68 



PLANT GROWTH AND NUTRITION 



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Test for grape sugar. 



A Test for Grape Sugar. — Place in a test tube the substance to 

 be tested and heat it in a little water so as to dissolve the sugar. 



Add to the fluid twice its bulk of 

 Fehling's solution/ which has been 

 previously prepared. Heat the mix- 

 ture, which should now have a blue 

 color, in the test tube. If grape sugar 

 is present in considerable quantity, the 

 contents of the tube will turn first a 

 greenish, then yellow, and finally a 

 brick-red color. Smaller amounts will 

 show less decided red. No other sub- 

 stance than sugar will give this reac- 

 tion. If Benedict's test ^ is used, a 

 colored precipitate will appear in the 

 test tube after boiling. 



Starch changed to Grape Sugar in 

 the Corn. — That starch is being 

 changed to grape sugar in the germi- 

 nating corn grain can easily be shown if we cut lengthwise through 

 the embryos of half a dozen grains of corn that have just begun 

 to germinate, place them in a test tube with some Fehling's solu- 

 tion, and heat almost to the boiling point. They will be found 

 to give a reaction showing the presence of sugar along the edge 

 of the cotyledon and between it and the endosperm. 



Digestion. — This change of starch to grape sugar in the corn 

 is a process of digestion. If you chew a bit of unsweetened cracker 

 in the mouth for a little time, it will begin to taste sweet, and if 

 the chewed cracker, which we know contains starch, is tested 

 with Fehling's solution, some of the starch will be found to have 

 changed to grape sugar. Here, again, a process of digestion has 

 taken place. In both the corn and in the mouth, the change is 

 brought about by the action of peculiar substances known as 

 digestive ferments, or enzymes. Such substances have the power 

 under certain conditions to change insoluble foods — solids — into 



1 Directions for making these solutions will be found in Hunter's Laboratory 

 Problems in Civic Biology. 



