FUNCTIONS OF LEAVES 167 



Carbon dioxide is produced in immense quantities by 

 the decay of vegetable and animal matter, by the respira- 

 tion of animals, and by all fires in which wood, coal, gas, 

 or petroleum is burned. 



Green leaves and the green parts of plants, when they 

 contain a suitable amount of potassium salts, have the 

 power of removing carbon dioxide from the air (or in 

 the case of some aquatic plants from water in which it is 

 dissolved), retaining its carbon and setting free part or all 

 of the oxygen. This process is an important part of the 

 work done by the plant in making over raw materials into 

 food from which it forms its own substance. 



EXPERIMENT XXXIV 



Oxygen-Making in Sunlight. Place a green aquatic plant in a 

 glass jar full of ice-cold fresh water, in front of a sunny window. 1 

 Place a thermometer in the jar, watch the rise of temperature, and 

 note at what point you first observe the formation of oxygen -bub- 

 bles. Remove to a dark closet for a few minutes and examine by 

 lamplight, to see whether the rise of bubbles still continues. 



This gas may be shown to be oxygen by collecting some 

 of it in a small inverted test-tube filled with water and 

 thrusting the glowing coal of a match just blown out into 

 the gas. It is not, however, very easy to do this satisfac- 

 torily before the class. 



Repeat the experiment, using water which has been well boiled 

 and then quickly cooled. Boiling removes all the dissolved gases 

 from water, and they are not re-dissolved in any considerable quantity 

 for many hours. 



1 Elodea, Myriophyllum, Chrysosplenium, Potamogeton, Fontinalis, any of 

 the green aquatic flowering plants, or even the common confervaceous plants, 

 known as pond-scum or " frog-spit," will do for this experiment. 



