112 



PHYSIO LOG Y. 



seen at some distance from the table as it rises in the tube. In 

 the experiment from which the figure was made for the accom- 

 panying illustration, the solution had risen 

 in 6 hours to the height shown in fig. 102. 

 In 24 hours it had risen to the height shown 

 in fig. 103. 



225. Why the solution of caustic 

 potash rises in the tube. Since no air can 

 get into the thistle tube from above or 

 below, it must be that some part of the 

 air which is inside of the tube is used up 

 while the wheat is germinating. From 

 our study of germinating peas, we know 

 that a suffocating gas, carbon dioxide, is 

 given off while respiration takes place. 

 The caustic potash solution, or the baryta 

 water, whichever is used, absorbs the car- 

 bon dioxide. The carbon dioxide is heavier 

 than air, and so it settles down in the tube 

 where it can be absorbed. 



226. Where does the carbon dioxide 

 come from ? We know it comes from the 



growing seedlings. The symbol for carbon dioxide is CO 2 . The 

 carbon comes from the plant, because there is not enough in 

 the air. Nitrogen could not join with the carbon to make CC>2. 

 Some oxygen from the air or from the protoplasm of the grow- 

 ing seedlings (more probably the latter) joins with some of the 

 carbon of the plant. These break away from their association 

 with the living substance and unite, making CC>2. The oxygen 

 absorbed by the plant from the air unites with the living sub- 

 stance, or perhaps first with food substances, and from these the 

 plant is replenished with carbon and oxygen. After the demon- 

 stration has been made, remove the glass plate which seals the 

 thistle tube above, and pour in a small quantity of baryta water. 

 The white precipitate formed affords another illustration that 

 carbon dioxide is released. 



Fig. 103. 



Apparatus to show 

 respiration of germinat- 

 ing wheat. 



