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. The sporids cannot ordinarily produce rust 

 directly upon wheat, probably because of the toughness 

 of the epidermis; but it has been claimed (by Plowright) 

 that when sporids germinate upon very young leaves of 

 wheat-seedlings they penetrate the epidermis and then 

 soon give rise to a red-rust stage. In such cases the 

 cluster-cup stage is omitted. Possibly the rusts upon 

 the spring wheat, oats, and barley in the Mississippi 

 Valley and on the Great Plains where barberry is rare 

 are sometimes propagated in this way. It has been 

 shown also that on the Great Plains the red rust lives 

 through the winter on the little wheat plants, and that 



