412 F. M. Rolfs 



color of the medium, while those in the lower temperature had developed 

 the normal yellow color. On the fifteenth day the heat of the incubator 

 was reduced to 25° C; on the sixteenth day one of the cultures had taker 

 on a yellow color, and by the eighteenth day all the cultures had developed 

 the normal color. From these results it is evident that 37° C. is close to 

 the limit of growth for the organism on Hiss-glucose. 



In a series of experiments, slant Hiss-glucose and bouillon cultures 

 were kept at 20°, 24°, 28°, 32°, 36°, and 40° C, respectively, for three 

 weeks. vSix-days-old bouillon cultures of four strains of the organism 

 were used for making the inoculations. -Two three-millimeter loops of 

 the fluid were used for each tube. Three tubes of bouillon and two of the 

 slant Hiss-glucose medium were used in each set. Five tubes of bouillon 

 and three of the Hiss-glucose medium were also inoculated and were 

 placed at room temperature (from 23° to 25° C), to serve as checks. All 

 strains of the organism on both kinds of media made decidedly the best 

 growth at 24° and at 28° C. They also made better growth in both 

 media at 20° than at 32° C, and the growth at 32° was considerably better 

 than at 36° C. There was no growth at 40° C. 



The growth on Hiss-glucose cultures kept at 20° and at 24° C. developed 

 the normal yellow color; tubes kept at 28° C. developed a growth a shade 

 lighter in color; those kept at 32° C. were much lighter, and had a mere 

 trace of yellow; in those kept at 36° C. the yellow shade had disappeared 

 entirely, and the growth was of the color of the medium. 



Loss of virulence. — Park (1905) states that " bacteria differ ... as to 

 the ease and rapidity with which they grow in any nutritive substance 

 and the amount of poison ihery produce. Both of these properties not 

 onl}^ vary greatly in different members of the same species, but each 

 variety of bacteria may to a large extent be increased or diminished in 

 virulence." 



In June, 1910, four cultures, one each fi'om apricot, nectarine, peach, 

 and plum twigs, were obtained at IMountain Grove, Missouri. All the 

 cultures were grown continuously^ on a Hiss-glucose medimn. It was 

 observed that the plum organism obtained at this time was the most 

 \'igorously growing strain in the laboratory. In the inoculation work in 

 Missouri during 1910 it was also the most aggressive on all four hosts. 

 In the work at Cornell University during 1911 the writer was much sur- 

 prised to find that not only had this strain lost its vigor on the various 



