WORK OF LEAVES: RESPIRATION III 



185. Oxygen is absorbed by the germinating wheat. 



The constituent of the air* which is removed under these con- 

 ditions is one which, on undergoing a chemical change in the 

 germinating wheat, unites with some element in the wheat to 

 form the carbon dioxide given off. From the abundance of 

 carbon dioxide given off it is plain that the carbon is the element 

 in the wheat with which the constituent absorbed from the air 

 unites. Nitrogen, it is plain, cannot unite with the carbon to 

 form carbon dioxide. It must then be the oxygen from the air 

 which is absorbed by the wheat, and which unites with the car- 

 bon in the wheat to form the carbon dioxide given off. The 

 oxygen absorbed by the germinating seeds unites to form some 

 of the substance of the seedlings, and the carbon and oxygen 

 which unite to form the carbon dioxide come from the sub- 

 stance of the seedlings. The oxygen absorbed then replaces that 

 which is given off as a waste product in the carbon dioxide. 

 The rising of the potash or barium hydrate solution in the tube 

 in this experiment indicates to us then that oxygen is taken from 

 the air during the respiration of the seeds. This is exactly 

 what takes place when we breathe, or in animal respiration. 

 Oxygen is inhaled and carbon dioxide is exhaled. 



186. Respiration in opening leaf buds or flower buds. 

 Leaf buds or flower buds are excellent objects to use in showing 

 the excretion of carbon dioxide. If a handful of the buds is 

 placed in a jar as described in paragraph 182 for the germin- 

 ating peas, in the course of ten to twelve hours they will be 

 ready for the test with the barium hydrate and the lighted 

 splinter or taper. The buds can be obtained throughout the 

 spring, summer and autumn in the open, and during the winter 

 in greenhouses. 



187. Leaves as organs of respiration. The leaves are 

 organs of respiration. The stomates provide the openings for 

 the interchange of gases. The gases diffuse through the inter- 

 cellular spaces and reach all parts of the leaf. The gas entering 



* See paragraph 168, where the constituents are given as oxygen, nitro- 

 gen, and carbon dioxide. 



