MEASURING PHOTOSYNTHETIC ACTIVITY 227 



It should be stated at the outset that none of the methods thus far 

 developed is entirely satisfactory, and undoubtedly several of these could 

 be greatly improved. As almost the entire interpretation of the process 

 of photosynthesis is based upon these methods and the literature of the 

 subject is filled with many erroneous assumptions and explanations which 

 have been often repeated, it seems not amiss to subject these methods to 

 a critical examination in the light of the experiences of various workers. 

 In general, the determination of the amount of carbon dioxide absorbed 

 has been the most satisfactory method. This is due first of all to the 

 relative ease with which carbon dioxide can be determined quantitatively, 

 but also because the carbon dioxide absorbed bears a directer relation to 

 the material synthesized than does the amount of oxygen liberated. Oxy- 

 gen is utilized in subsequent respiratory oxidation of organic material 

 and may be held in varying amounts combined with a large variety of 

 organic acids, thus being, as it were, drawn out of the direct path of 

 the photosynthetic process. This fact is most clearly brought out through 



CO 

 a study of the-^p- ratio. In using the principle of the amount of oxygen 



liberated the fact has very frequently been overlooked that this merely 

 represents the difTerence between the amount of oxygen produced in the 

 photosynthetic process and that used by respiration. 



It has not been possible as yet to estabhsh a satisfactory standard of 

 photosynthetic activity. The amount of carbon dioxide absorbed, of oxy- 

 gen emitted or of dry material formed have been calculated on the basis 

 of area illuminated, fresh or dry weight of the plant and on the amount 

 of light absorbed by the plant under investigation. For leaves the area 

 standard gives a fairly satisfactory means of comparison. But this method 

 cannot be applied to aquatic plants or only with great difficulty and lack 

 of precision and, of course, not at all to algae; and it is with the latter 

 that some of the best work on measuring the rate of photosynthesis under 

 different conditions has been done. The' standard of weight is most un- 

 satisfactory, for relatively slight changes in either the fresh or dry weight 

 of the leaves, changes which may in no manner be connected with the 

 photosynthetic process, may alter the results very materially. From the 

 point of view of the energy changes involved, the method of calculating 

 photosynthetic activity on the basis of radiant energy absorbed by the 

 plant ofifers many advantages, though this entails rather elaborate apparatus 

 and a number of corrections. 



2. The Liberation of Oxygen 



A variety of methods have been employed to detect and determine 

 the oxygen liberated in the photosynthetic process. They include (A) the 

 use of the leucobases, (B) luminous bacteria, (C) motile bacteria, (D) 

 the optical properties of hemoglobin, (E) counting the bubbles of lib- 

 erated oxygen from a plant submerged in water and (F) gas analytical 



