THE FUNGI : ARCHIMYCETES AND PHYCOMYCETES 231 



As the oogonium swells a separation into a dense central oosphere and 

 a less dense periplasm occurs, associated with changes in the position of 

 the nuclei. Up to this stage they have been more or less evenly distributed 

 through the oogonium, but with these changes in the cytoplasm one nucleus 



FERTILIZATION TUBE 



PERIPLASM 

 ANTHERIDIUM 



A 



OOSPHERE 



Fig. 223. — Peronospora parasitica. Sexual reproduction. A, Young oogonium with 

 receptive papilla and antheridium. B, Oosphere and periplasm, antheridium with 

 fertilization tube. C, Male nucleus passing through fertilization tube towards 

 oosphere. D, Sexual nuclei paired in oosphere. E, Early stage in fusion of sexual 

 nuclei. Oospore wall forming. Periplasm degenerating. F, Mature oospore with 

 fusion nucleus. {After Wager.) 



places itself centrally in the oosphere and functions as the female nucleus, 

 while the rest pass into the periplasm and eventually abort. The antheridium 

 now comes into contact with the oogonium, and a fine fertilization tube is 

 formed which penetrates through the periplasm and discharges a single 

 male nucleus, together with a small amount of cytoplasm, into the oosphere. 

 Conjugation of male and female nuclei takes place and a zygote is formed. 

 Fertilization stimulates the secretion of wall material which occurs in the 

 periplasm. The precise way in which this takes place varies in different 

 species. In general, it appears that within the oogonial wall a very thick 

 wall is laid down, forming the wall of an oospore (Fig. 224), and enclosing 

 the original oosphere. The periplasm may also contribute an exospore 

 wall to the oospore. 



The oospore is a resting spore, remaining unchanged until the decay of 

 the host tissues, and after that it may remain in the soil for some months before 

 germination takes place. Under favourable conditions it germinates by the 

 bursting of the wall and the development of a germ tube. In cases where 

 the Fungus parasitizes an annual plant the oospore is the means whereby 

 the Fungus passes the winter months after the death of the host, and germina- 

 tion takes place the following spring. 



