82 
MORPHOLOGY 
(see p. 82). The basidiospore develops a mycelium that penetrates the 
young host plant. 
(b) Uredinales 
General character. These are the well-known rusts, all of them 
being destructive parasites, whose mycelia live in the intercellular 
spaces of higher plants, especially in the leaves. The best-known form 
is Puccinia graminis, one of the wheat rusts, and an outline of its life 
history will serve as an illustration of the group. 
Wheat rust. The mycelium traverses the tissues of the young 
wheat plant, and during the growth of the host it sends to the sur- 
face numerous sporophores, each bearing a single spore, the uredo- 
s pore (fig. 191). The groups 
of uredospores (summer 
spores) on the surface of 
the host form reddish spots 
or lines, giving rise to the 
name rust or red rust. By 
means of the uredospores 
the disease spreads rapidly 
through the growing wheat, 
the spores falling on the 
surface of uninfectcd wheat 
plants and sending out 
germ tubes that penetrate 
the host and form ncv/ my- 
celia. As the wheat plants 
mature, the mycelium sends 
to the surface of the host 
another kind of spore, the 
teleutospore, which is two- 
celled and thick-walled (figs. 
192, 193). The teleuto- 
spores (winter spores) are 
the winter stage of the 
FIGS. 191-193. Wheat rust: 191, uredospores parasite, germinating in 
(summer spores) (after COULTER); 192, group of tne following spring, 
young teleutospores (winter spores), among which . . , 
there may be some uredospores (after CHAMBERLAIN) ; XKWKWKW 
193, mature teleutospores. nation of the teleutospore 
