THE FUNGI : ASCOMYCETES AND BASIDIOMYCETES 303 



Fig. 296. — Puccinia graniinis. Mature uredosorus with terminal 

 uredospores. {After Allen.) 



Host Relationships in the Rusts 



Not all species of Rusts require two host plants to complete their life 

 histories. In some all the stages in the life-cycle are passed through in a 

 single host plant. Thus we can contrast Puccinia graniinis, which is heter- 

 oecious, with those in w^hich there is only one host, a condition said to be 

 autoecious. In cases where there are two host plants it is usually found that 

 these plants belong to families of very different systematic position, the one 

 bearing the pycnidia and aecidia being usually of a less advanced type than 

 that on which the uredosori and teleutosori are formed. Thus the Barberry 

 belongs to the Berberidaceae, a family closely allied to the Ranunculaceae, 

 which is considered a primitive family of Dicotyledons, while the grasses 

 belong to the Gramineae,a specialized family of the Monocotyledons. There 

 are many similar examples among the Rusts. The common Rust of the 

 Nettle produces its aecidia and pycnidia on Urtica dioica, a primitive Dicoty- 

 ledon, but its uredospores and teleutospores are formed on Carex pendula, a 



