136 BIOLOGY FOR BEGINNERS 



Pure Cultures. Thus the number and kind of bacteria to be 

 found in the substance tested can be determined. Other gelatin 

 can be inoculated from some one kind of colony forming a pure 

 culture, so that further study can be made and slides can be pre- 

 pared for use under the miscrocope. 



When our mothers " put up " canned fruits or vegetables at 

 home, they go through the first part of this same process. They 

 boil the cans, covers, and rubbers, which sterilizes them. Then 

 they fill them while still hot with the fruit, which has been sterilized 

 by cooking; and finally seal the cans to keep any other bacteria 

 from getting in and causing the contents to ferment or " work." 



Useful Forms of Bacteria. Do not forget that bacteria do not 

 always mean disease, for as a matter of fact, there are many kinds, 

 without which we could not live. If we pull up a clover plant, 

 there are usually found attached to its roots, numerous small 

 round bunches, called tubercles. These are the homes of millions 

 of bacteria which have the ability to take the free nitrogen of the 

 air and combine it into soil compounds which other plants can 

 then use. These nitrogen compounds are absolutely essential to 

 life. No other plant forms can manufacture them from the air. 

 Therefore we see how important these bacteria are in keeping up 

 the fertility of the soil. Nitrifying bacteria are found on the roots 

 of all members of the clover family, such as peas, beans, and al- 

 falfa. It had long been known that plowing under a crop of clover 

 made the soil better for the other crops, but the reason was not un- 

 derstood till the nitrifying bacteria were studied. 



Other helpful bacteria are those which, like fungi, aid in decay 

 and therefore act as scavengers, removing harmful waste, and re- 

 turning it to the soil as plant foods. This process is utilized in 

 sewage disposal systems, where certain bacteria act on the city's 

 sewage changing it to an odorless and valuable fertilizer instead 

 of a dangerous and expensive waste product. 



The souring of milk, the making of butter and cheese, the 

 " ripening " of meats, and the fermentation of vinegar, sauer 

 kraut, and ensilage, are some food processes in which bacteria are 

 indispensable. The separation of hemp and flax fiber from the 



