272 
PLANT STUDIES 
finally deadened spots on the leaves, where the tissues have 
been killed. 
From this internal mycelium numerous sporophores 
arise, coming to the surface of the host and securing the 
scattering of their 
spores, which fall 
upon other leaves 
and germinate, the 
new mycelia pene- 
trating among the 
tissues and begin- 
ning their ravages. 
FIG. 239. A branch of Peronospora in contact with 
two cells of a host plant, and sending into them 
its large haustoria. After DEBARY. 
The sporophores, af- 
ter rising above the 
surface of the leaf, 
branch freely ; and many of them rising near together, 
they form little velvety patches on the surface, suggesting 
the name " downy mildew." 
FIG. 240. Peronospora, one of the Phycomycetes, showing at a an oogonium (o) con- 
taining an egg, and an antheridium (n) in contact; at b the antheridial tube pene- 
trating the oogonium and discharging the contents of the antheridium into the 
egg; at c the oogonium containing the oospore or resting spore. After DEBARY. 
In certain conditions special branches arise from the 
mycelium, which organize antheridia and oogonia, and 
remain within the host (Fig. 240). The oogonium is of 
the usual spherical form, organizing a single egg. The an- 
