264 



Science of Plant Life 



kind of spore is formed. This spore will not germinate on 

 the barberry, but it will infect wheat. Thus the stem rust 



■C E 



Fig. 155. Life history of the stem rust of wheat. In fields where wheat has been 

 grown, the stubble (^1) carries over the winter black spores (B), that germinate in early 

 spring, producing smaller spores (C). These infect the leaves of the common bar- 

 berry (£>). In the leaves of the barberry the fungus grows and produces cup-shaped 

 cavities filled with spores (E) that are carried by the wind to wheatfields and infect 

 the wheat plants. After growing in the wheat a short time, the fungus produces first 

 the red spores (G) that spread the disease to other wheat plants, and later the two-celled 

 black spores that carry the disease over the winter again. 



of wheat spreads from one wheat plant to another by means 

 of red spores, from wheat to the barberry by spores that are 

 produced the next spring, and from the barberry back to the 

 wheat by still another kind of spore. 



In the Northern states, from the Dakotas to New England, 

 the barberry stage is of special importance in the life history 

 of the rust. In the central United States where winter 

 wheat is grown, the red spores produced during the summer 

 drop to the ground and infect the wheat planted in the au- 

 tumn. In this way the rust may be perpetuated from year 



