84 MORPHOLOGY 



teleutospores, the saprophytic promycelium or basidium bearing ba~ 

 sidiospores, and the parasitic mycelium bearing aecidiospores), lives 

 upon two unrelated hosts, and produces four (perhaps five) kinds of 

 spores. It is natural that such a polymorphous plant should not have 

 been understood at first, and that the different phases should have 

 received different names. The mycelium bearing uredospores was 

 Uredo ; that bearing teleutospores, not known at first to be the 



FIG. 196. Wheat rust: an aecidium (clustercup) arising from the mycelium of the 

 barberry leaf, and showing the rows of aecidiospores. After CHAMBERLAIN. 



same mycelium, was named Puccinia; and the form parasitic on the bar- 

 berry was named Aecidium. Now the name Puccinia is retained for 

 the plant, and the other names are used for convenience in designat- 

 ing the respective stages. Not all rusts include two hosts in their 

 life history, and it is usual to distinguish rusts as autoicous (those having 

 one host) and heteroicous (those having more than one host). 



Alternation. Recently the nuclear changes in the life history of wheat rust 

 have been traced. In the aecidium, the cell which produces a row of aecidiospores 

 contains two nuclei, which have been brought together in a single cell somewhere in 

 the mycelium. In the subsequent cell divisions the two nuclei divide independently, 

 so that each aecidiospore contains two nuclei. This binucleate condition con- 



