128 



PLANTS AND ANIMALS 



food in order to live. They have taken in carbon diox- 

 ide and given out oxygen. They have had leaves, flow- 

 ers, stalks and roots, and have grown from seeds. Now 

 we are going to study a very queer plant which has no 

 roots, stalk, leaves, flowers nor seeds. This plant is 

 called yeast and all its needs are sugar and water and a 

 little heat. 



Yeast plants are so small that they cannot be seen 

 by the use of the simple microscope, but it is necessary 

 to use a very powerful microscope. An ordinary yeast 

 cake contains millions of these 

 little plants. If sugar, water, and 

 a little heat are supplied, each 

 tiny plant begins to send out a 

 little swelling called a projection 

 which increases in size until it is 

 nearly as large as the plant itself. 

 Then it breaks off and becomes a 

 separate plant. Such growth is 

 called budding. The illustration 

 shows the process of budding in 

 nine stages. 



When the yeast plants are growing they change the 

 sugar into carbon dioxide and alcohol. The process of 

 changing sugar in this manner is called fermentation. 

 Most yeast cannot grow without sugar. The following 

 experiment will prove that carbon dioxide is produced 

 when yeast grows, while Section 79 will discuss the 

 manufacture of alcohol. 



Experiment 62. Fermentation. 

 Cut supplied through United States Department of Agriculture. 



