ANIMALS 399 



of bacteria would be as large as the earth. Of course, this 

 would mean that each bacterium had plenty of room to 

 live in and plenty of food to live on and nothing to injure 

 it. These conditions are not found, and each bacterium has 

 to struggle for existence just as every other plant does. 

 As it is, however, bacteria are numberless. 



Some of the activities of soil bacteria we have already 

 studied. There are many other kinds of bacteria, and the 

 relations of many of them to man are of such importance 

 that they will be given further attention in another chapter. 



Molds are made up of many cells, and reproduce them- 

 selves by producing spores. If the mold on bread is allowed 

 to grow for a long enough time under favorable circumstances, 

 you will note a fine black powder that forms. The par- 

 ticles of this powder are spores (seedlike bodies) which will 

 themselves grow into molds if favorable conditions are 

 offered. Mushrooms reproduce by means of spores. 



Yeasts are single-celled plants, as are bacteria, but they 

 do not increase as bacteria do. A little bud forms on the 

 side of the yeast cell, which grows until it finally separates 

 from the parent cell. In this way a single yeast cell 

 may produce several other yeast cells, whereas a single 

 bacterium may only divide into two. 



Animals. Animals do not take -their energy directly 

 from .the sunlight, but indirectly from the latent energy 

 stored up in the foods prepared by green plants. These 

 foods may be eaten as stored by the plants, or they may 

 have passed through the medium of other plants and an- 

 imals. The energy thus stored up is liberated by com- 

 bining the carbon with oxygen. Carbon dioxide is freed. 



The green plants use this carbon dioxide again and, by 



