396 



SMITH'S INTERMEDIATE CHEMISTRY 



taken up by plants in the form of atmospheric carbon dioxide. 

 It is admitted through minute openings (stomata), situated 

 mainly in the surface of the leaves. 



The Reaction Involved. Comparison of the formulae of 

 carbon dioxide C02 and of any plant substance, like starch 

 (CeHioOs)*, shows at once that the latter contains a far smaller 

 proportion of oxygen, relatively to the amount of carbon, than 



does the former. Hence, during the 

 digestion or assimilation of the carbon 

 dioxide by the plant, this compound 

 must be reduced. In point of fact, the 

 chlorophyll (green coloring matter) and 

 protoplasm in the leaves act upon the 

 carbon dioxide, causing oxygen to be 

 liberated : 



6CO 2 + 5H 2 O -t C 6 H 10 5 + 60 2 1. 



This action goes on only in the sun- 

 light. The steps by which sugar, 

 starch, and cellulose are manufactured 

 by the plant out of water and carbon 

 dioxide, are not yet perfectly under- 

 stood. But the liberation of the oxygen is easily shown Jby placing 

 a green plant under water in a jar, and setting the jar in the sun- 

 light (Fig. 98) . Bubbles of gas appear on the leaves, grow larger, 

 and then detach themselves and rise to the top. The gas re- 

 lights a glowing splinter of wood, and is pure oxygen. 



The results of recent investigations suggest the following stages 

 of the reaction : 



(1) Carbonic acid, formed by the union of water and carbon 

 dioxide, is reduced to formaldehyde (p. 348), oxygen being liber- 

 ated: 



H 2 C0 3 - H.CHO + 2 . 



FIG. 98 



