15G PLANT PARASITES. 



placed in small quantities in test-tubes plugged with cotton (Fig. 79) and carefully 

 sterilized by heat. 



THE STUDY OF CULTURES. Let us see how the nutrient media are used in study- 

 ing the bacteria. 



In the first place, it is necessary to get from the various mixtures of several species, 

 as they are apt to occur in nature or in diseased parts, single species growing by them 



FIG. 79. FIG. 80. 



FIG. 70. PURE CULTURE OF BACTERIA ON NUTRIENT AGAR IN STERILIZED TUBES. 



Showing at the left a smooth, at the right a wrinkled growth upon the surface. 



FIG. 80. A CULTURE OF NACTERIA ON POTATO. 



selves, so that their life history and characters may be studied in detail. To show by 

 an example how this is done, we will suppose that we have a sample of milk containing 

 bacteria, and wish to learn how many there are and of what species, and to get them 

 into separate receptacles for study. We melt the nutrient gelatin or agar in one of 

 the test tubes,' which we know to contain no living bacteria because we have steril- 

 ized both the tube and its contents by heat and add to it a measured volume, usually 



