32<S TIIK JirsTS ol" .\0\A SCDTIA. — I'UASKK. 



hosts are inorpbolugically unlike, although the fact was not 

 noticed until attention was called to it in this way. 



By infection exjx^rinient^ it has ])ecn shown that Piicclnia 

 curonaia, which was previously regarded as one rust, can be 

 broken up into two species, one producing accia on Rhamnus 

 fraiigida and the other on IihrnrDius cathaiiicd. Again the 

 latter, Puccinin Lolii, may 1)0 broken u]) into specialized forms 

 based on their infecting power. This rust occurs both on oats 

 (Avena satira) and on grass, Festuca elatior; but spores from 

 Arena will not infect Festuca, nor spores from Festvca infect 

 Avena. Tii the same way the spores of P. fjrawinis from oats 

 will not infect rye nor the rcA'crse, although the spores produced 

 from the germinating teliospores of l)otli will infect the barberry. 

 The aecia produced on the barberrv from the specialized form 

 on rye will not infect oats noi- the aecin from the foi'iii oil oats 

 infect rye. 'Jdie forms remain true although no moridiological 

 differences exist. In some ca^es, however, the forms do not 

 seem fixed and the aecial host acts as a bridge, the aeciospores 

 from either form infecting Ijoth host plants. Different names 

 are given to these forms: biological species, physiological 

 species, sister species and sj^ecialized forms, are some of the 

 terms usefl. If the aecial stage occurs on different ]dants, even 

 if no morphological differences exist, the rusts are usually 

 regarded as separate s])ecies; but if the aecia are produced on 

 the same plant, and differences of infecting power exist, they 

 are regarded as S]>ecialized forms or fonu S])ecies. 



Sexual If I/. 



Sajjin-Troiiffv showed ihat in the prfnuycelinm. sporidia 

 an<l aeeial myeelinm to ilic base of the af'ciuni. eatdi cell con- 

 tains bnt one nuclens, Avhile in the aeciospores and the 

 suc<ieoding spores and nivcelium each coll contains two. These 

 fuse in the teliospore and he regards this fusion as a tru • 

 sexual jirocess. 



