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 {G-ymnosporangiiuri), which grows upon cedar trees. 

 Each stage produces spores which, after germinating in favor- 

 able places, produce the other stage of the rust. 1 



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 (iiredospores). A 

 section cut through the host leaf enables one to see that the 

 summer spores (fig. 198, .Z>) are formed upon the ends of hyplise. 

 The spore-bearing ends of hyphse are continuations of hyphae 

 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, (7) and 

 the young hypha3 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. 



