PLANT GROWTH AND NUTRITION 



61 



Test for starch. 



turns purple or deep blue. It has been discovered by experiment 



that starch, and no other known substance, will be turned purple or 



dark blue by iodine. Therefore, iodine 



solution has come to be used as a test 



for the presence of starch. 



Starch in the Bean. If we mash 



up a little piece of a bean cotyledon 



which has been previously soaked in 



water, and test for starch with iodine 



solution, the characteristic blue-black 



color appears, showing the presence of 



the starch. If a little of the stained 



material is mounted in water on a glass 



slide under the compound microscope, 



you will find that the starch is in the 



form of little ovoid bodies called starch grains. The starch grains 



and other food products are made use of by the growing plant. 



Test for Oils. If the substance 

 believed to contain oil is rubbed on 

 brown paper or is placed on paper and 

 then heated in an oven, the presence 

 of oil will be known by a translucent 

 spot on the paper. 



Protein in the Bean. Another 

 nutrient present in the bean cotyledon 

 is protein. Several tests are used to 

 detect the presence of this nutrient. 

 The following is one of the best 

 known : 



Place in a test tube the substance 

 to be tested; for example, a bit of 

 hard-boiled egg. Pour over it a little 

 strong (60 per cent) nitric acid and heat 

 gently. Note the color that appears 



a lemon yellow. If the egg is washed in water and a little 



ammonium hydrate added, the color changes to a deep orange, 



showing that a protein is present. 



Test for protein. 



