FUNGI: THE RUST FUNGI 285 



cells into the barberry leaf, and starts the cluster-cup stage again, 

 thus completing the life cycle. 



Fig. 260. Fig. 261. Fig. 262. 



Teleutospore germinating, Promycelium of germinat- Germinating sporidia enter- 

 forming promyceiium. ing teleutospore, forming ing leaf of barberry by my- 



sporidia. celium. 



Figs. 260-262. Puccinia graminis (wheat rust). (After Marshall- Ward.) 



451. Races of the wheat rust. There are some peculiarities 

 in the life of the wheat rust, which as stated above occurs on oats, 

 barley, rye, and many grasses. For example, the uredospores 

 formed on the oats can infect oats but cannot infect wheat, rye, 

 barley, and some grasses; and the same is true with the uredospores 

 from the barley, rye, and some of the grasses, they cannot infect 

 the oats and members of the other groups. The uredo- and 

 teleuto-spores on these different plants cannot be distinguished as 

 to form, size, etc. They constitute physiological races which 

 have become confined to certain groups of hosts. In the case of 

 all of these races, however, the sporidia from the teleutospores 

 can produce the cluster cups on the barberry. But cluster cups 

 originating from oat teleutospores cannot infect wheat, barley, 

 rye, etc., but only oats and the members of the oat group. 



452. How the wheat rust passes the winter in regions 

 where the barberry is absent. It has long been known that the 



