THE GRAPE DOWNY MILDEW (PLASMOPARA VITICOLA) 355 
fertilization forms an odspore which can germinate. This 
peculiar feature, called parthenogenesis, meaning reproduction 
by an egg without fertilization, has been mentioned before 
(page 50). 
The aquatic habit, reproduction by zoéspores, and character 
of sex organs support the theory that the Water Molds are 
degenerate forms with the Green Algae as their ancestors. The 
Fic. 305. — Leaf of the Grape, showing the downy areas caused by the 
Downy Mildew, Plasmopara Viticola. After W. H. Hein. 
coenocytic character of their hyphae suggests a close relationship 
with the Siphonales. 
Downy Mildews (Peronosporales). — These Fungi, which are 
parasites on the higher plants, cause some serious plant diseases 
of which the Grape Downy Mildew and Potato Blight are notable 
ones, and will serve to illustrate the habits of the group. There 
are about 100 species known and the order is so named because 
of the downy patches which they produce on the diseased portions 
of the host. 
The Grape Downy Mildew (Plasmopara Viticola). — The 
Downy Mildew of the Grape, shown in Figure 305, is a very 
