PLANTS FROM AN ECONOMIC STANDPOINT 349 



to treatment. When requested to do so the State Agri- 

 cultural department will usually furnish pamphlets 

 dealing with the crop pests of that state and the best 

 methods of destroying them. 



Yeast. Man has learned how to use yeast plants 

 so that they have an economic value. All fermentation 

 and the process of modern bread making depend upon the 

 yeast plant. 



Experiment 79. Add a fourth of a cake of compressed yeast 

 cake to a pint of water containing some molasses or sugar. Now 

 divide this mixture in three parts, placing each part in a small jar. 

 Cover the jars and place one of them on ice. Place another in a 

 moderately warm place, and put the third in a water bath and 

 subject it to a boiling temperature for ten minutes, after which 

 place it near the second jar. Two days later examine the jars. 

 Have any bubbles appeared in the jars? In which jar or jars? 

 What effect has heat upon yeast plants? What effect has cold? 

 Devise an experiment to determine the kind of gas released by 

 yeast plants. What does baking do to the yeast plants in bread? 



QUESTIONS 



1. In what ways do -trees cool the atmosphere? 



2. What precautions are now being taken to prevent forest fires ? 



3. Cypress lasts better when not painted. Why? 



4. Name two uses of the maple tree. 



5. How is science increasing the value of our food plants? 



6. How many bushels of corn were grown in the United States 

 last year? 



7. Where is hemp grown? 



8. Name ten weeds. 



9. How do fungi differ from common plants? 



10. How do spores differ from seeds? 



11. What are the worst plant diseases of your region \ 



12. Name some other plant rusts besides wheat rust. 



