382 LIFE ON THE EARTH 



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 119. 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 com- 

 bine the carbon and the oxygen of carbon dioxide taken 

 from the air with the hydrogen and the oxygen of water 

 taken from the soil, thus forming a substance which prob- 

 ably at first is grape sugar, but which in many leaves is 

 changed at once into starch. 



Experiment 120. 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 

 colorless, 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 121. 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 120. 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 120 that leaves exposed to 

 the sun contained starch, and in Experiment 121 that 

 leaves which had been deprived of sunlight did not have 

 starch. The starch disappeared while the plant was in 

 darkness. Not all of the oxygen from the carbon dioxide 

 and the water is used in the manufacture of starch by the 

 chlorophyll, and so some of the oxygen becomes a waste 



