COMPOSITION OF LIFELESS AND LIVING THINGS 25 



mineral matters containing nitrogen, sulphur, and phos- 

 phorus obtained from the soil, the living plant manufactures 

 protein. 



35. Do green plants give off a gas in sunlight? Labora- 

 tory Study No. 18. 



Into a glass cylinder containing water fresh from the faucet 

 put a small amount of water plant (Elodea, Spirogyra, or 

 Milfoil), holding it to the bottom of the tall jar by means of 

 a weight if necessary. Stand the cylinder in direct sunlight. 



1. Describe the preparation of the experiment. 



2. What do you observe coming off from the plant ? (These 



bubbles of gas have been proved to be composed of 

 oxygen.) 



36. Do green plants give off a gas when deprived of sun- 

 light? Laboratory Study No. 19. 



Place the glass cylinder prepared as directed above in 

 darkness for several hours (or, still better, a second cylinder 

 should be used for comparison). 



1. In what respects do Experiments 18 and 19 differ? 



2. Do you see any bubbles as long as the cylinder is kept 



in the dark? 



3. Under what condition, therefore, does a green plant give 



off oxygen? 



37. The oxygen supply for animals. We have seen that 

 starch is made of carbon dioxid (C0 2 ) and water (H 2 0). By 

 repeated experiments biologists have proved that in the pro- 

 cess of manufacturing carbohydrates more oxygen is present 

 in the C0 2 and H 2 than is needed. This is the oxygen we 

 have seen given off by the green water plant in sunlight. 

 Every green plant gives off oxygen into the air when manu- 

 facturing carbohydrates. Hence, in this process the carbon 

 dioxid is constantly being taken from the air and a fresh 

 supply of oxygen set free. 



