76 ELEMENTS OF GENERAL SCIENCE 



food. The more we think of this, the more fully we shall 

 be convinced that almost all the things that we eat come 

 directly or indirectly from plants. The next question is, 

 How do plants produce these food materials ? 



81. Source of plant food. We hear so much about the im- 

 portance of good soil that it is easy to make the mistake of 

 thinking that plants secure all their food from the ground. 

 As a matter of fact, although plants are dependent on the 

 soil for a large part of their water and also for foods which 

 furnish them with nitrogen, phosphorus, potassium, and other 

 necessary substances, the gain in weight which they make 

 during growth is far from being accounted for by the loss 

 of weight of the soil. A large part of the material must be 

 supplied from some other source. 



Another thing that leads to the same conclusion is that 

 some of the most important substances in plants, as carbon, 

 are not found in the soil or are very scarce there. Wood con- 

 tains a great deal of carbon, though we do not usually recog- 

 nize it as such, because there are other substances combined 

 with it. If the wood is heated sufficiently to burn or drive 

 off other substances, that which remains will lie a black mass 

 called charcoal. Although the soil and the water contain at 

 times a small amount of carbon, it has been shown that this 

 is not the source of the plant's carbon supply. There is only 

 one other source for the carbon, and that is the air, for the air 

 does contain some carbon in the form of carbon dioxide. It 

 is from this that the carbon of plants is secured. 



82. Food of plants. The food materials of plants are not 

 very different from those of animals. For instance, starch 

 and sugar are very common in our own food, and they are 

 also an important part of the food of our common plants. 

 The difference between the food habits of common plants and 

 those of animals is not so much in the sort of things that 

 they require as in the way in which they secure these thinga 

 The animal secures the starch, sugar, and other compounds 



