346 PLANTS WITHOUT CHLOROPHYLL 



8. Leaving the disease-causing bacteria out, are bacteria harmful or useful ? 

 Why? 



9. Explain the nitrogen cycle. 



10. Explain the use of rotation of crops. 



11. In what respect is the cold-pack method of canning superior to the older 

 method ? 



12. Distinguish between useful and harmful preservatives. 



Problem and Project References 



Hunter, Laboratory Problems in Civic Biology. American Book Company. 

 At wood. Civic and Economic Biology. P. Blakiston's Son and Company. 

 Buchanan, Bacteriology for Students in General and Household Science. The 



Macmillan Company. 

 Broadhurst, Home and Community Hygiene. J. B. Lippincott Company. 

 Conn, Bacteria, Yeasts and Molds in the Home. Ginn and Company. 

 Densmore, General Botany. Ginn and Company. 

 Gager, Fundamentals of Botany. P. Blakiston's Son and Company. 

 Marshall, Microbiology. P. Blakiston's Son and Company. 

 Transeau, General Botany. World Book Company. 



