990 The American Naturalist. [ December, 
and regularly crowded as the aecidiospores, so that when they 
burst through the epidermis of the host they constitute elon- 
gated or irregular shaped spore-dots (sori) instead of definitely 
outlined cups. Here again, the spores of this kind were re- 
garded by the earlier botanists as belonging to a distinct 
genus, Uredo : hence we commonly still speak of them as ure- 
dospores. They are also known as “ stylospores,” in allusion to 
the fact that they are stalked. (Figs. III and IV of Plate 
XXXII.) 
Still later, a third kind of spore is produced, often in the 
uredosori, which bear some resemblance to the uredospores in 
being stalked, and in some cases, one-celled (Uromyces, Melamp- 
sora), but differing often in being two or more celled, and usu- 
ally having a thicker wall. These are the last to develop upon 
the mycelium within the host, and when they have ripened, 
usually the parasite dies. Since these spores appear to complete 
the development of the parasite, they have long been known 
as teleutospores (res, ^completion.") They germinate (in 
many species after a period of rest through the winter months) 
by the production of a short, several-jointed filament (the 
promycelium), from each cell of which short lateral branches 
develop, upon whose summits single minute spores (sporidia) 
are formed by abstriction. When these sporidia germinate 
upon the proper host they form parasitic threads which pene- 
trate its tissues and give rise to the aecidia described above, 
thus completing the cycle of life. (Figs. V to XIII of Plate 
XXXII.) 
The life history here sketched may be taken as typical, but 
it is subject to several modifications, e. g., (a) the omission of 
the aecidial stage; (b) the omission of the uredo stage; (c) the 
omission of both the aecidial and the uredo stages. Moreover, 
in many species the aecidial stage occurs upon a different 
host from that which supports the uredo and teleutospore 
stages, this condition being known as heteroecism, a familiar 
example of which may be seen in one of the common rusts 
of wheat (Puccinia graminis), where the aecidiospores develop 
on the leaves of the Barberry (Berberis vulgaris), the uredospores 
and teleutospores alone occurring in the leaves and stems of 
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