RELATION OF AIR TO FOOD MANUFACTURE 75 



sometimes other substances may result. In any event the 

 formation of starch and sugar is preceded by the formation 

 of simpler substances which we do not need to discuss in this 

 connection. All of these compounds of carbon, hydrogen, and 

 oxygen which resemble sugar and starch are called carbohy- 

 drates. There are many different kinds of carbohydrates, and 

 the compound formed by the leaf from carbon dioxide and 

 water is always a carbohydrate of some kind. That such a 

 compound as starch does, in fact, contain carbon and water 

 may be proved as suggested above (sect. 72). 



80. The waste material. Not only is there a carbohydrate 

 formed, but there is also some oxygen left over. This oxygen 

 has been separated from the carbon and is therefore in exactly 

 the same condition as if it had never been united with it. It 

 is gaseous oxygen exactly like that in the air, and it may pass 

 out through the stomata into the air, where it mingles with 

 the other gases. It may be called the waste material of the 

 process of manufacture which we have just been studying. 

 Bubbles of oxygen may often be seen rising from plants 

 which are growing under water. It is escaping from land 

 plants in a similar way, though it is not visible in the air. 



81. The source of energy. A factory needs energy, and our 

 carbohydrate factory in the leaf must have energy to operate 

 it. Most factories that we are familiar with are operated by 

 energy derived from a waterfall, or from burning coal under 

 the boilers of a steam engine, or by electric power, but the 

 leaf derives its energy from a source that is different from all 

 of these and at first appears rather surprising the sunlight. 

 Light is as much a source of power or form of energy as is 

 electricity, heat, or falling water, but men have not yet learned 

 so well how to use it. We may cause light to do work on a 

 photographic plate, we know that it will change the colors 

 of the chemicals we use to dye our fabrics, and we may cause 

 it to do other kinds of work for us, but we have not learned 

 how to make it run our factories. The leaf is able to use this 



