PLATE 26 



Relation of soil temperature to the infection of onion seedlings by the smut fungus, 



Urocystis cepulae. 



Representative seedlings taken from the experimental culture pots, showing the 

 influence of soil temperature upon the amount of smut. All the pots alike contained 

 smut-infested soil. The air temperature and other aerial factors were the same for all. 

 Soil temperature was the only factor varied experimentally, the temperature grada- 

 tions extending from about 10° to 30° C. 



Note the abundance of smut at 10° to 22°, as shown in the upper row. A slight re- 

 duction occurred at 23° to 26°. At 27° to 29° the reduction is sharply marked. At 

 29° to 3 1 ° inhibition is complete. For the percentages of seedling infection and other 

 details see Table VI and the accompanying text. 



