240 ESSENTIALS OP BOTANY 



306. Occurrence. — Bacteria are to be found almost 

 everywhere. Although their extremely small size makes 

 them quite iuTisible without the microscope, they are 

 present in unimaginable numbers in most of the air we 

 breathe and much of the food that we eat and the water 

 that we drink. The commoner kinds are saprophytic and 

 are to be found on most kinds of fermenting or decaying 

 organic matter. Parasitic bacteria of many species occur 

 in the bodies of animals and plants. 



307. Cultures of Bacteria It is mucli easier to observe some of 



the effects produced by the growth of bacteria than to study the in- 

 dividual organisms. These are so small that in order to make out 

 the details of their structure one needs a more powerful microscope 

 than is usually found in school laboratories. 



Pure cultures of bacteria are commonly made in some preparation 

 of gelatine in sterilized test-tubes. Boiled potatoes serve a good pur- 

 pose for simple (but usually not pure) cultures. 



Select a few small roundish potatoes with skins entire and boil 

 in water for a sufficient time to cook them through. Cut them in 

 halves with a knife well scalded, or sterilized, i.e., freed from all liv- 

 ing organisms, in a flame, and lay each, with cut surface up, on a 

 saucer, covering each with a glass tumbler. The tumblers and 

 saucers should be well scalded or kept in boiling water for half an 

 hour and used without wiping. Sterilization may be improved by 

 baking them in an oven for an hour. 



308. Inoculation. — The culture media prepared as above may 

 now be inoculated. Uncover them only when necessary and quickly 

 replace the cover. Scrape a little material from the teeth, tongue, 

 kitchen sink, floor of the house or schoolroom, or any other place 

 you may desire to investigate. With the point of a knife blade or a 

 needle sterilized in a flame, inoculate a particle of the material to be 

 cultivated into the surface of one of the potatoes. Several cultures 

 may be made in this way and one or more left uninoculated as 

 checks. Another may be left uncovered in the air for half an hour. 

 Others may be made with uncovered potatoes. Number each cul- 

 ture and keep a numbered record. 



