516 Introduction to the Study of Science 



starch grains. If this is true, the presence of starch in the 

 leaves may be detected by the following test. 



Exercise : Test for starch. Prepare a thin starch paste by boil- 

 ing a little starch in about one hundred times its volume of water. 

 When this is cool add a few drops of iodine solution. 1 What is the 

 result? Boil a thin slice of potato in a small quantity of water for 

 five or more minutes. Cool the water and add a few drops of iodine 

 solution. Is the result similar to that with starch? Would you 

 infer that the potato water contains starch? Make the same test 

 with bread and any other food which you think may contain starch. 

 The tests show that iodine solution turns starch a deep violet-blue or 

 blue-black color. 



In the late afternoon get leaves from such plants as geranium, 

 bean, or clover; immerse them in boiling water for a few minutes, 

 and then transfer to warm alcohol in which they should remain for 

 several hours. Alcohol dissolves and removes the green pigment and 

 makes the leaves nearly white. After the color is removed, wash the 

 leaves in water and immerse them for a few moments in iodine solu- 

 tion. What is the result? Does the iodine reveal the presence of 

 starch? If you can find some striped grasses, treat them in the same 

 way. Do all parts of the grasses show starch? 



Conditions upon which leaves depend. The results indicate 

 that the leaves contain starch, one of the carbohydrates manu- 

 factured by the chloroplasts. It is necessary now to determine 

 by experiment the conditions upon which leaves depend in 

 doing this work. The following is a simple and convenient 

 way. 



Exercise. Place some rapidly growing plant like the bean or 

 geranium in the dark for two or three days. When you remove it 

 from the dark, observe the pale sickly appearance of the leaves. 

 Much of the green pigment has disappeared. Cut off one or two 

 leaves and test them as you did the others. Before restoring the plant 

 to the sunlight cover a portion of the upper surface of a large well- 

 exposed leaf with tin-foil or black paper. Some design may be cut 

 out of the tin-foil to expose a part of the leaf. Now place the plant 



1 To prepare iodine solution dissolve five grams of potassium iodide in three 

 or four ounces of water. Then add one gram of iodine and let the mixture 

 stand for ten or twelve hours. An occasional shaking will aid in dissolving the 

 iodine. Add enough water to make one quart. 



