586 Introduction to the Study of Science 



ance into the spherical, the rod-shaped, and the spiral. To the rod- 

 shaped belong the bacilli causing typhoid fever. 



Bacteria are the smallest plants, ranging from ^-^ to ^^ of 

 an inch in diameter. They reproduce by division and by spores, and 

 they live wherever the nutritive and other conditions are favorable. 



Many bacteria endure freezing. Pathogenic bacteria are killed by 

 boiling. Most kinds of bacteria, except in the spore stage, are 

 weakened or killed by sunlight. 



Food preservatives are germicidal or disinfectant (killing bacteria), 

 or aseptic (preventing their growth). They may be harmless to human 

 beings, as are spices and salt; or probably harmful, as benzoate of 

 soda; or poisonous, as formalin. 



Other aseptics and disinfectants are carbolic acid, which is danger- 

 ous when strong, and hydrogen peroxid, which is safe. 



There is often a mutual dependence among groups of bacteria. 



REVIEW QUESTIONS 



I. Mention five instances in which the work of bacteria is useful 

 to human life. 2. Where are bacteria found? How abundant are 

 they? 3. What is a common evidence of the presence of bacteria? 



4. To what group do the pathogenic bacteria belong? 5. What 

 are some diseases caused by bacteria ? 6. Why is the study of bacteria 

 important ? 7. What four facts do we need to know ? 8. To what does 

 this knowledge lead ? 9. Into what groups may bacteria be classified 

 in accordance with their relation to human welfare? 10. Where are 

 bacteria found in the human body ? Are they always harmful ? 



II. Name several industries which are dependent upon the work of 

 bacteria. 12. What groups of bacteria are especially important for 

 plants? 13. What important element of plant food do bacteria 

 contribute? 14. In what form does it exist abundantly? 15. How 

 do bacteria enable green plants to get their food? 16. How is the 

 free nitrogen of the air made available for certain plants? 17. What 

 use is made of nitrogen by green plants? 18. What use do animals 

 and men make of plant products? 19. What becomes of the waste 

 products of animal life? 20. How is nitrogen again restored to the 

 soil? 21. What do the denitrifying bacteria do? 22. Whaft does 

 the complete cycle of nitrogen show? 



23. What are the three main classes of bacteria? 24. What is 

 the characteristic of (a) the saprophytes, (6) the parasites, (c) the 

 independent bacteria? 25. (a) What are some of the independent 

 bacteria? (6) What are the sources of their food? (c) What are 



