TYPES OF CRYPTOGAMS; THALLOPHYTES 225 
of grasses, and many species of such rusts are recognized. A rust 
may have one, two, or three kinds of spores, and when three occur 
one is known as the cluster-cup stage and the others as red rust and 
black rust, according to the usual approximate color of the spores. 
A B 
Fic. 160. Spore-Formation in Potato-Blight (Phytophthora infestans). 
A, a well-developed group of stalks, proceeding from a mass of mycelium 
inside the leaf and escaping through a stoma; £6, a young, unbranched 
stalk. h, hyphe of mycelium; 0, stoma; s, spore. (Both figures 
greatly magnified, B more than 4.) 
The rust called Puccinia graminis, growing on wheat, has its cluster- 
cup stage on the leaves of barberry in June. The spores from the 
cluster-cups are carried by the wind to the wheat, where they germi- 
nate and in a few days produce the redrust. A little later the black 
