174 



PARASITIC FUNGI 



c.w. 



Fig. 17. — Pythium. A, hyphas in tissue of seedling; sp., young 

 sporangium. B, ditto, more highly magnified. The hyphse (A) 

 have penetrated the cell walls (c.w.) through narrow holes which 

 the growing t'ips have bored out. C, branched hypha emerged 

 into the air, forming sporangia [sp.). D, germination of 

 sporangium in water. The contents have divided to form a 

 number of zoospores which are enclosed in a bladder pushed 

 through a hole in the wall ; z, free zoospore ; zi germinating 

 zoospore. E, sexual reproduction (conjugation) ; o, young 

 female organ (spherical) and male organ (club-shaped) in contact ; 

 6, penetration of male organ ; c, fertilised egg (zygote) surrounded 

 by thick wall, male organ empty. F, germination of zygote 

 in water (similar to D). G, cress seedling attacked by Pythium 

 just above the soil surface. (After Marshall Ward) etc.) 



