PERONOSPORALES 199 



the swarmspores are fully formed. The vesicle may arise at the 

 apex or at the side of the sporangium. Frequently it is initiated 

 as a cylindrical tube which may attain considerable length 

 before becoming inflated at the tip to form a globose bladder. 

 The recent use of the term emission collar by Sideris (1929) 

 for the tubular or attenuated portion of the vesicle lying next 

 the sporangium seems unfortunate. The swarmspores escape 

 by the rupture of the vesicle, and are then seen to be reniform and 

 laterally biciliate (not uniciliate as usually pictured for P. 

 deharyanum). Some writers apply the term sporangium to the 

 vesicle which frees the spores, and designate the primary cell 

 from which it arises the prosporangium (presporangium of Edson, 

 1915). This terminology is confusing and in any case cannot 

 be appUed in the related genus Phytophthora, where in a given 

 species the vesicle is sometimes formed and sometimes absent. 



The oogonia and antheridia of species of Pythium are developed 

 on either the intra- or extramatrical mycelium. The sexual 

 process corresponds in general to that described for Peronospora 

 parasitica. Differentiation into ooplasm and periplasm occurs, a 

 coenocentrum is present, fusion of one male with one female 

 nucleus takes place, and the oospore germinates usually by a germ 

 tube. The details of the process are given by Trow (1901) 

 for P. ultimum, by Miyabe (1901) for P. deharyanum, by Pat- 

 terson (1927) for P. torulosum, and by Edson (1915) for P. 

 aphanidermatum. 



The genus Zoophagus Sommerstorff (1911), based on Z. 

 insidians Som., a species attacking rotifers, is apparently too 

 close to the subgenus Aphragmium of Pythium to permit of its 

 recognition as a separate genus. This species has been discussed 

 by Arnaudow (1918; 1925), Gicklhorn (1922), Mirande (1920), 

 and Sparrow (1929). The latter writer, basing his observations 

 on American material, gives us a clear conception of the species. 

 The fungus differs from Aphragmium, as thus far described, in 

 possessing "gemmae" in addition to the filamentous sporangia. 

 The swarmspores are said to be diplanetic, and the species is 

 heterothallic. 



3. Phytophthora de Bary (1876: 240). 



Although most authorities on the Phycomycetes have included 

 Phytophthora in the Peronosporaceae, it is here removed from 

 that family and incorporated in the Pythiaceae because of the 



