234 PHYLUM VII. CARPOMYCETEAE 



the winter without injury, and when spring comes (IV) 

 they germinate on the rotting straw forming a 4-celled 

 "promycelium" and producing several (usually four) 

 minute spores, called sporids. This is the fourth and 

 last stage of the rust. Such sporids as fall upon 

 Barberry-leaves germinate, and enter directly through 

 the epidermis, giving rise to cluster cups again. 



399. These stages (I, II, III) are so different in appear- 

 ance that for a long time they were regarded as distinct 

 plants, and received different names. Thus the first 

 stage was classified as a species of Aecidium, the second 

 as a species of Uredo, and the third as a Puccinia. We 

 still preserve these names by sometimes calling the spores 

 of the first aecidiospores (or aeciospores) and of the second 

 uredospores (or urediniospores), while the third name is 

 retained as the scientific name of the genus. 



400. For a long time many botanists did not believe 

 the statement that this Wheat rust lives for a part of its 

 life upon one host (barberry), and later upon another 

 (wheat), but now this fact (known as "heteroecism") is 

 well established not only for Wheat rust, but also for 

 many other species. 



401. It is now thought that the sporids cannot pro- 

 duce rust directly upon wheat, but that infection from 

 them must come through the cluster cups on the bar- 

 berry leaves. This suggests that there must be other 

 means of infection in the Mississippi Valley and the 

 Great Plains where the barberry is rare and even when 

 present often scarcely, if at all, infested with cluster 

 cups. It has been shown that on the Great Plains the 

 red rust lives through the winter on the little wheat 

 plants, and that its spores (uredospores) blow to the 

 north in the spring from field to field, and back to the 

 south in the autumn. Probably this is the more com- 



