82 



MORPHOLOGY 



The basidiospore develops a mycelium that penetrates the young 

 seedling of the 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 outhne 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- 



i^ore(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 uninfected wheat 

 plants and sending out 

 germ tubes that penetrate 

 the host and form new 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 

 191, uredospores parasite, germinating in 



(summer spores) (after Coulter); 192, group of |]^g following spring, 

 young teleutospores (winter spores), among which . . 



there may be some uredospores (after Chamberlain) ; Bastdtum. The germi- 



193, mature teleutospores. nation of the teleutospore 



Figs. 191-193. — Wheat rust: 



