THE FUNGI 251 



in the presence of appropriate Algse, a fertile thallus. 

 At present the weight of evidence appears to be against 

 the occurrence of a sexual process in the development 

 of the ascus-fruit of Lichens, though in a few members 

 of the group such a possibility is not excluded. 



Physcia then has served to illustrate, on the one 

 hand, a highly organised Ascomycete, with a complex 

 fruit containing a large number of asci ; while, on the 

 other hand, it has made us acquainted with the remark- 

 able phenomenon of symbiosis, or the associated life of 

 two distinct organisms, each performing certain physio- 

 logical 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. 



