Mayi3, igis Some Bacterial Diseases of Lettuce 385 



FURTHER TEST FOR ANAEROBISM 



The organism will not grow in an atmosphere deprived of oxygen. 

 A special flask from which the oxygen had been absorbed by a mixture 

 of pyrogallic acid and potassium hydroxid was used for the test (de- 

 scribed on p. 374). Transfers of the organism were made to + 15 bouillon 

 and beef-agar stabs and placed in the jar. At the end of two weeks, 

 when they were removed, there was a mere trace of growth in the stabs, 

 and there was a thin pellicle on the top of one of the bouillon cultures. 

 But there was no clouding and no green color so characteristic of this 

 organism. The controls showed good growth in one day. The organism 

 is a rapid grower, and it is likely that the oxygen in the culture tubes was 

 not absorbed promptly enough to exclude all growth. Seven days 

 after removal from the jar the bouillon cultures were clouded, but no 

 growth had taken place in the agar. 



TEMPERATURE REL.'^TIONS 



Thermal death point. — The thermal death point lies between 52° 

 and 53° C. when transfers are made from a thinly clouded culture which 

 does not contain clumps of bacteria and they are kept in the water bath 

 for 10 minutes. If an 18 to 24 hour old +15 bouillon culture which 

 is densely clouded is used there will be growth when the transfers are 

 subjected to 55° and 56° C. for 10 minutes. This is because of the 

 tiny masses of bacteria which hold together in clumps, the inner ones of 

 which are somewhat protected. 



MAxifiUM temperature. — The maximum temperature for growth is 

 38° C. 



Minimum temperature. — The minimum temperature for growth is 

 below 0° C. 



Optimum temperature. — The optimum temperature for growth is 

 25° to 26° C. 



The medium used for these three preceding temperature tests was 

 + 15 peptone-beef bouillon. 



relation to light 



The organism is not particularly sensitive to sunlight. The different 

 sets of plates for this test were poured from bouillon cultures that were 

 not heavily clouded. The plates were exposed to bright sunlight at noon- 

 day in June and July, one-half of each plate being covered with carbon 

 paper and placed, bottom side up, on sacks of cracked ice, the tempera- 

 ture of the bag being 8° to 14° C. No colonies appeared on the uncovered 

 side of the plates exposed for 40 minutes while from 50 to 70 colonies 

 appeared on the covered sides. In four separate tests colonies appeared 

 twice on the exposed side of 35-minute plates, and twice none appeared. 

 On 30-minute plates i to 10 colonies appeared on the exposed sides 



