60 EXPERIMENTAL PLANT PHYSIOLOGY 



shows how cupric-oxide may be reduced to cuprous- 

 oxide, which is a red-brown precipitate. 



The previous experiment has shown that grape-sugar 

 is a reducing sugar. Cane-sugar is not a reducing sugar, 

 but is converted into one when it is boiled with a trace 

 of mineral acid. The change which takes place is 

 termed hydrolysis and consists in the addition of a 

 molecule of water to the molecule of the non-reducing 

 sugar, by which it is changed or " inverted " into a re- 

 ducing sugar. Thus : 



Ci,H,Ai+H,0 = 2CeHiA 

 Cane-sugar. Water. Grape-sugar. 



Experiment 39 



Aim. — To determine whether the starch formed in 

 the leaf is converted into sugar. 



Method. — A few leaves are picked in the afternoon 

 of a warm, sunny day. They are then put on damp 

 blotting-paper in a well-corked bottle and placed in the 

 dark for about three days. 



When the leaves are picked they contain a large 

 quantity of starch. It has already been shown that 

 starch does not remain in the leaves if the plant is kept 

 in the dark. In this case, however, the removal of any 

 substance from the leaf is prevented by its separation 

 from the plant. 



At the end of three days the following tests are 

 made : 



1. Some of the leaves are tested for starch. 



2. Some of the leaves are tested for sugar. 



3. Some leaves freshly gathered from the plant are 

 tested for sugar. 



Observations. — No starch is found in the leaves 

 although they have been separated from the plant. 



Those, however, that were submitted to the second 

 test were found to contain sugar. 



No sugar reaction is obtained in the case of the 

 freshly gathered leaves. 



