Metabolism; Digestion and Translocation 255 



that is, producing dextrose and levulose. The hydrolysis 

 of the polysaccharids may involve a series of changes in 

 which water is taken up and ultimately n molecules of 

 hexose sugar (or sugars) are split off. 



143. Sugars. - The commoner forms of sugar found 

 in plants are sucrose (cane-sugar), glucose (dextrose), 

 and fructose (levulose). Glucose and fructose are ap- 

 parently important products in the metabolism of cells 

 usually, but these compounds are often promptly used 

 in general metabolism, especially in the building up of 

 other products. There may be no accumulation of them 

 in the plants where they are being constantly manufac- 

 tured. Accumulation does occur, however, particularly 

 in ripening fruits, such as the grape, peach, prune, and 

 date. In the raisin the characteristic brown nodules of 

 this sugar may be seen. Indirectly these sugars are of 

 much commercial value, for sweetness and flavor together 

 determine the prices paid for fruits, prices which are in 

 general far above their actual food value. 



The monosaccharids are reducing sugars, precipitating 

 heavy metals from solutions of their salts upon heating. 

 Maltose also possesses this quality, but cane-sugar does 

 not. A standard test for reducing sugars is obtained by 

 Fehling's solution (see Laboratory work), consisting of 

 copper sulfate in an alkaline solution of potassium sodium 

 tartrate. Upon boiling with this solution a brick red 

 precipitate of cuprous oxide is produced. 



Sucrose is the form in which sugar commonly accumu- 

 lates in plant cells. From the stems of the sugar cane and 

 from the root of the sugar-beet there were extracted during 

 1909 nearly 15,000,000 tons of commercial sugar. The 



