240 ESSENTIALS OF BOTANY 
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306. Occurrence. — Bacteria are to be found almost 
everywhere. Although their extremely small size makes 
them quite invisible 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 much 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. 
