The Breathing of Plants^ or Respiration 63 



The phosphorus begins to bum, that is, it unites with the 

 oxygen, forming dense white fumes. After a time the fumes 

 disappear. Where have they gone ? The water has absorbed 

 them and formed phosphoric acid. What else has happened ? 

 The water has risen and fills about one-fifth of the jar. The 

 acid occupies less space than the gas fumes, so the weight of 

 the air outside the jar forces the water up to take the place of 

 the oxygen. The nitrogen does not burn, but fills the remainder 

 of the jar. 



Water also absorbs CO., and forms carbonic acid; but 

 there is such a small amount that you would not be able to see 

 the water rise. 



It is oxygen that we need for breathing. Nitrogen dilutes 

 the oxygen enough for us and for land-plants to breathe. But 

 fish and water-plants require the oxygen diluted still more with 

 water. If oxygen were not diluted with nitrogen we should 

 feel as uncomfortable " as a fish out of water." 



When a match burns, the oxygen combines with the carbon 

 of the wood and makes smoke or carbon dioxide (COo). So 

 when we breathe, the oxygen that we inhale combines with 

 the carbon substance of our bodies and forms carbon dioxide, 

 which we exhale. 



Similarly, when a plant breathes, the oxygen it inhales com- 

 bines with the carbon substances of the plant, and the plant also 

 exhales carbon dioxide. Plants and animals also exhale water. 



How do we know that plants inhale oxygen and exhale 

 carbon dioxide ? 



Respiration or Breathing in Germinating Seeds. — 

 Soak a handful of peas in water for twenty-four hours. Remove 

 from the water and put them in a glass jar. Cover tightly and set 

 aside for twenty-four hours. Light a splinter, uncover the jar, 

 and thrust the flame into the jar. It is extinguished. The 

 oxygen which was in the jar has been used by the seeds. 

 They have given off a gas which suffocates the flame. Place 

 a short piece of lighted candle in the bottom of a tumbler. 

 Pour the gas over them. It is heavy and puts out the flame. 

 What is it ? 



To show that Carbonic Acid Gas or Carbon Dioxide 



