128 



PHYCOMYCETEAE 



Fig. 40. Peronosporales, Family Pythiaceae. (A-D) Pythium dictyosporum Racib. 

 (A) Formation of zoospores within vesicle at the open tip of the discharge tube of the 

 filamentous sporangium. (B) Zoospores. (C) Young oogone and antherid. (D) Mature 

 oogone and oospore. (E) Pythium torulosum Coker and Patterson, showing the toruloid 

 zoosporangium and vesicle. (F, G) Pythium proliferum de Bary. (F) Spherical zoo- 

 sporangium with vesicle. (G) Proliferated zoosporangium. (A-D, courtesy. Sparrow: 

 Mycologia, 23(3):191-203. E-G, courtesy, Matthews: Studies on the Genus Pythium, 

 Chapel Hill, Univ. North Carolina Press.) 



plants, mainly roots and stems and in one species in the leaves. In the 

 forms parasitic in land plants the mycelium is mainly intracellular, usu- 

 ally killing the cells rapidly, in some cases appearing to kill them in 

 advance of the arrival of the hyphae. 



The mycelium is slender and, as in the Saprolegniales, is a branching 

 coenocyte. Septa are formed to set off the antherids and oogones and, 

 usually, the zoosporangia, and to separate the empty older portions of the 

 mycelium from those containing protoplasm. Sexual reproduction has 

 been studied by Trow (1901), Patterson (1927), Edson (1915), and others, 

 and has been found to resemble most closely that in the Rhipidiaceae. 

 The oogone arises terminally, less often intercalarly on longer or shorter 

 unmodified hyphae. It is ovoid or spherical and smooth or papillate or 

 even echinulate. Sometimes it is tubular-elongate at one or both ends. 

 Of the several nuclei in the oogone, all but one migrate to the periphery 



