SECTION II 



THE BREATHING OF THE PLANT 



In the breathing process of animals a larger propor- 

 tion of carbon-dioxide is given back to the air than is 

 taken from it. This was proved in Experiment 21, 

 where it was seen that the amount of atmospheric 

 carbon-dioxide is being continually increased as a result 

 of the breathing of animals. 



Now plants, like animals, are living things. They 

 feed. They have the capacity for growth. Do they 

 also breathe ? 



The first step towards finding an answer to this 

 question is to determine whether or no plants give out 

 carbon-dioxide. 



Just at first this may look like a contradiction of 

 what has been already learnt. But it is not neces- 

 sarily so. 



It has been seen that, given certain conditions, a 

 plant takes in carbon-dioxide from the air in its feeding 

 process (pa^e 56). But this absorption of carbon- 

 dioxide for food does not, in any way, hinder the plant 

 from giving off the same kind of gas in the absolutely 

 distinct process of breathing. 



In trying to find out, however, whether there is 

 evolution of carbon-dioxide, great care must be used 

 to prevent the plant from taking back, as food-material, 

 the carbon-dioxide that it may have given out by 

 breathing. If the reabsorption of the carbon-dioxide 

 is not prevented, the lime-water test will, of course, 

 fail. 



Obviously, then, the plant must not be allowed to 

 obtain food from the air while breathing experiments 



70 



