54 



PLANT GROWTH AND NUTRITION 



drops of iodine solution/ we find that the mixture in the test tube 

 turns purple or deep blue. It has been learned after many experi- 

 ments that starch, but no other known substance, is turned purple 



Colors seen in test for starch, 



A, before; B, after, a 

 grain of corn has been tested 

 with iodine. 



or dark blue by iodine. Therefore, iodine solution is used as a 

 test for the presence of starch. 



Starch in the Bean. — If we mash up a little piece of a bean coty- 

 ledon which has been previously soaked in water, and test with 

 iodine solution, the characteristic blue-black color appears, show- 

 ing the presence of starch. If a little of the stained material is 

 mounted in water on a glass slide under the compound microscope, 

 we shall find that the starch is in the form of little ovoid bodies 

 called starch grains (figure, page 60). The starch grains and 

 other food products are made use of by the embryo. 



Test for Oils. — If a substance is rubbed on brown paper or is 

 placed on paper and then warmed in an oven, the presence of oil 

 will be shown 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 



1 Iodine solution is made by simply adding a few crystals of iodine to 95 per 

 cent alcohol ; or, better, take by weight 1 gram of iodine crystals, i gram of iodide 

 of potassium, and dilute to a dark brown coior in weak alcohol (35 per cent) or 

 distilled water. 



2 The proportion of oil in beans is small, ft is better to try this test on a walnut. 



