10 TEXTBOOK OF PLANT PHYSIOLOGY 



ments of the plant are supplied by the carbon dioxide of the atmos- 

 phere and that the organic substances of the soil play no impor- 

 tant role in carbon nutrition. 



The absorption of carbon, which is necessary for the structure 

 of the plant, from the carbon dioxide of the air, is one of the most 

 important processes of plant life, as well as of the whole organic 

 world. Since animals as well as man lack the power of utilizing 

 the carbon dioxide of the air, they can obtain organic compounds 

 only after these have been manufactured by plants. Hence, 

 the above-mentioned process justifies a close and detailed 

 study. 



In order that the plant may absorb from the air the carbon 

 dioxide necessary for its nutrition, certain external conditions are 

 required. In the first place, the plant must receive sufficient light. 

 In the dark winter season, or in the depth of a room far from a 

 window, water cultures fail. Exact weight determinations have 

 shown that under such conditions the plant not only shows no 

 increase in dry matter, but actually loses weight. Another essen- 

 tial factor is the green color of the plant. Plants that are not 

 green, as for instance fungi or some parasites as Orobanche or 

 Lathraea squamaria, are unable to utilize carbon dioxide. The 

 same is true of the non-green parts of the plant, such as the roots 

 which receive all their food from the leaves. The green leaves, 

 therefore, must be regarded as the special organs of plant nutri- 

 tion as far as the carbon dioxide of the air is concerned. 



2. Methods of Detecting and Studying the Process of Nutrition 

 with Carbon Dioxide. Liberation of Oxygen by the Plant. — At the 

 present time, we not only have at our disposal numerous methods 

 by means of which we are able to detect the process of assimilation 

 of carbon from carbon dioxide by the plant, but also we have 

 methods to study this process quantitatively. The principle of 

 these methods is as follows: 



In order to determine the amount of carbon dioxide which 

 is assimilated by the plant, the following procedure is used: 

 Through a jar, containing the plant, an air current is passed in 

 which the amount of carbon dioxide has been determined before- 

 hand. The non-utilized portion of carbon dioxide is then absorbed 

 by an alkali. This method shows that in bright light a branch of 

 a plant is able to absorb almost all of the carbon dioxide from the 

 passing air, if the current does not move too fast. Boussingault 



