196 FIRST YEAR SCIENCE 



The great function of the leaf is to manufacture plant 

 foods. The leaf is so constructed that air can enter it 

 and come in contact with its living cells, as does the water 

 coming up from its roots. The circulation of water in 

 the leaf was seen in Experiment 99. There is in the 

 living cell of the leaf a green substance called chlorophyll. 

 This has the power to utilize the energy of sunlight and 

 to combine carbon dioxide, a gas which makes up a small 

 part of the air, with water from the roots, forming a sub- 

 stance which probably at first is grape sugar, but which in 

 many leaves is changed at once into starch. 



Experiment 100. Boil a few fresh bean or geranium leaves for a 

 few minutes in a beaker of water. Pour off the water and pour on 

 enough alcohol to cover the leaves. Warm the alcohol by putting the 

 beaker in a dish of hot water. When the leaves have become color- 

 less, remove from the alcohol and wash. Place the leaves in another 

 beaker and pour on a solution of iodine. (This solution can be made 

 by dissolving in 500 cc. of water, 2 grams of potassium iodide and 

 gram of iodine. The solution should be bottled and kept.) If the 

 leaves turn dark blue or blackish, starch is present. 



Experiment 101. Place a thrifty geranium or other green plant 

 in darkness for two or three days and then treat the leaves as was 

 done in Experiment 100. Do they show the presence of starch ? 

 The direct presence of the sun's energy in the form of light is neces- 

 sary for the formation of starch in the leaves. 



It was found in Experiment 100 that leaves exposed to 

 the sun contained starch, and in Experiment 101 that 

 leaves which had been deprived of sunlight did not have 

 starch. The starch disappeared while the plant was in 

 darkness. Carbon dioxide is composed of carbon and 

 oxygen ; and water of hydrogen and oxygen. In the 

 manufacture of starch by the chlorophyll some -of the 

 oxygen is not used and becomes a waste product which 

 the leaves throw off. This is seen in .Experiment 102. 



