ORGANIC COMPOUNDS OF PLANTS 187 



all differ in both physical and chemical properties. This 

 is because the elements, C, H and O, are put together in 

 different ways in the three compounds. For example, a 

 pile of bricks may be put together to form one structure, 

 and then again in different ways to form other structures; 

 in each structure there are the same number and kinds 

 of bricks. So in the molecules of starch, dextrin, inulin 

 and cellulose, there are the same number and kinds of 

 atoms, but in each they are combined in a different way. 

 In the study of the composition of plants, organic com- 

 pounds are frequently met with which have the same 

 general composition, but different chemical and physical 

 properties. Whenever two compounds have the same 

 general formula and percentage composition, but differ- 

 ent chemical and physical properties, the difference is 

 said to be one of structural composition. 



Sugar 



242. Classification of Sugars. As commonly used, 

 the term sugar is applied to the product obtained from 

 sugar-cane or sugar-beets. As used in chemistry, it in- 

 cludes a large class of compounds of which maple-, cane-, 

 and beet- sugar are examples of only one division. The 

 two main classes of sugars present in plant bodies are 

 sucrose and dextrose; occasionally, other sugars are found. 



The sucrose group includes cane-, beet-, maple-, milk-, 

 and malt-sugar. These sugars have the general formula 

 C 12 H 22 O n , and are characterized by the molecule contain- 

 ing 12 atoms of carbon. The dextrose group includes 

 glucose, levulose, galactose, and all sugars having the 

 general formula C 6 H 12 O 6 . This group is characterized by 



