PLANT CONSTITUENTS — CARBOHYDRATES 275 



Whether cane sugar is originally formed in the plant from 

 the monosaccharoses, glucose and fructose, is uncertain. From 

 its wide distribution in plant juices it seems probable that it is a 

 translocation form of carbohydrate from which the glucose 

 and fructose in fruits are formed. 



In all plants where cane sugar is present it is an important 

 food material possessing approximately the same value as 

 starch. Because of its abundance in sugar cane and sugar 

 beet it is one of the most valuable of the carbohydrates found 

 in agricultural crops. The amount present in some of the 

 common crops will be seen from the table given later. 



Maltose. — This disaccharose is a transition substance in 

 the conversion, by diastase, of reserve starch into glucose and is 

 found chiefly in sprouting grain or malt. It has also been 

 found in the sap of green leaves. As it is simply transition 

 material in plants it plays no large part in crops as a food sub- 

 stance. In malted grain produced artificially for the manu- 

 facture of alcohol by fermentation it is present in large amount. 

 The waste mash from distilleries contains some unchanged 

 maltose and glucose and is valuable as an animal food. In 

 preparing some human cereal foods the grain is sometimes 

 malted and in these maltose sugar is present. 



Monosaccharoses. — The only two monosaccharoses that 

 are present as such in plants are the two hexoses glucose and 

 fructose. These two sugars occur in the leaf sap and, as has 

 been stated, glucose is probably the first photosynthesized carbo- 

 hydrate. Except in their physiological relations, the most 

 important occurrence of glucose and fructose is not in the leaves 

 but in the juice of fruits. In most fruit juices the two sugars 

 occur together, from which fact we derive their common names 

 of grape sugar and fruit sugar. In these juices they have 

 probably been produced by the hydrolysis of cane sugar, which 

 is undoubtedly their precursor. In almost all plant juices and 

 in seeds some glucose and fructose are usually found. When 

 present in plants, their food value is practically that of starch 

 and the other sugars. 



