FUNCTIONS 231 



bohydrate synthesis there is certain to be an excess of 

 oxygen ; that is, if the molecules CO 2 and H 2 O are used 

 in equal numbers (as is believed to be the case), there are 

 two atoms of hydrogen for every three of oxygen, and 

 since in the finished product there must be but one atom 

 of oxygen for each two of hydrogen, there is an excess of 

 oxygen under such circumstances. 



The oxygen given off by plants as a by-product of active 

 photosynthesis under good illumination is much greater 

 in amount than the oxygen consumed by them in respira- 

 tion. Since, therefore, the green parts of plants give off 

 more oxygen than they take in, early observers were led 

 to believe that plants breathe in a way which is just the 

 opposite of the breathing of animals. It was believed, 

 since plants absorb carbon dioxide and give off oxygen, and 

 since animals absorb oxygen and give off carbon dioxide, 

 that there is herein an admirable arrangement whereby 

 the supply of both of these gases is maintained in the air. 

 No doubt the oxygen given off by plants does to some 

 extent aid animal life by keeping up the oxygen content 

 of the air, and no doubt animal life aids plant life to some 

 extent by being a source of carbon dioxide, but to say 

 that " plants breathe one way and animals another " is 

 to state the matter very improperly. Plants respire in the 

 way we do just as truly as we do ourselves, but they also 

 give off oxygen in large quantities in connection with a 

 process which does not occur in us. 



The real, inner organs of photosynthesis are, as you 

 know, the chloroplasts. They are composed of chlorophyll 

 and protoplasm. (See page 74.) Light is essential to 

 their work. Just what part of this process is done by the 

 chlorophyll and just what part by the protoplasm is not 



