134 



PHYCOMYCETEAE 



vague that a few mycologists, e.g., Fitzpatrick (1923), have suggested 

 uniting the two genera, which would then have to take the older name 

 Pythium. However the two genera may still be kept separate on the basis 

 of the method of zoospore formation: within the zoosporangium (con- 

 idium) in Phytophthora and in an external vesicle in Pythium. In the rare 

 cases where a vesicle is formed in the former the zoospores are produced 

 within the zoosporangium before passing into the vesicle. Thomas (1942, 

 1943) made a chemical investigation of the composition of the cell walls 

 of these two genera. In both of these the major part of the wall consists 

 of cellulose with an outer deposit of fatty substance. Outside of this there 

 is a layer of some pectic compound in the case of Pythium and of some 

 other carbohydrate in Phytophthora. After the carbohydrates and fatty 

 substances are all dissolved the hyphae still retain their form and this 

 residual matter proved to be chitin. This is particularly interesting in 

 view of the conclusions of von Wettstein (1921) that cellulose and chitin 



Fig. 43. Peronosporales, Family Pythiaceae. Pythiogeton transversum Minden. (A) 

 Portion of plant with a zoosporangium not quite mature, a zoosporangium discharging 

 its contents, and an empty zoosporangium. (B) Portion of the discharged contents 

 separating into zoospores. (C) Young oogone and antherid. (D) Oogone and antherid 

 after fertilization. (E) Mature oospore. (After von Minden: Mykologische Unter- 

 suchungen and Berichtc, 1(2). •146-255.) 



