The Respiration of Plants 85 



haled. In plants, as in animals, this process takes place in all 

 living cells, but in plants there are no special organs like the 

 lungs in animals, which assist in the inhalation and exhalation 

 of the gases. Respiration has been regarded as a form of 

 combustion, but it is a more complex and a vital process and 

 so cannot be directly compared to the burning of a piece of 

 wood. 



How do we know that plants inhale oxygen and exhale 

 carbon dioxide ? 



Ex. 32. 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 

 into the tumbler (it is heavier than air). It puts outthe flame. What is it ? 



Ex. 33. To show that Carbonic Acid Gas or Carbon Dioxide is 

 given off when Plants Respire. Make some baryta-water by dissolving 

 barium hydroxide in water, and allow it to settle. Pour some of the clear 

 water into a test-tube ; breathe into it through a glass tube. The water 

 becomes cloudy and a white film of barium carbonate collects on the sur- 

 face. 1 Place some baryta-water in a shallow dish. Uncover the jar of 

 peas and pour the gas into the dish. A white film appears on the surface. 

 The peas have exhaled the same gas which you breathed into the jar, 

 carbon dioxide. 



Ex. 35. Another pretty experiment shows that carbon dioxide is given 

 off by plants while respiring. Place heads of flowers daisies just open- 

 ing are good ones in a flask. Invert the flask in soda-limewater, which 

 also absorbs carbon dioxide. Pour some mercury into the soda-limewater. 

 In a short time the mercury will be drawn up into the neck of the flask as 

 the carbon dioxide is absorbed. The mercury will continue to rise as long 

 as the carbonic acid gas is absorbed by the solution. 



Place the flask in the dark and see if the mercury continues to rise. 

 Mark the height at which the mercury stands at night, and again in the 

 morning. A change in temperature affects the height. When the flask 

 cools, the gas within occupies less space and the mercury rises. 



Plants get energy to do work by respiring. Un- 

 less our rooms are supplied with plenty of fresh air we become 

 dull and unable to work. Breathing decreases our weight but 



1 Limewater may be used instead. The results are not quite so good. 

 These liquids should be kept corked. 



