330 NATURE STUDY AND AGRICULTURE 



the opposite of animal respiration. Real respiration in 

 plants was entirely overlooked because the volume of the 

 gases used in food making is so much greater than that used 

 in respiration as to quite obscure the latter process except 

 at night. 



We should now see clearly that all living things "breathe 

 in" oxygen and " breathe out" carbon dioxide, for this 

 means respiration which is essential to all life. But green 

 plants, quite in addition to this process, also can do the 

 work of food making, upon which all the rest of the living 

 world depends, and which involves its own characteristic 

 intake and outgo of gases quite independently of respira- 

 tion. 



It is well to contrast sharply this photosynthesis with 

 respiration, for they are very often confused. Photo- 

 synthesis requires light, involves an intake of carbon 

 dioxide and an outgo of oxygen, goes on only in cells con- 

 taining chloroplasts, manufactures food, and can be sus- 

 pended periodically (as at night). Respiration does not 

 require light, involves an intake of oxygen and an outgo of 

 carbon dioxide, goes on in every living cell, consumes food, 

 and must go on while life goes on. 



The processes just described (photosynthesis, diges- 

 tion, assimilation, respiration) have to do with the ordinary 

 life and growth of the tree, and they are often spoken of 

 together as the work of nutrition. The activities of plants, 

 however, include not only nutrition, but also reproduction. 

 The work of nutrition provides for the maintenance of the 

 individual; the work of reproduction provides for the 

 maintenance of the race. The tree, which we have selected 

 as a representative plant, provides for reproduction by the 



