OOMYCETES 



83 



51, 7, lower left) and allows its nucleus with most of the protoplasm to 

 pass over. Fusion of the two nuclei takes place very late during the 

 maturation of the oospore. The wall of the mature oospore consists of 

 three layers, the original membrane, a thin hyaline primary endospore 

 and a thick secondary endospore which consists of reserve material 

 which is dissolved at germination. This takes place through a hypha 



Fig. 51. — Phytophthora erythroseptica. Development of oospores. 1. Oogonium 

 beginning to penetrate antheridium. 2. 2 hr. 20 min. later, the oogonium begins to swell 

 beyond the antheridium. 3. Same as 1. 4. Older stage. 5,6. Development of oogonium 

 after it has grown through the antheridium. 7, 8. Fertilization. 9. Immature oospore. 

 (1, 2 X 420; 3 to 9 X 800; after Pethybridge, 1913, and Murphy, 1918.) 



which, according to circumstances, may develop to a mycelium or cut 

 off a conidium. 



The species of the Peronosporeae, e.g., Plasmopara alpina (Rosenberg, 

 1903), P. densa, Sclerospora graminicola, Peronospora vernalis (Ruhland, 

 1904), P. Alsinearum, P. effusa (Berlese, 1898), P. Ficariae (Berlese, 1898; 

 Kruger, 1910) and P. parasitica (Wager, 1900; Kruger 1910) show 

 essentially the same steps in development as Phytophthora erythroseptica; 

 only their antheridia are paragynous and at the time of fertilization 

 contain all the nuclei of which only one enters the oogonium. In regard 



