THE LIBERATION OF ENERGY IN PLANTS 67 



and usually light is seen. The energy set free by the oxida- 

 tion of fuel may be transformed at one time into light, at 

 another time into motion, or again into heat. 



It is probably true that the liberation of energy in living 

 plants is somehow due to the action of oxygen. The pro- 

 cess, however, is doubtless extremely complicated, and just 

 what takes place no one knows. Certainly oxygen in some 

 form is essential for the life of every plant and animal. That 

 this is true of plants, the following experiment will show. 



79. To prove that seeds need air in order to grow. 



Laboratory Study No. 39. Demonstration or Home Work. 



Secure two wide-mouthed bottles and place in the bottom 

 of each a wet sponge or some wet blotting .paper, and pour 

 enough water in each bottle just to cover the sponge or paper. 

 Fill both bottles with pea seeds that have been soaked in 

 water for twenty-four hours. Insert a tightly fitting cork 

 into the mouth of one of the bottles to exclude the air. Leave 

 the other bottle open to the air, and add enough water from 

 day to day to make up for the loss by evaporation. Put 

 both bottles in a warm place. 



1. Describe this experiment, showing in what respects the 



conditions are the same for both groups of seeds. 



2. In what one respect do the two groups of seeds differ ? 



3. At the end of several days examine both bottles of seeds, 



and state your observation concerning the amount of 

 growth in each bottle. 



4. State clearly your conclusion as to the necessity of air for 



growth of pea seedlings. 



80. Relation of oxygen and carbon dioxid to oxidation. 

 We have now demonstrated that seedlings will not grow 

 without air. Biologists have proved conclusively that oxygen 

 is the element in the air that is essential for the work of all 

 plants, and that without it they die. Hence, we may con- 

 clude, as in the case of the furnace, that the necessary energy 



