THERMAL DEATH POINT. 35 



The colonics moasurod from 1 to 8 nun. in diiinu'tcr. Colonies wore 

 also visible in the plates at 2(» and 30 and at 37.5^0., but they were 

 smaller— scarcel}' larger than pin points. Similar tests were made of 

 other temperatures above and below 35° C. with like results. Since all 

 oTowth above and below 35° C. is slower than at this temperature, it 

 appears that 35° C. is the optimum temperature for the growth of the 

 calla-rot oraanism. In thirtv-four hours the colonies at 35° C. had 

 the characteristic radiating- form, while those at and above 37.5° C. 

 were round. 



THERMAL DEATH TOIXT. 



The thermal death point is the lowest temperature at which the life 

 of the oro-anism will be destroved when a fresh culture is exposed to 

 that tempcn-ature for ten minutes. To determine that point with the 

 calla-rot oro-anism fresh beef-broth cultures were made from a 24-hour- 

 old culture of l)eef broth, each culture consisting of 10 c. c. of broth 

 inoculated with a 1-nnn. loop of the 2-i-hour-old culture. The tubes 

 containing these fresh cultures were placed in water at constant tem- 

 perature for ten minutes. In the first experiment three sets of tubes 

 were used. One set was exposed to a temperature of 40°, another set 

 was exposed to 49.20 ^ and the third set was exposed to 49.40° C. 

 After exposing the tu])es to these temperatures they were placed at 

 room temperature of a])out 20° C, and at the expiration of eighteen 

 hours all control tulles were clouded and all exposed tubes were clear. 

 Six hours later set 1 (49° C.) was clouded slightly; sets 2 and 3 were 

 still clear. Twenty-four hours later— i. e., forty-eight hours from the 

 time the tubes w^ere exposed to the heat— all inoculated tubes were 

 clouded. In the second experiment three sets of tubes were again 

 used. After inoculating in the same manner as above, one set was 

 exposed for 10 minutes to a temperature of 49.50°, another to 50°, 

 and a third to 50.20° C. Several inoculated tubes were left untreated 

 for control. At the expiration of twenty-four hours all control tubes 

 were clouded, and all exposed tubes were clear. Twenty-four hours 

 later four tubes in set 1 (49.50- C.) were clouded and two were clear. 

 All tubes in sets 2 and 3 (12 m all) were still clear. At the expiration 

 of two weeks all tubes in sets 2 and 3 were still clear, and the two 

 tubes in set 1 were also clear. Agar plates were made from the clouded 

 tubes that were heated to 49.50- C, and in all cases pure cultures of 

 the calla organism were obtained, as indicated by the shape of the 

 colony and by the fact that inoculations into calla plants produced the 

 characteristic symptoms of the disease. Several sets of cultures were 

 subsequently exposed to a temperature of 50° C. for ten minutes, but 

 always with the result that they all remained clear indefinitely, while 

 a i)art, at least, of the cultures exposed below 50° C. clouded in a 

 longer or shorter time, showing that 50° C is the thermal death point 

 for this organism. 



