CULTURES OF POTATO. 209 



which the colonies are growing. In some instances 

 this liquefaction spreads laterally and downward, caus- 

 ing a saucer-shaped excavation ; while in others the 

 colony sinks almost vertically into the gelatin and may 

 be seen lying at the bottom of a funnel-shaped depres- 

 sion. These differences are constantly employed as 

 one of the means of differentiating otherwise closely 

 allied members of the same family of bacteria. (See 

 Fig. 34.) Studies upon the spirillum of Asiatic 

 cholera and a number of closely allied species, for 

 example, reveal decided differences in the form of 

 liquefaction produced by these various organisms. 

 The minutest detail in this respect must be noted, and 

 its frequency or constancy under varying conditions 

 determined. 



Cltltuees 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 lob- 

 ulated, 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 ap- 

 pearance, again it may be moist and glistening. Some- 

 times bubbles, due to the fermentative action of the 

 growing bacteria on the carbohydrates of the potato, 

 are produced. 



A most striking form of development on potato is that 

 often exhibited by the bacillus of typhoid fever and the 

 bacillus of diphtheria. After inoculation of a potato 

 with either of these organisms there is usually no naked- 

 eye evidence of growth, though microscopic examina- 



14 



