64 THE NATURE AND WORK OF PLANTS 



point. Hold the tube with this end pointing di- 

 rectly toward the sun, and put the eye at the other 

 end. Can any light be seen through the leaf ? If 

 so, add another leaf and test again. How many 

 leaves are necessary to take up all the light? If 

 several kinds of leaves are tested in this way, it 

 will be found that those of different species vary 

 greatly in their light-absorbing power. Do you 

 notice any connection between this capacity and 

 the place in which the species grows naturally? 



74. Formation of food in ehlorophyl-bearing organs. 

 — The carbon dioxide of the air is used by the 

 plant in making food. This compound consists of 

 two volumes of oxygen and one of carbon. It is 

 taken into the leaf through openings on the lower 

 surface and passes through the thin walls of the 

 cells containing chlorophyl. It is then split in two 

 parts, and the carbon is combined with the oxygen 

 and hydrogen of water to form a sugar. Most of 

 the oxygen is thrown off. All of these steps may 

 not be followed, even in the best-equipped laboratory, 

 but if the leaf of any water plant growing in the 

 sunlight is observed, it may be seen to give off small 

 bubbles of oxygen. This may be best seen by plac- 



