t;s 



ELEMENTS OF GENERAL SCIENCE 



there are other substances combined with it. If the wood is 

 heated sufficiently to burn or drive off other substances, that 

 which remains will be a black mass called charcoal. Although 

 the soil and the water contain at times a small amount of car- 

 bon, 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. 

 72. 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 ; 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 re- 

 quire as in the way in which they secure these 

 things. The animal secures the starch, sugar, 

 and other compounds from a plant or from 

 some other source, where they may be found 

 in such a condition that it can use them 

 directly. The plant, on the other hand, is able 

 to make these compounds from their elements. 

 Such compounds as starch and sugar are 

 composed of three simple substances, car- 

 bon, hydrogen, and oxygen. We can easily 

 prove that this is the composition of starch 

 by heating some in a test tube (fig. 37). The 

 heat decomposes the starch, and we have carbon and water. 

 Water, we already know, is composed of hydrogen and 

 oxygen, and therefore we have here the three substances. 

 73. The place where food is made. If a green leaf is exam- 

 ined early in the morning, little or no starch will be found in 



FIG. 37. Decom- 

 position of starch 



When starch is de- 

 composed by heat, 

 a black deposit of 

 carbon remains in 

 the bottom of the 

 tube, and water 

 condenses on the 

 inside of the tube 



