258 GENERAL MORPHOLOGY OF PLANTS 



the fungus. In the onion mildew (Peronospora schleideniana) the 

 spore germinates by a germ tube which forms the mycelium. 

 In the grape downy mildew (Plasmopara mticola) the protoplasm 

 of the spore (conidium) first divides into several smaller bodies 

 which form bean-shaped zoospores with two lateral cilia. These 

 escape from the conidium (really a spore case in the downy mil- 

 dews), swim about for a time, then come to rest, germinate and 

 produce mycelium again if they are favorably situated. The 

 spores of the potato blight germinate in both ways. Successive 

 crops of the asexual stage are rapidly formed, and the disease 

 spreads. 



417. Sexual reproduction. This takes place by the forma- 

 tion of the sexual organs, sperm and egg cases (antheridia and 

 oogonia), and the fertilization of the egg. The egg case is a 



Fig. 217. 



Fertilization in Peronospora alsinearum; tube from anther idium carrying in the sperm 

 nucleus in figure at the left, female nucleus near; fusion of the two nuclei shown in the two 

 other figures. (After Berlese.) 



short branch which swells out into a large rounded body. A 

 single egg is formed from the centrally located protoplasm, leav- 

 ing a layer of protoplasm (periplasm) around the outside which 

 does not 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 carries in the sperm nucleus, which fuses with the egg nucleus 

 to bring about fertilization (fig. 217). 



418. 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 



