PERONOSPORALES 1015 



undifferentiated portion of the mycelium set off by cross walls and 

 bearing an evacuation tube (Fig. 86 B, p. 1030), (b) a complex of irregular 

 digitate or lobulate intercommunicating elements set off by cross walls 

 from the rest of the hyphae and having an evacuation tube (Fig. 86 F), 

 and (c) a bursiform, utriform, spherical, or somewhat ovoid terminal 

 or intercalary structure set off by septa from the more or less differen- 

 tiated hyphae, with or without a discharge tube (Fig. 87 C, E, p. 1032). 

 Intergrades between some of these types apparently occur and make 

 difficult identification of an organism on this basis alone. Somewhat 

 specialized sporangiophores characterize Phytophthora. 



In the aquatic Pythiaceae, with the exception of Phytophthora and 

 Diasporangium, the zoospores are formed outside the sporangium, 

 from an amorphous mass of protoplasm. In Pythium and in Zoophagus 

 this process takes place in a delicate somewhat spherical vesicle pro- 

 duced at the tip of the discharge tube (Fig. 83, p. 1016). In Pythiogeton 

 (Minden, 1916; Drechsler, 1932), however, the vesicle is elongate and 

 quickly disappears, leaving the protoplasmic mass, previously concen- 

 trated in its distal part, to undergo cleavage into zoospores free in the 

 water (Fig. 88 AC, p. 1045). Under humid conditions and occasionally 

 when submerged (Minden, op. cit.; Drechsler, op. cit.) the protoplasmic 

 mass may, in Pythiogeton, be shot out with explosive force. Sometimes 

 this ejaculation occurs at the beginning of the evacuation of the spo- 

 rangium, at other times, after the contents have completely emerged 

 into the vesicle. 



Zoophagus and many of the aquatic species of Pythium exhibit an 

 undifferentiated sporangium resembling that in certain species of 

 Lagenidium. The sequence of changes as they occurred prior to and 

 during the formation of zoospores in P. adhaerens (Sparrow, 1931b), 

 parasitic on the green alga Rhizoclonium, is as follows: Numerous 

 vacuoles appear in the densely granular contents of the mycelium, but 

 after several hours decrease in size and at last disappear. In the finely 

 granular protoplasm narrow cross walls laid down at considerable inter- 

 vals along the mycelium may be observed. These blocked-off hyphal 

 portions are the initials of the zoosporangia. The tips of certain hyphae 

 (one for each continuous mycelial segment) now show striking mod- 

 ifications, which indicate that the contents will be evacuated through 



