255 FORMS OF THE RUSTS 



forming a small spore, known as the basidiospore. This basldial 

 stage completes the life history of the fungus for the basidiospores 

 are carried to the leaves of the barberry and begin again the life 

 cycle of the parasite by forming the cluster cups. It is held by 

 many that this sequence in spore formation represents an alter- 

 nation of generation. According to this view the aecium is the 

 result of a sexual process. Therefore the aeciospore would be 

 the beginning of the sporophyte or asexual generation, which 

 terminates in the teliospore. The germination of the teliospore 

 marks the beginning of the sexual generation and it is interesting 

 to note that we have four cells produced as in the tetraspores of 

 the red algae. 



It is not surprising that this story was not unravelled for a 

 long time and that these different stages of the parasite were 

 known as distinct species. The first clue to the relationship was 

 gained in England where it was observed that the wheat fields 

 to the leeward of the barberry bushes were especially infested with 

 rust. For this reason, a law was passed early in the history of 

 Massachusetts compelling the destruction of the barberry bushes. 

 This suggestion of relationship between the cluster-cup stage of 

 the barberry and the rust of the wheat finally led to the inocu- 

 lation of wheat plants with aeciospores and this resulted after 

 a week or more in the appearance of the characteristic rusty 

 streaks on the leaves of the wheat. 



(b) Features in the Life History of other Rusts. — Considerable 

 variation characterizes these rusts, not all of them having so 

 elaborate a life history as that outlined above. One of the most 

 common species of Puccinia affecting the wheat is perennial in 

 the wheat and possibly in other grasses where it produces ure- 

 diniospores in the spring, thus, the aecial, telial and basidial 

 stages are eliminated. In the apple rust, the uredinial stage is 

 missing. This disease produces on the leaves of various members 

 of the apple family, yellow patches in which are formed tube-like 

 cluster cups (Fig. 165, A). The aeciospores are only capable of 

 infesting the juniper, in the branches of which they produce 

 gall-like swellings (Fig. 165, B) known as cedar apples and also 

 sometimes bushy outgrowths known as witches' brooms. In the 



