16 REPRODUCTION 



do have, all the powers necessary to sustain individual 

 life, and equally to reproduce and keep the species going. 

 They get food, and use it both to grow and to supply 

 energy. They are sensitive to all the important external 

 conditions of life. They are able to adjust themselves to 

 these conditions. 



2. Where They Are Found. As one might expect, 

 these minute one-celled plants and animals must live in 

 moist places. Otherwise they would dry up and lose the 

 internal water that is absolutely necessary to active life. 

 Many of them can dry up for days and months and come 

 again into activity when water returns, but, of course, 

 they are not active during this period of dryness. We 

 find them in all waters, both fresh and salt; in the bodies 

 of larger plants and animals, where they may produce 

 diseases; in decaying organic matter, where they assist 

 the decay and live upon the products of it. 



They are found in all parts of the world and are among 

 the most interesting of all living things. They are usually 

 transparent under the microscope. Many of them are so 

 small, however, that they can scarcely be seen at all, even 

 by means of our microscopes of highest-powers. 



The great improvement of the compound microscope 

 has made these simplest organisms one of the most 

 interesting fields of study in the whole natural realm. 

 You are able to watch all their activities, and even to 

 see them passing through the various stages of their life 

 cycle. Sometimes this cycle is completed in a few hours. 



3. Their Importance in Nature. Aside from the mere 

 fact that they are interesting to look at and to study, 

 these lowly plants and animals are extremely important 

 to man, and to the other plants and animals that live on 

 the earth. Some of the plants, minute as they are, are 

 green like the leaves of trees. A good example of this is 

 the plant Pleurococcus which you may see as a green 

 stain on the north side of fences and trees. Each plant 



