164 PLANT PHYSIOLOGY 



matter but also a means of storing energy in the form of a com- 

 bination of carbon and hydrogen capable of being oxidized. 

 This potential energy is accumulated from the radiant energy 

 falling on the leaf, which is caught by the chlorophyll and trans- 

 formed into chemical energy. The thermodynamics of the 

 assimilation process, which plays such an important role in the 

 general energy exchange in the world, will be discussed in detail 

 in Art. 41. The potential energy accumulated by the assimila- 

 tion process can be gauged by the immense reserves deposited 

 in the form of peat, coal, and mineral oil, which represent the 

 chief motive power of the world's industry. 



38. Methods of Detection and Study of Photosynthesis. — At 

 present there are numerous methods by means of which it is 

 possible not only to detect the process of assimilation of carbon 

 dioxide by the plant but also to study this process quantitatively, 

 as the following discussion will indicate. 



To determine the amount of carbon dioxide that is assimilated 

 by the plant, an air current in which the amount of carbon 

 dioxide has been determined beforehand is passed through a jar 

 containing the plant. The unused portion of carbon dioxide is 

 then absorbed by an alkali. This method shows that in bright 

 light leaves are able to absorb almost all of the carbon dioxide 

 from the passing air if the current does not move too fast. 

 Boussingault was the first to employ this method about a hundred 

 years ago. In recent times, it has been considerably improved 

 by other investigators. This method has been used widely in 

 studies by Kostytchev, Lundegardh, Boysen Jensen, and others. 

 Of the many types of absorbers, most simple and convenient is 

 the one designed by Ordojan. It consists of a glass tube about 

 50 cm. in length filled with barium hydroxide solution. At 

 the base of this tube is a finely perforated metal sieve, which 

 breaks the air current into very fine bubbles, thus facilitating 

 more complete absorption of carbon dioxide. The part of the 

 plant to be investigated is placed in a special glass chamber, 

 through which the air current is drawn rapidly by means of an 

 aspirator. The air then enters into the absorber through a 

 lateral glass tube connected with the absorber at its base, and 

 the carbon dioxide is absorbed by the barium hydroxide solution. 



Simultaneously, by means of a similar aspirator and absorber, 

 determinations are made of the carbon dioxide contained in the 



