FUNGI, DOWNY MILDEWS 285 



take part in the formation of the egg. The sperm case is a 

 slender branch which rests against the wall of the egg case and 

 develops a slender fertilization tube, which penetrates to the egg. 

 This tube carries in the sperm nucleus, which fuses with the egg 

 nucleus to bring about fertilization (fig. 228). 



431. The white rust. An example of white rust is the one on 

 cruciferous plants like the mustard, turnip, cabbage, and shep- 

 herd's purse. This white rust (Cystopus candidus= Albugo 

 Candida) is very common on the shepherd's purse, deforming the 

 stems, leaves, flowers, and fruit. The mycelium is intercellular, 

 and branched haustoria penetrate the cells. 



Fig. 229. 

 Ripe oospore of Peronospora alsinearum. 



432. Asexual stage. The sporophores are short, are de- 

 veloped in great numbers, and crowded underneath the epi- 

 dermis. These sporophores bear chains of spores (conidia; fig. 

 230), and the mass bursts through the epidermis, giving a white 

 rusty appearance. The spores germinate by the formation of 

 zoospores as in the grape downy mildew. 



433. Sexual reproduction. This processs is very much as 

 described for the downy mildews, but in some species many sperm 

 nuclei from the sperm case enter the egg and pair off to fuse with 

 the many nuclei in the egg. 



