i8o 



BACTERIA IN RELATION TO PLANT DISEASES. 



In case of agar and gelatin there are numerous variations due to inadvertent 

 changes in the culture-medium, 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- 



Fig. 143* 



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

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

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

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

 even after many days, but they are promptly and strongly radiate when grown at 

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

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

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

 exposed in the thermostat at 37 C. might be due to physical changes in the surface 



*Fic. 143. Iris-rhizome rot. The same as 142, but sown thinly and kept for 4 days at 25 C. 



