328 THE RUSTS OF NOVA SCOTIA. -FKASER. 



hosts are morphologically unlike, although the fact was not 

 noticed until attention was called to it in this way. 



By infection experiments it has been shown that Puccinia 

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

 broken up into. two species, one producing aecia on Rhamnus 

 frangula and the other on Rhamnus cathartica. Again the 

 latter, Puccinia Lolii, may be broken up into specialized forms 

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

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

 Avena will not infect Festuca, nor spores from Festuca infect 

 Avena. In the same way the spores of P. graminis from oats 

 will not infect rye nor the reverse, although the spores produced 

 from the germinating teliospores of both will infect the barberry. 

 The aecia produced on the barberry from the specialized form 

 on rye will not infect oats nor the aecia from the form on oats 

 infect rye. The forms remain true although no morphological 

 differences exist. In some cases, however, the forms do not 

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

 from either form infecting both host plants. Different names 

 are given to these forms: biological species, physiological 

 species, sister species and specialized forms, are some of the 

 terms used. If the aecial stage occurs on different plants, even 

 if no morphological differences exist, the rusts are usually 

 regarded as separate species; but if the aecia are produced on 

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

 are regarded as specialized forms or form species. 



Sexuality. 



Sapin-Trouffy showed that in - the promycelium, sporidia 

 and aecial mycelium to the base of the aecium, each cell con- 

 tains but .one nucleus, while in the aeciospores and the 

 succeeding spores and mycelium each cell contains two. These 

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

 sexual process. 



