THE BREATHING OF THE PLANT 



73 



air-tight ; it is well, therefore, to seal it with paraffin- 

 wax. 



The conditions of the experiment must now be con- 

 sidered. There is a definite amount of air available for 

 the use of the peas, namely, the amount contained in 

 the bottle and the glass-tubing. If this amount is 

 increased the liquid inside the tube will be pressed 

 down below the level of that out- 

 side it ; if, on the other hand, the 

 amount should decrease, the level 

 of the liquid inside the tube will 

 rise; while, if no change occurs in 

 the volume of air, the liquid inside 

 the tube will remain at the same 

 level as that without. It is known 

 already, from the last experiment, 

 that carbon-dioxide will be given 

 out ; this does not bring about any 

 increase in the volume of gas in the flask, because it 

 is at once absorbed by the potash solution. Therefore, 

 if any change is observed in the volume of air in the 

 flask, such change must be due to the oxygen taken up 

 by the peas. 



Observation. — The coloured liquid rises slowly up 

 the tube. 



Inference. — A plant absorbs oxygen from the air. 



{Note. — No account has been taken of the amount of 

 carbon-dioxide originally in the flask ; this is present in 

 so small an amount as to be practically negligible.) 



It has thus been proved that plants breathe just as 

 animals do ; that is, they take oxygen from the air, and 

 give back carbon-dioxide in its place. 



Fig. 35 



Experiment 44 



Aim. — To determine the amount of oxygen that is 

 taken in the process of respiration. 



Method. — The apparatus set up for Experiment 43 

 (Fig. 35) serves to show very roughly the amount of 



