GENERAL PRINCIPLES 



oats, etc., so much, have a complicated life history. One of 

 the most complicated is that of the common wheat rust. In 

 the spring the winter spores (teleutospores) 

 germinate, produce a short mycelium on which 

 four small spores of another kind (sporidia) are 

 borne. These germinate on the surface of the 

 barberry leaf, enter the mesophyll by the 

 stomata and develop a mycelium. What is 

 called a cluster cup is then formed just under 

 the lower epidermis. This is filled with spores 

 (aecidiospores), and when the epidermis finally 

 breaks, the spores are set free. These then 

 germinate on the wheat leaf and in its tissues 

 a fourth kind of spore appears in such masses 

 as finally to burst the epidermis and produce 

 long, narrow orange-colored pustules filled with 

 summer spores (uredospores). These may 

 germinate in the same way and produce new 

 generations of uredospofes. Later in the season 

 still another kind of spore appears among the 

 summer spores, or in clusters by itself. These 

 have thick, dark-colored walls, and make black 

 patches on the leaf. These are the winter 

 spores, teleutospores, which germinate in the 

 next spring and start a new cycle. 



750. The common cedar apple is the teleuto- 

 spore-bearirig stage of a rust which has its 

 cluster cup on the leaves of the haw. 



751. These parasites are fungi. They are 

 typical plant parasites and often greatly damage 

 the host, as, e. g., the wheat rust. Less im- 

 portant, economically, are the phenogamous 



parasites. The Indian Pipes are common flowering plants 

 growing on the roots of other plants. The parasite has no 



FIG. 229. A 

 fungus, Cordy- 

 ceps ravenelii, 

 parasitic in the 

 grub of a beetle, 

 L a c h n o s terna. 

 Two long stro- 

 mata of the 

 fungus are seen 

 growing from the 

 body of the grub. 

 (From Folsom 

 after Riley.) 



