Jan. 13, 1919 Morphology of Urediniospores of P. graminis 



49 



of a given form studied. The "mode averages" were obtained by find- 

 ing the arithmetic mean of all modes of a given biologic form cultured on 

 various congenial hosts. 



Table III. — Comparative sizes of urediniospores of biologic forms of Puccinia graminis 



Table III shows distinctly the considerable variation in the size of 

 spores of the different biologic forms. The uredinospores of P. graminis 

 tritici are the largest of all, while those of P. graminis tritici-compacti 

 are less than i/x shorter and only a fraction of a micron narrower. 

 The other forms vary more perceptibly. The spores of P. graminis secalis 

 approach those of P. graminis avenae in length, the latter resembling 

 those of P. graminis tritici in width. The spores of P. graminis phlei- 

 pratensis are similar in width to those of P. graminis secalis, but con- 

 siderably shorter; while P. graminis agrostis has smaller uredinospores 

 than any other biologic form of P . graminis studied. 



Relative to shape, the six biologic forms discussed in this paper could 

 be classified in two principal groups; the ellipsoid-cylindric group, con- 

 sisting of P. graminis tritici, P. graminis tritici-compacti, and P. graminis 

 secalis; and the ovoid-subglobose group, including P. graminis avenae, 

 P. graminis phlcipratensis, and P. graminis agrostis. Stakman and 

 Piemeisel {14) made an identical classification of these forms on the basis 

 of their parasitism. 



It is interesting to note that the morphological differences between the 

 individual biologic forms of Puccinia graminis are fully as great and 

 distinct as those between many generally recognized species of fungi. 

 Because of similar morphological variation in certain biologic forms of 

 Erysiphe graminis, expressed by distinctive characteristics in the color 

 of the conidia and in some cases also in their size, Salmon (9) concluded 

 that those forms were "incipient morphological species." The same 

 may be true of the biologic forms of Puccinia graminis. 



INFI.UENCE OF HOST 



If biologic forms of Puccinia graminis are incipient species, they are 

 probably evolving gradually. If the change is sudden and accidental, 

 finding the evidence may be merely a matter of chance. If, however, 

 92802°— 19 3 



