RESPIRATION 119 



189. 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 carbon 

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

 absorbed by the germinating seeds encourages chemical change 

 in the substance of the seedlings, the carbon dioxide formed 

 from the union of carbon and oxygen is a waste product of this 

 change. The oxygen absorbed then in general replaces that 

 which is used in forming the carbon dioxide. The rising of the 

 potash or barium hydrate solution in the tube in this experi- 

 ment 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 in- 

 haled and carbon dioxide is exhaled. 



190. 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 186 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. 



191. Openings in plants for the interchange of gases. 

 In the leaves the stomates (paragraph 144) form 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 172, where the constituents are given as oxygen, nitrogen, 

 and carbon dioxide. 



