OOMYCETES 91 



justifiable to separate the uniflagellate forms (Monoblepharidaceae and 

 Blastocladiaceae) from the biflagellate (Ancylistaceae, Saprolegniaceae 

 Leptomitaceae and Peronosporaceae), the more so as other characters 

 go parallel with the number of flagella. The uniflagellate zoospores are 

 chiefly amoeboid, the biflagellate not; the uniflagellate families during 

 growth give no cellulose reaction, the biflagellate always; the protoplasm 

 in the former is often reticulate, in the latter homogeneous and periplastic. 



The Oedogoniaceae or the Chytridiales are possible sources of the 

 uniflagellate forms. The type of fertilization suggests the Oedogoniaceae 

 but this family is much more highly organized and has multiflagellate 

 zoospores and sperms. The uniflagellate condition of the zoospores and 

 the habitual agreement of the Blastocladiaceae with Macrochytrium sug- 

 gest the Chytridiales, only in this case, the sperm fertilization in Mono- 

 blepharis would be a new process. On account of the lack of intermediate 

 forms, this difficulty seems greater than that which would arise in the 

 derivation from Oedogoniaceous uniflagellate algae. 



Derivation of the biflagellate forms is still more difficult. The 

 Chytridiales, so far as known, can hardly be considered, as biflagellate 

 forms have not been observed in them. Atkinson (1909) attaches 

 Lagenidium to Polyphagus, and considers the Oomycetes and Zygomycetes 

 have developed in parallel lines from the Ancylistaceae and those in turn 

 from the Rhizideae; it is difficult, however, to support this connection 

 between Lagenidium and Polyphagus. Similarly Scherffel (1925) con- 

 siders an ascending series, from the Monadineae (Pseudosporeae) 

 through the Archimycetes, Chytridiales and Ancylistaceae (Ectrogella) 

 to the Saprolegniaceae, which has developed parallel with the algae under 

 discussion. Direct derivation, however, from the Vaucheriaceae accord- 

 ing to Bary and Tavel, or lower biflagellate Siphonocladiales and Siphon- 

 ales according to Davis, is more satisfactory. The structure of the 

 zoospores might be explained by the assumption of a new diplanetism 

 and oogamous fertilization by the development of a conjugation tube. 

 Besides, certain Leptomitaceae, Rhiphidium and Araiospora, show in habit 

 and position of sexual organs, a striking similarity to the Vaucheriaceous 

 Dichotomo siphon. As Atkinson has already emphasized, however, it is 

 questionable whether much significance may be attached to this agree- 

 ment; for there is a great possibility of convergence phenomena, e.g. in 

 the Chlorophyceae, the Codiaceae have developed independently forms 

 very similar to the Vaucheriaceae. 



It seems to be the best solution to connect the Oomycetes with the 

 Chlorophyceae. This is pure conjecture, however; the true ancestors in 

 which separation took place have mostly died out and it is only by acci- 

 dent that analogies could be found on which to base conclusions. The 

 finding of some new tropical species, however, might supply an entirely 

 new point of view. 



