128 INTERRELATIONS OF PLANTS AND ANIMALS 



The growth of the same starch may be represented as 

 follows : 



Growth = Carbon dioxide + water = starch -f free oxygen 

 6 C0 2 + 5H 2 = C 6 H 10 5 + 6 2 



4. What is meant by ' fertilizing the soil ' ? 



5. What would happen to the plant nourishment in the soil 

 if there were no decaying organic matter to add to it ? 



6. Write a paragraph, telling how some bacteria may be 

 useful to man. 



7. Write another paragraph, showing that but for useful 

 bacteria the world would be a desert, and man himself could 

 not exist. 



c. Some Methods of Fighting Harmful Bacteria 



1. DISINFECTION OR STERILIZATION 



Materials. Nutrient gelatine or agar-agar, milk, 5% carbolic 

 acid, 4% formalin, and lysol if possible. Steam or hot air steri- 

 lizer. Ice box or cracked ice. 



Observations. 1. Inoculate each of six tubes of gelatine 

 with bacteria by touching colonies of bacterial growth with a 

 sterilized platinum wire or a needle, and transferring to the 

 gelatine. Cork each one with sterilized cotton. Label the tubes 

 1 to 6. Place No. 1 in a warm place. What is the result in 

 three days ? Five days ? 



2. Cover the surface of No. 2 with carbolic acid solution, 

 seal, and set it in a warm place. What are the results in three 

 days ? Five days ? 



3. Cover the surface of No. 3 with 4 % formalin and set it 

 in a warm place. What are the results in three days ? Five 

 days? 



4. Cover the surface of No. 4 with corrosive sublimate solu- 

 tion. What are the results in three days ? Five days ? 



5. Cover No. 5 with 70% alcohol, and set it in a warm place. 

 What are the results in three days ? Five days ? 



6. Sterilize No. 6 in the sterilizer for one half hour. Seal at 



