144 A TEXT-BOOK OP BOTANY 



of fertilization, a heavy- walled oospore is formed within the 

 oogonium (Fig. 133, C). The infected leaves containing 

 the oospores 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 Alga3, and suggest clearly that they are Algae 

 that have become parasitic or saprophytic. In the whole 

 group the filaments of the mycelium are coenocytic, as are 

 the bodies of the group of green Algse to which Vaucheria 

 belongs ( 68). They reproduce by spores, which are usu- 

 ally scattered by the wind, and also produce oospores. 

 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 Algae ( 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 



