FUNGI 149 
84. Rusts.—Rusts are destructive parasites that attack 
almost all seed-plants, but those that attack the cereals 
are of special importance. Wheat, oats, rye, and barley 
all have their rusts; and in the United States there is 
a yearly loss of several million dollars on account of the 
ravages of the wheat-rust alone, scarcely a field being en- 
tirely free from the pest. Naturally these parasites have 
been investigated persistently; but while very much has 
been learned about their life-histories and behavior, no 
remedy has been discovered. It has been found that certain 
varieties of wheat resist the rust better than others, and 
that varieties ripening early escape serious injury; and 
these facts may lead to the breeding of resistant and early 
races. 
The life-history of a rust is usually very complex, since 
there are several phases in the history, and all the phases 
may not occur on the same host plant. Since wheat-rusts 
are better known than any other, : 
one of them may be used to illus- 
trate the life-history. 
While the leaves and the stems 
of wheat are growing, the mycelium 
of the parasite is burrowing among 
the tissues of infected plants. About 
the time of harvest, numerous sporo- 
phores arise from the mycelium and 
reach the surface of leaves and ‘ 
stems, each sporophore producing *"%140--The summer spores 
at its tip a reddish spore (Fig. 140). 
These are the summer spores, and they occur in such great 
numbers that they form rusty-looking lines and spots, 
giving name to the disease. The summer spores are scat- 
tered freely by the wind; and those falling upon other 
plants germinate immediately, the new mycelium pene. 
trating the host plant and beginning its ravages. By 
