THE FUNGI 253 



apothecium; in other cases similar bodies are formed 

 on other parts of the thallus. 



Physcia then has served to illustrate, on the one 

 hand, a highly organised Ascomycete, with a com- 

 plex fruit containing a large number of asci; while, 

 on the other hand, it has made us acquainted with 

 the remarkable phenomenon of symbiosis, or the asso- 

 ciated life of two distinct organisms, each performing 

 certain physiological functions for the benefit of the 

 other partner. 



TYPE XXII. PUCCINIA GRAMINIS. 



The group of Fungi represented by this type is a 

 comparatively small one, and shows a narrow range of 

 diversity as compared with a great Order like the 

 Ascomycetes. The plants, however, are of much 

 interest, for they afford one of the very best examples 

 of typical parasitic Fungi, which have adapted them- 

 selves exclusively to life at the expense of other plants. 

 Some of them, and especially that species which we 

 have chosen as our type, are extremely injurious to 

 important crops, and so possess a very considerable 

 practical interest. The life - history of these parasites 

 is singularly complicated, at least in their more perfect 

 representatives. 



Puccinia graminis is the cause of the rust or mildew 

 of Wheat and other cereals, the two forms of the 

 disease being, as we shall see, stages of one and the 

 same malady. We will begin with the stage known 

 as Rust. 



