Ch. Ill, 3] 



SYNTHESIS OF FOOD 



23 



bon dioxide of the atmosphere is available, but otherwise not. 

 Carbon dioxide cannot pass through the walls of the water- 

 proof epidermis (at least not in appreciable quantity), but 

 it enters the leaf through the slit-like openings, the stomata, 

 the function of which is thus 

 explained. From the stomata 

 it moves along the air pas- 

 sages to every part of the 

 chlorenchyma. 



The formation of grape 

 sugar from carbon dioxide 

 and water is expressed by 

 the following equation, which 

 exhibits the extremes, though 

 not the intermediate steps, of 

 the process. 



6 C0 2 + 6 H 2 - C 6 H 12 6 + 6 2 



Now this equation implies 

 that in the formation of the 

 sugar, free oxygen is pro- 

 duced in volume precisely 

 equal to that of the carbon (se * n G in t^ifwhereby ItTaTbe 



dioxide absorbed. This theo- proved that oxygen is released by 



retical deduction can readily Th^^ £*«,. water 



be tested by experiment, by plant is caught in the water-filled 

 ™™~~ r r • j. j test-tube supported above, and sub- 



means of appliances pictured seqU entiy tested, 

 herewith (Figs. 6 and 7) ; and 



thus the actual production of oxygen, in the indicated vol- 

 ume, is conclusively proved, and all parts of this photo- 

 synthetic equation are found exactly true. It expresses 

 concisely and accurately one of the greatest of all natural 

 processes. 



The absorption of carbon dioxide and release of oxygen 

 thus shown to occur in the photosynthetic formation of 

 grape sugar in leaves explains the widely known fact that 



