374 EXPERIMENTS WITH PLANTS 



aside for two or three days, and if no growth appears, 

 proceed to roake a bacteriological examination of milk. 

 For this purpose obtain some milk freshly drawn; 

 some milk which has stood for from twelve to twenty - 

 four hours, and some sour milk. Make so-called "stab 

 cultures " by dipping the point of the sterilized needle 

 into the milk, and then plunging it straight down 

 through the center of the gelatin to the bottom; the 

 needle must be sterilized in the flame after each stab. 

 The plugs should be replaced at once, and the vials 

 set aside. In which do you see the first signs of 

 growth; in which is the growth most abundant; what 

 do you conclude as to the relative number of bacteria 

 in the three kinds of milk ? 



Stab cultures are especially interesting because some 

 of the bacteria are placed deep in the gelatin w^here 

 they are deprived of air, while others are left on the 

 surface. Some kinds of bacteria die if deprived of air; 

 other kinds die if exposed to the air, while there are 

 still other kinds which can live under either condition. 

 In a stab culture we have an opportunity to judge 

 which of these classes of bacteria w^e have. Thus, a 

 culture which grows like Fig. 212 (a), indicates the 

 first class; if like Fig. 212 (&), the second class; if 

 like Fig. 212 (c) , the third class (or a mixture of the 

 first two). 



In order to determine whether bacteria use up oxygen 

 and produce carbon dioxide, we may take an infusion 



