4i8 



General Botany 



produces disease on two different kinds of host plants. The stem 

 rust of wheat, for example, produces patches of red spores (ure- 



FiG. 259. Life history of the stem rust of wheat. In fields where wheat has been 

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

 spring, producing smaller spores (C) . These infect the leaves of the common barberry (D) . 

 In the leaves of the barberry the fungus grows and produces cup-shaped cavities 

 filled with spores (£) that are carried by the wind to wheat fields 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. 



diniospores) which will infect other wheat plants. It pro- 

 duces also black spores (teliospores) which live over winter on the 

 stubble and straw, and which germinate the following spring and 

 produce spring spores (basidiospores) that infect the barberry. 

 On the barberry leaves the fungus produces a cup-like depression 

 within which a fourth kind of spores (aeciospores) are formed. 

 These spores will not germinate on the barberry, but they will 

 infect wheat. Thus the stem rust of wheat spreads from one 

 wheat plant to another by means of red spores. The following 

 season it may spread from wheat stubble to the barberry by 

 basidiospores produced by the teliospores, and from the barberry 

 back to the wheat by still another kind of spore (Fig. 259). 

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



