386 
SOME GENERAL PLANT PROBLEMS 
some near at hand, and some long distances away. When 
moisture makes it possible, each spore sprouts and forms 
a new center of infection. These 
spores, called summer spores, are 
red or orange colored and egg- 
shaped. In the autumn another 
kind of spore is produced, namely, 
the black or winter spore. These 
appear in black pustules on the 
stems and leaves of the plants 
which serve them as host. These 
spores are longer, and have thick 
walls, an adaptation which en¬ 
ables them to live over the winter 
in stubble and straw. They are 
made up of two cells, and usually 
are not carried by the wind. 
When they germinate in the 
spring, each of the two* parts 
produces four round, colorless 
bodies called sporidia. These are 
blown about by the wind, but 
they are not able to propagate 
the disease unless they fall upon 
the barberry, where they produce 
a plant so different in appearance 
that it was for many years con¬ 
sidered a separate plant and given 
a different name. On the leaf or 
fruit of the common barberry, the 
sporidia produce yellow circular 
spots containing reddish spores in 
long chains. From the shape of the spot, this is often called 
the cluster cup stage. This stage is most active from May 
until midsummer. The spores from the plant on the barberry 
Figure 354. — Heads of Wheat 
Unaffected by Black Rust. 
