74 TEXT-BOOK OF FUNGI 



to the wall of the oogonium. At the point of contact the 

 wall separating the oogonium from the antheridium be- 

 comes very thin, and a slight hyaline protoplasmic papilla 

 is formed, which slightly projects into the antheridium. 

 This structure, called the receptive papilla, seems to be in 

 some way connected with the perforation of the oogonial 

 wall and the formation of the fertilising tube. After the 

 differentiation of the ooplasm, a denser mass termed a 

 coencentrum appears in its centre, and one of the nuclei 

 formed by division in the oogonium comes into close 

 contact and is eventually embedded in it. The fertilising 

 tube of the antheridium enters the oogonium at the point 

 where the receptive papilla is formed, and increases in 

 length until it reaches the coenocentrum with its embedded 

 female nucleus. At this stage the apex of the antheridial 

 tube opens, and the male nucleus escapes and comes into 

 contact with the coenocentrum. The male and female 

 nuclei do not fuse at once, but remain apart until the 

 oospore is nearly ripe, when fusion takes place, con- 

 sequently the mature oospore is uninucleate. Peronospora 

 parasitica is the only known member of the Perono- 

 sporeae having retarded nuclear fusion in the oospore. 

 This feature is, however, not uncommon, and may occur 

 in Spirogyra, Cosmarium, and Basidiobolus'. In some 

 instances fusion is retarded until germination commences, 

 as in Pofyphagus. 



In Cystopus candidus, another member of the Perono- 

 sporeae, fertilisation, as described by Wager, conforms in 

 essential features with that recorded for Peronospora. The 

 oogonia and antheridia are both multinucleate, and the 

 nuclei undergo karyokinetic division. After the zonation 

 period in the oogonium and the formation of a coeno- 



