144 A TEXT-BOOK OF BOTANY 
of fertilization, a heavy-walled odspore is formed within the 
odgonium (Fig. 133, (). The infected leaves containing 
the odspores fall and gradually decay, thus liberating the 
oospores, which are free to germinate during the next spring 
and infect new leaves. 
The downy mildews include some very destructive par- 
asites, attacking potatoes (potato-rot), grape-vines, lima 
beans, lettuce, onions, cucumbers, melons, radishes, etc. 
Various means have been discovered for holding these dis- 
eases in check. 
81. Alga-like Fungi.— Mucor and Peronospora are repre- 
sentatives of a large group (Phycomycetes) of Fungi that 
most resemble Alge, and suggest clearly that they are Alge 
that have become parasitic or saprophytic. In the whole 
group the filaments of the mycelium are ccenocytic, as are 
the bodies of the group of green Alge to which Vaucheria 
belongs (§ 68). They reproduce by spores, which are usu- 
ally scattered by the wind, and also produce odspores. 
Some of them, represented by Mucor, have similar gametes, 
that are brought together in a way that suggests the Spiro- 
gyra group among the green Alge (§ 69); while the others, 
represented by Peronospora, produce eggs and sperms, as 
in the case of Vaucheria, though, since there is no water 
connection, the sperm reaches the egg through a tube. 
Mucor also illustrates the saprophytes, and Peronospora 
the internal and destructive parasites. 
82. Mildews.—The true mildews are very common 
parasites on leaves and other parts of seed-plants, the 
mycelium spreading over the surface like a cobweb. They 
are often called powdery mildews in contrast with the 
downy mildews (§ 80), since in most cases they look like 
patches of whitish powder on the leaves. A very com- 
mon form occurs on lilac leaves (Fig. 134), which nearly 
always show the whitish patches from early summer until 
fall. Other common mildews attack such valuable plants 
