318 
SMALLEST PLANTS {BACTERIA) 
Milk, one of' the most important articles of food, is a 
possible source of danger from harmful bacteria which may 
get into it in various ways. Milk should be kept cold, and 
should be used before it is too old. The harmless bacteria 
in milk form lactic acid and cause the milk to sour. The 
growth of these bacteria can be checked by pasteurizing the 
milk. Ice cream, if too old, is dangerous, for the slow- 
growing bacteria have had a chance to develop. 
The men who did the most to make the study of bacteria 
possible were Leeuwenhoek, who improved the microscope; 
Pasteur, who discovered bacteria in milk, and Koch, who 
found the way to make a pure culture and to test cows for 
tuberculosis. Many students are devoting their lives to 
the study of the various bacteria. 
Every one should know the main facts about bacteria so 
that he may not have a foolish fear of them, but may be 
able to take reasonable precautions against the harmful 
kinds. Since a healthy body is the best safeguard against 
harmful bacteria, we should observe the laws of hygiene in 
order to keep well, and at the same time avoid, when 
possible, the bacteria which produce disease. 
QUESTIONS 
What are the main points of likeness between a bacterium and 
a flowering plant? What has the pleurococcus which the bacterium 
lacks? How can food be protected from harmful bacteria? In what 
respects are bacteria harmful to milk? In what respects helpful? 
Why are a few harmful bacteria not injurious in a healthy body? If 
one bacterium divides every half hour, and all live, how many will there 
be at the end of twenty-four hours? (Solve by arithmetic or by 
algebra.) Why does an apple with a broken skin decay more rapidly 
than one in which the skin is not broken ? Why should one not put 
ice into water to cool it ? 
REFERENCES 
Calkins, Biology, pages 37-40. 
Conn, Biology, pages 26, 80, 232, 235. 
