i8o 



BACTERIA IN RELATION TO PLANT DISEASES. 



In case of agar and gelatin there are numerous \'ariations due to inadvertent 

 changes in the cuhure-mediuni, especially if this is made by students. The 

 media should be made by competent, experienced persons, and then the descriptions 

 of the behavior of the organism on it should be broad enough to include slight 

 differences in the aspect of the colonies, streaks, and stabs, which often depend on 

 chemical and physical conditions within the control of the experimenter, e. g., on 

 the water-content, on age of the medium, amount of moisture in surface-layers, 

 kind of peptone, kind of gelatin, length of exposure and degree of heat during 

 .sterilization, etc. The dense or thin sowing of the plate may sometimes make a 

 very decided difference in the aspect of the colonies. Fig. 142 shows a densely- 



143.-- 



sown plate, the colonies round or roundish. Fig. 143 shows the same organism, 

 and from tlie same set of plates, but thinly sown and two days older. Here the 

 colonies are radiate. In case of Bacillus aroidccc when grown on agar-plates, 

 near the maximum and minimum temperature limits, the surface-colonies are round 

 even after many days, but they are prompt!}' and strongly radiate when grown at 

 or near the optinnim temperature (see figs. 144, 145). When very thin sowings of 

 this organism were exposed to the Ingh temperature, tlie colonies were also round. 

 It occurred to the writer that tlie round colonies obtained on the agar-plates 

 exposed in the thermostat at 37° C. might be due to ph}'sical changes in the surface 



'Fir,. 143. — Iris-rhizomc rot. Tlic same as 142, but suwii thinly and kept for 4 days at 25° C. 



