68 



Journal of Agricultural Research 



Vol. XVI, No. 3 



Table "Kll .—Comparative sizes of urediniospores of Puccinia graminis as affected by 

 common hosts. Summary of Tables X and XI 



It is evident from Table XII that barley can not change or unify the 

 wheat and rye forms; neither can oats do so with the oats and timothy 

 forms. This substantiates the writers' results on the constancy and 

 stability of the biologic forms of P. graminis in general. Only uncon- 

 genial hosts appear to have the property of changing the morphology 

 of urediniospores as expressed by size or shape. And even in such 

 cases the urediniospores resume their original size and shape when 

 grown again on congenial hosts. 



EFFECT OF PHYSICAL FACTORS 



The attempts to change the spore morphology by means of physical 

 factors are given in Tables XIII to XVII. While some of the observa- 

 tions on the effect of these factors on the development of the rust are 

 repetitions of those previously made by other investigators and while 

 some of the others may seem perfectly obvious, they are made to show 

 the correlation, if any exists, between the vigor of the fungus and the 

 morphological characters of the spores. At the same time some of the 

 results on the development of the rust under various conditions are 

 valuable in themselves. 



Effect of Temperature 



In the present experiment on temperature wheat seedlings were 

 inoculated with fresh urediniospores of P. graminis tritici in the usual 

 manner, given normal germination conditions, and then exposed to 

 various temperatures. The high temperature was obtained by means 

 of an electric heater put under a glass bell jar where the plants were 

 kept continuously. For low temperature the plants under a bell jar, as 

 in the above case, were kept either in an unheated greenhouse or outside, 

 according to existing conditions. The temperatures were recorded 

 by thermographs from which the records were then computed. A set 

 of control plants was kept under normal greenhouse conditions (Table 

 XIII). 



