72 BACTERIA IN RELATION TO PLANT DISEASES. 



better, 011 larger Petri dishes rilled with pounded ice ; otherwise, in case of 30 to 60 

 minute exposures, the temperature may rise nearly or quite to that of the thermal 

 death-point of the organism, and then we shall have the effect of heat complicating 

 that of light. To avoid errors it is always best to take one-half of each dish as a 

 check (rather than the whole of a separate dish), and the rise of temperature should 

 be carefully recorded. In some tests made by the writer in Washington in May the 

 temperature of the plates exposed in the open air to the sun for 45 minutes (without 

 ice) rose from 25 to 51 C. Figs. 61 and 62 show the effect of sunlight upon thin 

 sowings of Bacillus am vlozwiis and Bacterium phascoli in poured-plate (Petri-dish) 

 cultures. 



VITALITY ON VARIOUS MEDIA. 



By this I mean the determination of the resistance of organisms to their own 

 decomposition products. This varies greatly. Much may be learned by the study 

 of old cultures. Do not discard test-tube cultures until after many weeks. Examine 

 frequently. Make transfers from tubes which have been inoculated for a year or 

 more. Determine whether this vitality is due to spores or persists in the ordinary 

 vegetative rods. On what kinds of media does a particular organism live longest? 

 Can length of life be increased by occasionally neutralizing decomposition products 

 (acids) with sterile carbonate of lime ? or by occasional additions of food ? Some 

 bacteria are veritable revelers in filth ; others are extremely sensitive ; all are soon 

 under abnormal conditions in our culture-tubes. 



Another way of keeping bacteria alive for a long time is by reducing their growth 

 to a minimum. Stock-cultures, especially of perishable organisms, should, generally 

 speaking, be kept in the ice-box at temperatures under 15 C. This greatly reduces 

 the always heavy burden of keeping alive cultures of organisms which are not 

 in immediate demand for actual experiment. Some will also remain alive a long 

 time when sealed airtight. Particular organisms may be kept a long time in par- 

 ticular media, e. g., Bacterium vascularum in diluted peptonized cane-juice gelatin, 

 Bad. Stcwarti in milk, etc. Some organisms are quite resistant to their own 

 decomposition products, e. g., Bacillus coli, Bad. pcricarditidis. In the cool box 

 B. coli will often live a year in agar stab cultures. 



MIXED CULTURES AND MIXED INFECTIONS. 



The behavior of mixed cultures and mixed infections may be tested in various 

 fluids, making poured plates from time to time ; in tubes of agar, potato, and other 

 solid media ; in crossed streaks on agar or gelatin plates ; and in the plants themselves. 



When two bacteria, or a bacterium and a fungus, are sown together in a culture- 

 medium, there may be (i) antagonism, with the crowding out of one species ; (2) a 

 more or less complete indifference, both organisms growing well ; or (3) a distinctly 

 favorable effect, i. e., a marked increase in growth or in pathogenic effect due to 

 the presence of the second organism. The antagonism may result in the prompt 

 destruction of one of the organisms, or only in a retardation or inhibition which 

 finally disappears after the first organism has made its growth and subsided. In 

 some cases the favorable effect of one organism upon another is due to the fact that 

 it prepares food for it out of an unfavorable substratum, e.g., maltose from starch. 



