32 ELEMENTARY STUDIES IN BOTANY 



Substances are said to be either organic or inorganic. An 

 organic substance is one that is made by a living body ; an 

 inorganic substance is one that is usually made quite inde- 

 pendently of a living body, as air, water, compounds in the 

 soil, rock material, etc. The manufacture of food consists 

 in taking these inorganic substances and making from them 

 organic substances. It is this that green plants are able 

 to do, and they manufacture food not only for their own 

 use, but also for the use of plants that are not green as well as 

 for the use of all animals. The food used by plants does not 

 differ from that used by animals ; the difference is that green 

 plants have the added power of manufacturing food. A 

 miller uses flour for his bread, just as every one else does, 

 but he differs from others in also being equipped to manu- 

 facture his flour. 



There are several general kinds of food, but the peculiar 

 work of green plants has to do with only one of them, the 

 kind called carbohydrates. If this name does not happen 

 to suggest any kind of food, such common carbohydrates as 

 sugar and starch will make the kind clear to every one. 

 The importance of the manufacture of carbohydrates, which 

 is the peculiar work of green plants, is recognized when it is 

 known that in the manufacture of the other foods carbohy- 

 drates must be used. This means that although carbohydrates 

 are not the only kind of food, they are the necessary start for 

 all other kinds. 



23. The raw material. It is important to know the 

 inorganic substances a green plant uses in the manufacture 

 of carbohydrates. They cannot be rare substances, or 

 vegetation would not be so common. They are water and 

 carbon dioxide. The former needs no explanation; the 

 latter is often called " carbonic acid gas," and is the so- 

 called " impure " gas that accumulates in badly ventilated 

 rooms. Carbon dioxide is everywhere in the air, in very 

 small proportion (about three parts in 10,000), and is more 



