254 INTRODUCTION TO BOTANY 
of these plants the different stages were thought to be differ- 
ent plants, and were so named. For example, the rust that 
develops on leaves of the apple tree early in the spring forms 
spores. These spores produce the parasite known as cedar 
apple (Gymnosporangium), which grows upon cedar trees. 
Each stage produces spores which, after germinating in favor- 
able places, produce the other stage of the rust. 
Wheat and oat rust is probably the best-known and most- 
feared member of the rust sub-division of fungi. In the United 
States it does damage to our wheat and oat crops every year to 
the amount of at least $15,000,000, and probably much more. 
The first conspicuous appearance of rust in the late spring 
or early summer is in the form of reddish-brown patches upon 
stalks and leaves of wheat and oats (fig. 198). The patches are 
composed of large numbers of summer spores (wredospores). A 
section cut through the host leaf enables one to see that the 
summer spores (fig. 198, 2) are formed upon the ends of hyphe. 
The spore-bearing ends of hyphze are continuations of hyphee 
which have pushed their way among the leaf cells from which 
they have absorbed their nourishment. At the time summer 
spores are formed, the host plant is usually thoroughly infested 
with the mycelium. The summer spores are readily carried about 
by currents of air or by contact with animals. If placed upon 
wheat or oat plants, these spores germinate (fig. 198, C) and 
the young hyphee penetrate the host and produce new mycelium. 
Later in the summer the same mycelium which produced 
summer spores produces a heavy-walled, two-celled spore 
(fig. 198, D) known as the winter spore (teleutospore). When 
formed in large quantities, these spores appear as blistery 
patches much like those made by the reddish summer spores, 
except for the difference in color. The winter spores are scat- 
tered over the ground and upon wheat and oat straw. After 
1“ The Cedar-Apple Fungi and Apple Rust in Iowa,’ Bulletin 84, Iowa 
Agr. Exp. Sta., 1905, 
“The Asparagus Rust : its Treatment and Natural Enemies,” Bulletin 129, 
N. J. Agr. Exp. Sta., 1898. 
