Jan. 13,1919 Morphology of Urediniospores of P. graminis 51 



Many inoculations were made with P. graminis tritici on wheat, barley, 

 rye, and Hordeum jubatum (Table IV). All except rye are very sus- 

 ceptible. On congenial . hosts the spores remained quite constant in 

 shape, size, and color, irrespective of their origin and subsequent history. 

 On rye, however, an uncongenial host, both the uredinia and the uredini- 

 ospores became appreciably smaller, especially in length. 



These results are not in accordance with those of Freeman and John- 

 son (j, p. 28), who say: 



The host-plant exercises a strong influence, not only on the physiological and bio- 

 logical relationship, but in some cases even on the morphology of the uredospores. 



The difference in results might possibly be explained by supposing that 

 Freeman and Johnson worked with a mixed strain, or that they did not 

 measure enough spores. Their rust, however, may actually have 

 changed. It will be readily seen from Table IV that the writers were 

 not able to change the dimensions more than about i n, which is within 

 the range of experimental error. 



The color of the urediniospores of P. graminis triciti is pale cadmium- 

 yellow; their shape predominantly elliptic to ovoid; size limits 23 to 

 42 by 15 to 25 n, and average modes 32.36 by 19.82 n. 



Results obtained by Stakmanand Piemeisel {14) showed that the biologic 

 form of rust, P. graminis tritici-compacti, discovered west of the Rocky 

 Mountains on several different grasses and on club wheat, varied para- 

 sitically from P. graminis tritici, found east of the Rockies. Many com- 

 mon aestivum wheats, such as Haynes Bluestem and Fife, are resistant 

 to this biologic form, while barley is fairly tolerant and the club wheats 

 inoculated and Pacific Bluestem are very susceptible. Spore measure- 

 ments of over a dozen strains (Table V) indicate that, whereas the spore 

 sizes on the susceptible hosts vary but little (less than i /x) from those of 

 P. gratninis tritici, yet they are on the average nearly 2 ix shorter on the 

 tolerant hosts and almost 4 /x shorter on the resistant ones. The width 

 of the spores does not seem to be influenced by the host. 



Identical results were obtained with P. graminis tritici-compacti, found 

 in the summer of 1 917 in Louisiana and Alabama, on several soft wheats. 

 The constancy of size is remarkable, and, like the western strain, the 

 southern strain, too, exhibits a special affinity for club wheats, while the 

 most of the hard wheats are resistant. 



The color of the urediniospores of P. graminis tritici-compacti is prac- 

 tically the same as that of P. graminis tritici. The spores are slightly 

 shorter, and ovoid to ellipsoid in shape. 



