PROBLEM 



Our Relationship to Other Orgmimns 



383 



Fig. 332 Fruits of the cot- 

 ton plant. Three have hjirst 

 open showing where the 

 cotton grows. ]Vhy is cot- 

 ton an i?nportant crop 

 plant F (u. s. department of 

 agriculture; 



ment. Some of them are rich in proteins 

 and much cheaper than meats or fish. 

 Plants also supply us with such minor 

 foods as spices, sugar, tea, coffee, flavor- 

 ing, and substances for making beer and 

 other alcoholic liquors. Before reading 

 further do Exercise 2. 



The other uses of plants are many and 

 important. Much of our clothing is made 

 from cotton. From trees we obtain lum- 

 ber, wood pulp for the manufacture of 

 paper, firewood to heat homes and for 

 cooking, and the raw materials for many 

 manufactured products. One of the most 

 important of these is rubber. From the 

 forests we obtain also turpentine, rosin, 

 tar, tannin for tanning leather, cork, and 

 many other products. And from the for- 

 est trees that lived millions of years ago 

 we get our coal. Some stems have fibers 

 that are used for weaving linen (flax) or 

 for making rope (jute, hemp, and 

 others). Some supply us with drugs; 

 quinine, for example, is extracted from 

 the bark of the cinchona tree originally 

 found in Peru. (You will find The Fever 

 Bark Tree by M. L. Duran-Reynals in- 

 teresting reading.) 



How we use microorganisms. We think 

 so often of microorganisms as causes of 

 disease that we tend to forget how use- 

 ful some of them are. You read in the 

 last Problem of the bacteria and molds 

 that restore nitrates to the soil and are 

 useful in the carbon cycle. You have al- 

 ready learned that yeasts ferment sugar, 

 changing sugar to alcohol and carbon 

 dioxide. In breadmaking the carbon di- 

 oxide raises the dough, making the bread 

 light. A special kind of yeast is used in 

 this process. Other kinds of yeasts are 

 used to produce alcohol. See Exercises 

 3 and 4. Some kinds are used in the brew- 

 ing of beer where grains are acted on by 

 yeasts; others in the fermenting of grapes 

 to produce wines. Recently, too, yeasts 

 have come into use for their vitamin con- 

 tent, and certain kinds of yeasts have 

 been developed that contain very large 

 percentages of protein, providing essen- 

 tial proteins at a smaller cost than meat. 



Among other microorganisms that we 

 use are the bacteria that give flavor to 

 food, especially meat. Almost all meat is 

 tough. One method of making it tender 

 is to let it "hang" for a while. Frequently, 



