PERONOSPORALES 1033 



undifferentiated mass into a delicate vesicle produced by the tip of the 

 discharge tube where cleavage and maturation takes place, capable of 

 repeated emergence before finally encysting and germinating; plants 

 probably always monoecious; oogonia terminal or intercalary, spherical 

 or subspherical when terminal, ellipsoidal to limoniform when interca- 

 lary, smooth- walled, or variously echinulated, for the most part forming 

 a single egg with or without conspicuous periplasm; antheridia none or 

 one to several, hypogynous, monoclinous or diclinous, allantoid, clavate, 

 globose, suborbicular or trumpet-shaped, terminal or intercalary, borne 

 on a short or long stalk, or sessile, usually one to four (may be 

 lacking or if present up to twenty-five) to an oogonium, forming a dis- 

 tinct fertilization tube; oospores usually borne singly within the oogo- 

 nium, plerotic or aplerotic, wall smooth or reticulate, thin or inspissate, 

 the granular protoplasm usually bearing a conspicuous reserve globule 

 and a lateral refringent body, upon germination forming one or several 

 germ tubes, or zoospores. 



Saprophytic and parasitic on plant and animal material in water and 

 soil. 



KEY TO THE SPECIES OF PYTHIUM l 



Sporangium filamentous 



Sporangium undifferentiated from the vegetative hyphae; without 

 inflations 

 Sex organs formed 



Wall of oogonium smooth 



Antheridium delimited by a septum 

 Oospore wall smooth 

 Oospore plerotic (i. e., filling the oogonium) 



Antheridium monoclinous and diclinous; one or two 

 to an oogonium, slightly inflated, clavate, and 

 crooknecked; hyphae with lateral swollen out- 

 growths; in normal soil (Fig. 86 A) 



P. monospermum Pringsheim, 2 p. 23 m 

 Antheridium either monoclinous or diclinous, one to 

 an oogonium; in marine algae 



1 See also the new species Pythium thalassium Atkins (1955), parasitic and sapro- 

 phytic in eggs of the pea crab (Pinnotheres pisuni). 



2 Page numbers followed by "m" are those of Middleton's (1943) monograph. 

 Note further that these species are not entered in the "List of Substrata" (p. 1073). 



