THE ELEMENTS OF PLANT FOOD 



37 



FIG. 8. Granules of Po- 

 tato Starch. 



'^ 



merce is obtained from potatoes and from corn. It is 

 made up of small grains which, 

 when seen under the microscope, 

 show each grain to be made up of 

 concentric rings with the nucleus at 



one side. These 



grains when 



heated in water 



nearly to the boil- 



ing point form a 



pasty mass. This is caused by the 



swelling and bursting of the starch 



grains. Fine white flour contains about 

 74 per cent of starch, rice contains about 79 per cent, 

 and potatoes about 16 per cent. 



Note. The presence of starch in solution may be shown by the 

 iodine test. Dissolve a few grains of iodine in alcohol. Put into a 

 glass of water a drop of boiled starch. Pour into this mixture a 

 few drops of the dissolved iodine. The mixture will be colored blue. 

 The color will be apparent if only a very small amount of starch is put 

 into the water. 



Boil the colored solution and note that the color disappears. It 

 reappears on cooling. 



Dextrin (C 6 H 10 O 5 ). When heated to about 212 

 starch may be changed to a soluble form, known as 

 dextrin. The chemical composition remains the same 

 as starch, but the atoms are differently arranged in the 

 molecule. Dextrin is soluble in cold water and makes 

 a mucilage. The mucilage on postage stamps is made 

 largely of dextrin. The starch of the loaf of bread has 

 been changed into dextrin in the brown crust. The 

 ease with which toast is digested is due to the same 

 change in the starch. 



