CLASSIFICATION 61 



lated, as in Monoblepharis; or the gametes may be brought into 

 contact by passage of the male gamete through a fertihzation 

 tube into the oogonium. In Polyphagus both gametes pass into 

 a swelling in the fertilization tube, and this swelling becomes 

 transformed into the zygote membrane. In Rhizopiis nigricans 

 the two gametangia become transformed into the zygote wall. 

 In the Saprolegniales and Peronosporales the antheridial con- 

 tents flow into the oogonium, and the oogonial wall remains as a 

 cyst for the developing oospores. 



Both gametes may be uninucleate, as they are in Synchytrium 

 and Monoblepharis. In other species one gamete is uninucleate 

 and the other multinucleate, and in still others, as in the AIu- 

 corales and some species of Albugo, both are multinucleate. 



Organs of locomotion of zoospores. Among aquatic Phyco- 

 mvcetes motility is induced by organs called cilia. The studies 

 of Couch (1941) show^ that there are two distinct structural types 

 of ciha: the "whip-lash" type and the "tinsel" type. The whip- 

 lash type consists of a long, quite rigid, basal portion, the handle, 

 and a short, thin, upper portion, the lash. The tinsel type con- 

 sists of a central axis, from w^hich extend short lateral hairs. 

 Among true chytrids the ^\'hip-lash cilium is situated posteriorly. 

 The zoospore may rotate on its own axis, as seen by dark-field il- 

 lumination, and the cilium will then appear alternately as a single 

 and a double image. If the image is single, the cilium is undulat- 

 ing in a plane vertical to the observer; if it is double, in a plane 

 horizontal to the observer. In Rhizidioinyces apophysatiis there 

 is a single anterior tinsel cilium. In species of Olpidiopsis, 

 Lagenidium, Saprolegnia, and Pythium, wdth biciliate zoospores, 

 the anterior cilium is of the tinsel type and the posterior cilium 

 of the whip-lash type. 



Classification. The most recent comprehensive monograph 

 of aquatic Phycomycetes is that of Sparrow (1943). In it are 

 descriptions of 475 species and 10 varieties, belonging in 112 

 genera. It does not include the Peronosporales, as described in 

 this book, nor the Alucorales and Entomophthorales, all species 

 of which are terrestrial. Sparrow's excellent treatise will serve 

 for years as a taxonomic handbook for students of aquatic 

 fungi. The number of flagella and the place of their attachment 

 are basic in the separation of orders in his keys. 



