72 TEXT-BOOK OF FUNGI 



oogonium become reduced, due to disintegration, and at 

 the period of ' balling,' or the breaking up of the proto- 

 plasm present in the oogonium into a number of spheres 

 oospheres, each oosphere contains only one nucleus. The 

 single nucleus of the oosphere is associated with a well- 

 developed centrosome and astrosphere ; there is also 

 present a coenocentrum, or ovocentrum, as it is termed 

 by Trow. After the formation of oospheres, fertilisation 

 tubes from the antheridia grow into the cavity of the 

 oogonium, and when they reach the oospheres, these 

 acquire a cell-wall and a second nucleus. The antheridial 

 tubes often produce a lateral branch in the oogonium, 

 which comes in contact with and fertilises a second 

 oosphere. The male nucleus after its entry into the 

 oosphere acquires a distinct centrosome and astrosphere. 

 When the male nucleus reaches the central region of the 

 oospore, the dense protoplasm accompanying it goes to 

 swell the centrally situated ovocentrum, which now en- 

 closes two nuclei, two centrosomes, and two astrospheres. 

 The fusion of the male and female nuclei does not take 

 place until centrosomes, astrospheres, and coenocentrum 

 have disappeared. 



Fertilisation takes place in Peronospora parasitica as 

 follows, according to Wager. Both oogonium and 

 antheridium are multinucleate. Previous to fertilisation 

 the protoplasm in the oogonium becomes differentiated 

 into a distinctly marked peripheral, granular, homogeneous 

 portion termed periplasm, and a vacuolate central portion, 

 the ooplasm. This period has been termed the stage of 

 zonation. All the nuclei pass into the periplasm and 

 undergo mitosis. The nuclei in the antheridium also 

 undergo division, and this organ becomes closely attached 



