396 TEXTBOOK OF BOTANY 



20. What kinds of floor coverings (carpets, rugs, linoleum, cork, 

 or simply hardwood flooring) are best for rooms used by large num- 

 bers of people ? Why ? 



21. Why do many cities and some states require the testing of 

 cattle for tuberculosis ? 



22. Why should the surface of a wound be kept open^for a time 

 instead of being allowed to heal over as soon as possible ? 



23. Why do teeth that are not kept clean decay? 



24. What substances are used as " antiseptics "? How are they 

 used? 



26. Why are milk, meat, and other foods kept in refrigerators 

 in warm weather? 



26. Why are fruits and vegetables heated or boiled before being 

 canned ? 



27. Why must the cans in which they are placed be tightly 

 sealed? 



28. Why do preserves containing a large proportion of sugar 

 keep well without being tightly sealed? 



CHAPTER III 



1. Observe the greenish patches on the bark of 'trees or on the 

 shaded parts of a stone wall. How is the green substance dis- 

 tributed? Is it a thin or a thick layer? Is it more abundant on 

 one side of a tree than on another? Why? Can you scrape it 

 off in sheets? Remove some of the material, bring it into the 

 laboratory and place it in a damp place or sprinkle it with water. 

 What changes take place in its appearance? 



2. Mount a small portion of the green material in water, tear it 

 apart carefully and examine it under the microscope. Look for 

 small green cells which are alone or in twos or fours. These are 

 cells of Protococcus. Do you ever find more than four cells in a 

 colony ? If so, are they all in one layer ? 



3. Can you see that each cell is surrounded by a transparent 

 wall ? This appears most clearly on the sides where two cells are 

 attached to each other. 



4. Is the green color scattered all through the cell? If not, 

 can you see that it is contained in a body (the chloroplast) with a 

 definite outline? 



