THE FOOD OF THE PLANT 59 



Inferences. — Grape-sugar can be recognized by the 

 red-brown precipitate formed when a solution of it is 

 boiled with a few drops of copper sulphate solution and 

 excess of potash; in the case of cane-sugar the same 

 precipitate is formed in the solution, but not until it 

 has been boiled with an acid. 



These reactions may cause a little difficulty at first, 

 but the tests must be satisfactorily applied before pro- 

 ceeding further. In order to give more practice several 

 plant-storage organs should be tested for sugar. 



Experiment 38 



Aim. — To test for sugar in the carrot, onion, beetroot, 

 turnip. 



Method. — Extracts of the vegetables are made and 

 each is tested as in Experiment 37. 



Observations. — Grape-sugar is present in the carrot, 

 onion, and turnip. 



Cane-sugar is present in the beetroot. 



The chemical changes that underlie these reactions 

 will easily be understood by those who have some 

 knowledge of the subject. 



Copper sulphate and potash react, giving a flocculent 

 precipitate of^ copper-hydroxide : 



CuS0,-f-2 KOH = Cu(OH)2-i-K2SO, 



The copper-hydroxide precipitate is then dissolved 

 again by the addition of excess of potash. 



Copper-hydroxide is made up of cupric-oxide and 

 water. Thus : 



Cu(OH)2 = CuO-FH20 



Now certain sugars have the power of taking oxygen 

 from other substances and "reducing" them to less 

 oxidised forms. The equation 



4CuO = 2Cu„0-hO„ 



