CHYTRIDIALES 59 



phlyctis laevis (Sparrow, 1939a) the outer wall of the tip of the short 

 tube likewise disappears before discharge. Here, however, a well-defined 

 papilla remains on the surface of the mass of zoospores. This circum- 

 stance might be considered evidence for the cytoplasmic origin of the 



papilla. 



Little can be said with certainty concerning the external factors which 

 induce the discharge of the mature sporangia. Under some conditions 

 they will remain mature but undischarged for relatively long periods, 

 whereas under others they may evacuate their zoospores as soon as 

 they are formed. In general, however, sudden changes in environmental 

 conditions may precipitate sporulation, as in the algae. Thus if spo- 

 rangia are transferred into fresh clean oxygenated water from the quiet 

 water in which they have matured and which presumably contains 

 various metabolic products accumulated during the vegetative phase 

 of the fungus, discharge can frequently be brought about (Couch, 

 1939a). Changes in temperature, though most often noted in the liter- 

 ature in connection with the inducing of sporulation in biflagellate 

 Phycomycetes (Coker, 1923), no doubt act in a similar fashion here. 

 There seems to be a certain periodicity in the sporangial discharge of 

 Phlyctochytrium hallii and P. biporosum (Couch, 1932), since sporangia 

 of these species have been observed to release their zoospores usually 

 in the late afternoon or evening. 



Even less is known of the processes within the sporangium that cause 

 the discharge of the zoospores. Possibly, the intake of water by the mass 

 of mature spores creates a pressure too great for the modified material 

 of the papilla to resist. If this hypothesis is correct, however, the per- 

 meability of the sporangium wall to water must vary, since in undisturb- 

 ed water mature sporangia often remain undischarged for days or even 

 weeks. In the blastocladiaceous fungus AUomyces, Ritchie (1947) found 

 that the sporangium at the time of discharge had lost 22 per cent of 

 its original volume, this presumably by excretion of water. In sporangia 

 of Phlyctochytrium biporosum about to discharge, Couch (1932) ob- 

 served that, after a period of slight motion, the spore initials become 

 quiescent and clear narrow spaces appear between them. This results 

 in a flattening out of those spores next to the wall, as though pres- 

 sure is being applied to them. Simultaneously, the sporangium swells 



