GROWTH 



219 



bonic acid to the plant under the glass bell ; and, 

 still more, how can we learn that the plant actually 

 uses it ? 



We know that the activity of plants and animals is 

 diametrically opposed with regard to the atmospheric 

 gases. Plants absorb carbonic acid and give off 



Fig. 64. 



oxygen ; animals absorb oxygen and give off carbonic 

 acid. Hence, if we placed an animal under the glass 

 bell along with the plant both would thrive together. 

 But we can substitute an apparatus for the animal 

 (fig. 64, B), which, so far as the exchange of gases goes, 

 will act precisely in the same way as an animal would. 

 It will be in a certain sense an artificial animal. This 

 is how the apparatus is arranged. A liquid greedily 

 absorbing the oxygen of the air is poured into a glass 



