CYSTOPUS, OR WHITE RUST. 141 



four of them died; the fifth continued to be infected with Cystopiis 

 during the whole time it was under observation — viz., about six 

 months. The forty-five remaining plants had ordinary culture. 

 They passed the winter without growing, and in the spring they 

 developed normally. 



" A fresh batch of seeds of Lepidium was sown in March. 

 Sixty of the young plants had ordinary culture in flower-pots, and 

 their growth was normal. A dozen young plants were put in 

 flower-pots and the soil only w^atered, with the precautions already 

 described, with water charged with oogonial zoospores. The 

 plants developed normally. On March 20th, eleven plants 

 received on their coytledons drops of water charged with oogonial 

 zoospores. On April 2nd, five of them had fine pustules of Cys- 

 topus on their cotyledons, and the other six produced pustules 

 either on their cotyledons or leaves at various intervals. None of 

 these plants succumbed to the attacks of the parasite, but flowered 

 and fructified normally. It is, however, easy to conceive that 

 degeneration of the invaded plants is produced by the vegetation 

 of the fungus." ^ 



The first development of Cystopiis seems, therefore, to be some- 

 what difficult, because it can only take place in the cotyledons ; 

 but this is somewhat compensated for by the enormous quantity 

 of reproductive organs that are formed. Thus, a single plant of 

 Capsella or Lepidium infected with Cystopiis can easily produce a 

 million conidia, each containing from five to eight zoospores, 

 besides a large number of oospores, each containing about a 

 hundred zoospores. We must also remember that the oospores 

 or winter resting-spores germinate on the ground in the spring, 

 and are thus ready to attack the cotyledons as they make their 

 appearance above ground. 



The definition of Cystopiis given by Cooke is, " Receptacle 

 consisting of thick branched threads ; conidia concatenate, at 

 length separating ; oospores deeply seated on the mycelium." De 

 Bary says : — " Conidiophores growing in large bunches, the 

 conidia being developed in single rows, in basipetal order." Cys- 

 topiis possesses much fewer species than Peronospora. Four only 

 have been described as British, and some four or five more have 



* ec 



Ann, cU Sc. Nat. Bot.," Ser. iv., Vol, xx, 



