DEVELOPMENT OF PLANTS 237 



thick dark walls and from one to several spores are developed at 

 the end of the hyphae (Fig. 160, D). Consequently, when the 

 epidermis is ruptured, these spores form rusty black blotches on 

 the leaves. This third stage is known as the telial, since it ends 

 the season's growth. These spores, called teliospores, are resting 

 spores and tide the fungus over the winter. They germinate 

 in the spring quite independent of any plant and being dependent 

 upon the food stored in the spore, they only form short hyphae 

 which usually become divided into four cells (Fig. 161, B). 

 This structure is known as the basidium. Each cell of the basid- 

 ium sends out a delicate tube into the end of which the cell 

 contents passes, thus forming a small spore, known as the basidio- 

 spore. This basidial stage completes the life history of the fun- 

 gus 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 alternation of generation. According 

 to this view the aecium is the result of a sexual process. There- 

 fore the aeciospore would be the beginning of the sporophyte 

 or asexual generation, which terminates in the teliospore. 



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- 



