218 
THE PLANT WORLD. 
The True Rusts. 
The true rusts are also parasitic fungi. The mycelium grows 
in the tissues of many cultivated and wild plants. Here and 
there it forms cushions or cups containing spores which are 
somewhat yellowish, or orange, or reddish, or brown or black, in 
color, according to the stage of the fungus or the kind. 
The wheat rust ( Puccinia graminis) is one of the common 
ones, and many persons are, of course, familiar with its appear¬ 
ance. It has a very interesting life history since it appears in a 
number of different forms which at one time were supposed to 
be different kinds of fungi. These forms are as follows: 
The red rust form (uredospore stage). This occurs through¬ 
out the summer and often does great damage. Examine with a 
lens affected stems and leaves. Observe the ruptured epidermis 
where the cushions of spores are formed and note the spores 
which appear as a yellowish powder or granular substance. 
These are summer spores and serve to spread the disease since 
wind carries them to other plants. Here they germinate and the 
mycelium enters at a stoma and forms another crop of red rust 
spores and so on through the season. 
The black rust form (teleutospore stage). This form often oc¬ 
curs later in the season, or as the grain is ripening. Examine 
with a lens and compare with the red rust. The spores of this 
form are winter spores and live through the winter. In the 
spring they germinate and produce on a short mycelium a third 
form of spore, the sporidium. This is minute and light and easily 
carried by the wind. One of the singular things about the wheat 
rust and others of its kind is that these sporidia cannot infect the 
wheat plant and produce the disease (in some rust diseases whose 
life history is different the sporidia can infect the same kind of 
host) but they can produce a disease in another kind of host. 
The cluster cup form on the barberry. The sporidia from the 
black rust form germinate on the barberry and the mycelium 
enters a stoma where it spreads in the leaf tissue forming a di¬ 
seased, yellowish spot. When this form is mature examine with 
a lens the spots on the upper surface of the leaf. Note the 
