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 
