THALLOPHYTES— FUNGI 



277 



the formation of an oospore which is liberated by the decay of 

 the host tissues. The oospore may produce zoospores or de- 

 velop a mycelium. 



Plasmopara viticola is the cause of the downy mildew of the 

 grape and may be considered as a type of the order Peronospo- 

 rales. The mycelium gi-ows within the leaf and fruit, draws its 

 food supply from them and iinally forms a doAvny white growth 

 on the under surface of the leaf. This growth is made up of 

 conidiophores or fruiting filaments. They bear conidia, which 

 produce free-swimming biciliate zoospores. These zoospores 



.^^ 



Fig. 126. — Albugo Candida showing the formation of the conidia spores just below the epider- 

 mis of the host plant and also the antheridium and archegonium. 



swim for a few minutes m the moisture on the surface of the 

 leaf, come to rest, lose their cilia and produce new hyphse which 

 penetrate the leaf through the stomata. The oogonia and anthe- 

 ridia are very similar to those of the Albugo. The late blight of 

 the potato is in reality a mildew and is caused by a fungus 

 (Phytophthora infestans) . It is very similar to the grape mil- 

 dew and is one of the most important plant diseases. 



The Zygomycetes are isogamous, aerial, and reproduce by 

 non-motile asexual spores and usually by conjugation. They are 

 well illustrated by the common bread mold, Khizopus nigrimns, 

 although they inchide several other genera. 



RMzopus nigricans is the bread mold fungus (Fig. 127). 



