STUDY OF CULTURES ON POTATO. IQQ 



tion produced by these different organisms. The 

 slightest detail in this respect must be noted, and its 

 frequency or constancy under different conditions 

 determined. 



Cultures on Potato. — A very important feature 

 in the study of an organism is its growth on sterilized 

 potato. Many organisms present appearances under 

 this method of cultivation which alone can almost be 

 considered characteristic. In some cases coarsely lobu- 

 lated, elevated, dry or moist patches of development 

 occur after a few hours ; again, the growth may be finely 

 granular and but slightly elevated above the surface of 

 the potato; at one time it will be dry and dull in 

 appearance, again it may be moist and glistening. 

 Sometimes there is a production of bubbles, owing to 

 fermentation brought about by the growth of the organs. 



A most striking form of development on potato is 

 that possessed by the bacillus of typhoid fever and the 

 bacillus of diphtheria. After the inoculation of a potato 

 with either of these organisms there is no naked -eye 

 evidence of a growth in either instance, though micro- 

 scopic examination of scrapings from the surface of the 

 potato reveals an active multiplication of the organisms 

 which had been planted there. The potato is one of 

 the most important differential media which we possess 

 for this work. 



Reaction Pkoduced by Organisms in their 

 Growth. — The reactions produced in the media by 

 different organisms in the course of their growth are 

 very valuable as means of differentiation. 



In some cases these changes are so marked that they 

 are readily detected by the coarser reagents ; again they 

 are so slight as to require the employment of the most 



