MATERIALS FROM PLANTS AND ANIMALS 417 



308. Food. For people the world over, food is the largest 

 single item in the cost of living. We have already seen 

 (page 107) that all of our food comes either directly from the 

 bodies of chlorophyl-bearing plants or indirectly, by way of 

 animals that feed upon green plants. A very small portion of 

 the total food supply of human beings comes from fungi (mush- 

 rooms), which in turn get their nourishment from the remains 

 of other living things. We also make use of organisms in the 

 preparation of food. For example, the yeast ^ used in baking, 

 and the molds used in curing cheese, are not themselves parts 

 of the food we eat, but they are essential agents in making food. 

 The nitrogen-fixing bacteria (see page 300) also add indirectly 

 to our food supplies, since they enable the plants of the bean 

 family to utilize nitrogen from the atmosphere, and since they 

 help to keep the soil usable for other food crops. 



309. Fibers and hairs. The bulk of our clothing material is 

 woven from various plant and animal fibers, although such 

 fibers are used for other purposes than the making of clothing— 

 for cordage, burlap, sailcloth, aeroplane wings, bunting, and so 

 on. Cotton, flax, and hemp are the most important fiber plants. 

 In the manufacture of cotton we use the fine fibers surrounding 

 the seeds after the fruit is ripe ; in making linen and hemp we 

 use the bast fibers from the stem. 



Wool is the most valuable of animal fibers. Besides the wool 

 of the sheep, goat's hair and camel's hair are also used to a 

 considerable extent. Silk is made from the fine thread spun by 

 the larva of the silk moth as it passes into the pupa stage. At- 

 tempts have been made to utilize the corresponding silk of cer- 

 tain large American moths, but so far without success ; and up 

 to the present time experiments with the cocoon silk of various 

 spiders have also failed to establish workable methods. 



^In recent times compressed yeast (a mixture of yeast plants with starch 

 grains) has been offered to the public as food for supplying vitamins. While 

 it is true that yeast does contain vitamin B, we can more conveniently and 

 more economically get what we need of this substance in connection with a 

 normal mixed diet (see pages 147, 105). 



