THE CARBOHYDRATES 15 



a-naphthol, sulphuric acid gives a red colour which rapidly 

 changes to purple. This is known as the Molisch reaction ; 

 it is very delicate, and is the best general test for carbohydrates. 

 It is due to the formation of furfural by the action of the acid 

 on the carbohydrate. 



Occurrence and Functions. Carbohydrates are typically 

 vegetable products. They form the chief structural element 

 of plants, and are stored up in large quantities in seeds, roots, 

 tubers, etc. Usually from 50 to 90 per cent, of the dry 

 matter of plants consists of compounds which belong to the 

 carbohydrate group. All the soluble compounds and some 

 of those which are insoluble in water are readily digestible by 

 animals. The remainder are either wholly or partially in- 

 digestible. The digestible carbohydrates may be transformed 

 into other compounds and stored up as tissue in animal bodies. 

 They readily undergo oxidation, and the energy produced 

 may be used to perform work or maintain the temperature of 

 the body. They are, in fact, amongst the most important 

 nutrient substances in the food of farm animals. 



Carbohydrates are produced by animals only in insignificant 

 quantities. They occur only in the juices and fluids, i.e. they 

 play no structural part, and may be regarded, for the most 

 part, as transformation products. 



Classification. The carbohydrates which occur in nature 

 and are of agricultural importance may be divided into three 

 principal groups as follows : 



A. Sugars, e.g. cane sugar, milk sugar, etc. 



B. Amyloses, or polysaccharides, e.g. starch, cellulose, 

 dextrine, etc. 



C. Mucilaginous substances of unknown or indefinite 

 composition. 



Sugars. The sugars are all soluble in water and can be 

 crystallised from saturated solutions. They diffuse through 

 semi-permeable membranes, exert considerable osmotic pres- 

 sure, and are therefore easily absorbed by animals. They are 

 neutral to litmus and exhibit a characteristic sweet taste. 

 Sugars are strictly non-volatile. When heated in the dry 

 state they are changed into a black, fusible, amorphous, soluble 



