OOMYCETES 



79 



The conidia and sporangia of the Peronosporaceae behave very 

 differently on germination. In Pythiomorpha gonapodioides the zoo- 

 spores are formed in the zoosporangium and are discharged by the 

 solution of the papilla and the rupture of the inner membrane. They 

 come to rest, encyst, and germinate either by hyphae or swarming. 

 The second swarm stage is non-motile. Secondary sporangia are some- 

 times formed by proliferation of the primary sporangia as in Saprolegnia 

 (Kanouse, 1925). In Pythiogeton at the evacuation of the sporangium 

 the whole content passes out into a sac in 20 to 60 seconds (Fig. 49, 1); 



Fig. 48. — Plasmopara viticola. A, conidiophores with oogonia and oospores at the left; B, 

 haustoria; C, germinating conidia. (After Millardet.) 



this ruptures and discharges the naked masses of protoplasm into the 

 air (Fig. 49, 2). Shortly there appear indentations which penetrate 

 deeper so that after 15 to 20 minutes the outlines of the zoospores are 

 visible (Fig. 49, 3). These round off and swim away. Also in Pythium, 

 the sporangia or conidia discharge their undivided contents into a sac 

 where it then falls apart into swarm spores which are liberated by the 

 bursting of the sac (Fig. 44, 4 and 5, 9 to 11). Under certain conditions 

 P. debaryanum may germinate by a germ tube. In P. ultimum zoospore 

 formation is entirely suppressed and only tube germination is known. 

 After Pythium, the process of germination becomes continually simpler; 

 thus in Pythiomorpha the zoospores are formed already in the sporangium. 



