230 LEAVES 



vegetables, etc. It is a chemical classification, which 

 means that it is based on the composition of the mole- 

 cules. Meat is composed of both fat and protein, vege- 

 tables are composed mainly of carbohydrate and protein, 

 and milk is composed of all three. Our bodies appear to 

 be best nourished when our food includes all three of these 

 chemical classes of food, and no one of them in undue 

 proportion. 



Sugar and starch are the most common examples of the 

 carbohydrate class of foods, and both are abundant in 

 plant tissues. Plants are able to change sugar into starch 

 and starch back again into sugar. This is important. 

 Sugar is soluble and so moves readily as a solute through 

 the plant ; starch is insoluble and so does not move through 

 the plant. Since its insolubility makes it independent of 

 the laws of osmosis, starch is an excellent form in which 

 to store food. Storage generally depends upon insolu- 

 bility, just as movement generally depends upon solubility. 

 The cells of a potato are full of starch, and starch is the 

 principal form of food found in storage organs generally. 



Fats and proteins are also quite insoluble, so it is not 

 surprising to find that sugar is the usual transfer form of 

 food in plants. No form of food is more readily soluble ' 

 or movable than sugar, and out of it any living cell appears 

 to be able to form proteins or fats, just as they are able to 

 transform it into starch. 



The molecules of carbohydrates are composed of atoms 

 of carbon, hydrogen, and oxygen. These elements are 

 obtained by the plant from carbon dioxide (CO2) and from 

 water (H2O). In the molecules of carbohydrates there 

 are nearly always just twice as many atoms of hydrogen 

 as of oxygen. Since this is so, it is evident that in car- 



