74 THE FEEDING OF ANIMALS 



found in the whey, from which the milk-sugar of com- 

 merce is obtained. Very soon after milk is drawn, unless 

 it is heated to the point of sterilization, or is treated with 

 some antiseptic, the lactose begins to diminish in quantity, 

 being converted into lactic acid through the action of 

 lactic-acid organisms (bacteria). Sour milk, therefore, 

 is different from sweet in containing less sugar or none 

 at all. 



100. The sugars as a class. — ^When considered from 

 the standpoint of efficiency, the sugars are among the 

 most valuable of aU the carbohydrates, although in quan- 

 tity they are less important than the starches, at least 

 in raw food materials. 



Unlike starch, they are foimd in solution in the sap of 

 growing plants. It is probable that these are the forms in 

 which carbohydrate material is transferred from one part 

 of the plant to another. It is easy to see that some such 

 medium of exchange is necessary. The actual production 

 of new vegetable substance takes place in the leaves. 

 When, therefore, cell-walls and starch grains are to be 

 constructed in the stem and fruit, the building-material 

 must be carried from the leaves to these parts in forms 

 which will readily pass through intervening membranes. 

 Excepting certain soluble compounds, closely related to 

 starch, the sugars appear to be the only available bodies 

 fitted for this office. 



It is very seldom that a plant contains only a single 

 sugar. Generally two or more sugars are found together. 

 This is especially the case in the corn plant, sorghum, and 

 the fruits, and the proportions of each depend somewhat 

 on the stage of growth of the plant. 



101. Other more complex poly-saccharides. — ^This 

 group includes a large nimiber of carbohydrates that may 



