PLANTS WITHOUT CHLOROPHYLL 145 



Light. — If we cover one half of a petri dish in which bacteria 

 are growing with black paper and then place the dish in a light 

 warm place for a few days, the growth of bacteria in the light part 

 of the dish will be found to be checked, while growth continues in 

 the covered part. It is a matter of common knowledge that disease 

 germs thrive where dirt and darkness exist and are killed by any 

 long exposure to sunlight. This shows us the need of light in our 

 homes, especially in our bedrooms. 



Air. — We have seen that plants need oxygen in order to per- 

 form the work that they do. This is equally true of all animals. 

 But not all bacteria need air to live ; in fact, some are killed by 

 the presence of air. Just how these organisms get the oxygen 

 necessary to oxidize their food is not well understood. The fact 

 that some bacteria grow without air makes it necessary for us to 

 use the one sure weapon we have for their extermination, and that 

 is heat. 



Heat. — Experiment. — Take four cultures containing bouillon, in- 

 oculate each tube with bacteria and plug each tube with absorbent cotton. 

 Place one tube in the ice box, a second tube in a dark closet at a moderate 

 temperature, a third in a warm place (about 100° Fahrenheit), and boil the 

 contents of the fourth tube for ten minutes, then place it with tube num- 

 ber two. In which tubes does growth take place most rapidly ? Why? 



Bacteria grow very slowly if at all in the temperature of an ice 

 box, very rapidly at the room temperature of from 70° to 90° 

 and much less rapidly at a higher temperature. All bacteria 

 except those which have formed spores can be instantly killed as 

 soon as boiling point is reached, and most spores are killed by a few 

 minutes boiling. 



Sterilization. — The practical lessons dra^vn from sterilization 

 are many. We know enough now to boil our drinking water if 

 we are uncertain of its purity; we sterilize any foods that we 

 believe might harbor bacteria, and thus keep them from spoiling. 

 The industry of canning is built upon the principle of sterilization. 



Canning. — Canning is simply a method by which first the 

 bacteria in a substance are killed by heating and then the 

 substance is put into vessels into which no more bacteria may 

 gain entrance. This is usually done at home by boiling the fruit 



BUNTEB, CIV. BI. 10 



