Jan. 13, 1919 Morphology of Urediniospores of P. graminis 75 



ment and, as shown in Table XIX, there seemed to be no appreciable 

 effect on the size of the spores, although the size of the uredjnia gradually 

 and persistently became larger, which was due to the additional shed- 

 ding of spores and coalescence of adjacent uredinia. The color of the 

 uredinia became darker with age and the spores lost their coherent 

 floccose consistency and by the least disturbance were separated from 



the uredinia. 



GENERAL DISCUSSION 



The data presented in this paper provide ample e\ddence to show 

 that the morphology of biologic forms is but slightly and only tem- 

 porarily changed in response to biotic and physical factors. Resistant 

 host plants and unfavorable cultural conditions affecting the normal 

 development and vigor of the rust fungus may also affect the size of its 

 urediniospores. But as soon as the unfavorable factors are removed the 

 fungus resumes its normal functions and regains its original structure. 



No host which is congenial to a given biologic form can, under favor- 

 able cultural conditions, exert any perceptible influence on the mor- 

 phology of the rust spores. P. graminis avenae appears to deviate 

 from this rule in so far as shape and size of urediniospores are concerned. 

 The shape varies considerably on any host to which the rust may be 

 confined, while the spore sizes appear to be peculiar of the particular 

 host the rust parasitizes. 



The attempts to split up the various biologic forms of P. graminis 

 studied into a number of different morphological strains by culturing 

 them for long periods of time on several different but definitely con- 

 genial hosts have utterly failed. The attempts to unify the spore sizes 

 of different biologic forms by culturing them continuously for a con- 

 siderable length of time on the same host were also unsuccessful. 



Adverse environmental conditions, such as resistant host varieties, 

 affect the virulence and spore size of the rust fungus. Excessive heat 

 is more injurious to the rust growth and affects the size of urediniospores 

 more effectively than does very low temperature. High humidity 

 during the incubation period appears to be an indispensable condition; 

 the difference in humidity later is probably of lesser importance. Defi- 

 ciency of soil moisture and sunlight, and other ecological factors affect- 

 ing the host plant unfavorably, appear to be equally unfavorable to the 

 rust parasite. ' 



The results show that P. graminis is quite stable and can not be ex- 

 pected to change rapidly. This is true both of its parasitic capabilities 

 and of its morphologic characters. The facts presented in this paper 

 give additional support to the rapidly accumulating body of data which 

 show that the biologic forms studied are fairly constant. Whether 

 this will apply equally well to the large number of forms recently found 

 on varieties of wheat is a question which can be answered only by future 

 investigation. 



