144 



PLANTS WITHOUT CHLOROPHYLL 



Foods preferred by Bacteria. — If bacteria are living and 

 contain no chlorophyll, we should expect them to obtain protein 

 food in order to grow. Such is not always the case, for some 

 bacteria seem to be able to build up protein out of simple inorganic 

 nitrogenous substances. If, however, we take several food sub- 

 stances, some containing much protein and others not so much, we 



will find that the bacteria cause 

 decay in the proteins almost 

 at once, while other food sub- 

 stances are not always attacked 

 by them. 



What Bacteria do to Foods. 

 — When bacteria feed upon a 

 protein they use part of the 

 materials in the food so that it 

 falls to pieces and eventually 

 rots. The material left behind 

 after the bacteria have finished 

 their meal is quite different 

 from its original form. It is 

 broken down by the action of 

 the bacteria into gases, fluids, 

 and some solids. It has a characteristic "rotten" odor and it 

 has in it poisons which come as a result of the work of the bac- 

 teria. These poisonous wastes, called ptomaines, we shall learn 

 more about later. 



Conditions Favorable and Unfavorable to the Growth of Bacteria. — ■ 

 Moisture and Dryness. — Experiment — Take two beans, remove the sldns, 

 crush one, soak the second bean overnight and then crush it. Place in 

 test tubes, one dry, the second with water. Leave in a warm place two 

 or three daj^s, then smell each tube. In which is decay taking place ? In 

 which tube are bacteria at work ? How do you know ? 



Moisture. — Moisture is an absolute need for bacterial growth, 

 consequently keeping material dry will prevent the growth of 

 germs upon its surface. Foods, in order to decay, must contain 

 enough water to make them moist. Bacteria grow most freely 

 in fluids. 



Growth of bacteria in a drop of impure 

 water allowed to run down a sterilized 

 cultvure in a dish. 



