 spores 1. PLASMODIOPHORA, p. 22. 



II. Restin); spores united in small clusters or more or less 



compact cystosori 



A. Spores usually in tetrads or dyads. Zoosporangia un- 

 known 2. TETRAMYXA, p. 37. 



I?. Spores usually in octads. Zoosi)orangia numerous, 

 small, oval, and spherical with or without exit 

 papillae 3. OCTOMYXA, p. 40. 



C. Cystosorus predominantly spherical to ellipsoidal and 



hollow; often variable in size and shape. Zoospo- 

 rangia small 4. SOROSPHAERA, p. 41. 



D. Cystosorus predominantly disc-shaped, two-layered 



and flattened; often variable in size and shape. 

 Zoosporangia unknown 



5. SORODISCUS, p. 46. 



E. Cystosorus oval, spherical, and s])onge-like, lacking a 



central cavity but traversed by prominent canals 

 and fissures. Zoosporangia numerous or few, small, 

 oval and spherical, or large and irregular 



6. SPONGOSPORA, p. 54. 



F. Cystosorus indefinite in size and shape 



1. Zoosporangia small, oval and spherical; producing 

 few zoospores 7. LIGNIERA, p. 58. 



3. Zoosporangia usually large, elongate, lobed and 

 irregular with prominent exit tubes 



8. POLYMYXA, p. 63. 



PLASMODIOPHORA 



Woronin, 1877. Arb. St. Petersburg Nat. Gesell. 

 8:169. 



Osti'iiffliUeUa Ferdinandsen and Winge, 1914. Ann. 



Bot. 28: 64. 



(plates 2. 3. 4.) 



Resting spores lying free in host cell, not united 

 in cystosori, variable in size and shape, usually pro- 

 ducing one zoospore in germination. Zoospores an- 

 teriorly biflagellate and heterocont, becoming inter- 

 mittently amoeboid, infecting the host as an amoeba 

 ( ?), dividing and budding (.''), and eventually form- 

 ing multinucleate plasmodia, which cleave into uni- 

 nucleate segments. Cleavage segments developing 

 into small zoosporangia which produce few zoo- 

 spores. Secondary zoospores reinfecting host and 

 forming additional ])lasmodia. Sporogenous ))lasmo- 

 dium partly or com])letely filling host cell, moving 

 slowly in amoeboid fashion within the host cell and 

 in migrating from cell to cell ; occasionally luider- 

 going schizogony into uni- and multinucleate me- 

 ronts; rarely encysting; cleaving into resting spores 

 at maturity. 



