198 A YEAR IN SCIENCE 



Some of these we call plants and others animals. The 

 question naturally arises : What are the differences 

 betAveen these two groups of organisms? We have 

 just learned that the difference is not one of compo- 

 sition, for in each the living substance is protoplasm. 

 The difference most evident, is in the relative powers 

 of motion. Animals, we may say, can move from place 

 to place, but plants can not. This difference, however, 

 does not hold, for some animals are attached and can 

 not move. The more common of such animals are 

 corals and sponges. Motion is also present in plants. 

 If a plant is placed in front of a window the leaves 

 move toward the light ; a root will move toward water ; 

 and the cells (spores) from which some plants are 

 formed are capable of swimming about as rapidly as 

 some animals do. 



The chief difference between plants and animals is 

 one depending upon the kind of food-materials which 

 each requires. Plants, w r e already know, secure some 

 of their raw food-materials from the soil. It is com- 

 monly believed that they secure most of their food 

 supply from this source, but this is not Avholly correct. 

 The plant takes water from the soil, and a very small 

 part of the soil itself is used. The remainder of the 

 materials required are supplied by the air in the form 

 of carbon dioxide. Out of these raw materials, water, 

 carbon dioxide, and small quantities of minerals from 

 the soil, a plant is able to make its own food. An ani- 

 mal, on the other hand, can not make its food. 



