THE NATURE OF PHOTOSYNTHESIS 99 



will be shown under the discussion of the effect of light intensity accord- 

 ing to Harder 's experiments, there is a portion of the curve where more 

 than one factor is limiting. 



Benecke considers that any factor is in "absolute minimum" only when 

 that factor is entirely absent, e.g., light is in "absolute minimum" when 

 the plant is in darkness. If we start with very weak light and then grad- 

 ually increase the intensity, the light factor is in "relative minimum" the 

 moment of first illumination. Other factors can at the same time be in 

 "relative minimum." The difficulty of obtaining strictly mathematical 

 results lies in the experimental complexities involved, in the fact that it 

 is almost impossible to alter only one factor without at the same time 

 affecting some other factor. Even if corrections can be applied to such 

 factors as temperature on bicarbonate dissociation or solubility of carbon 

 dioxide, it is impossible to correct for the effect of external agencies on 

 internal conditions, as e.g., the action of light on chlorophyll and certain 

 stimulatory effects. 



The principles involved in the theories of limiting factors can be more 

 clearly elucidated in the discussions of the influence of various individual 

 factors in the following sections. 



b. The Influence of Light. 



Investigations on the influence of light on photosynthesis have been 

 directed in two main courses. One of these has been chiefly concerned 

 with the effect of different intensities of illumination on photosynthetic 

 activity and the other with the influence of different frequencies or wave 

 lengths. Some of the earlier work on both of these phases of the problem 

 is contradictory and confusing. This is due, first of all, to a lack of 

 realization on the part of the older workers of the interrelation of the 

 various factors affecting photosynthesis, which was first clearly recognized 

 in Blackman's theory of limiting factors, and secondly, to technical diffi- 

 culties in the measurement of light intensity. With a clearer understand- 

 ing of the influence of other factors involved in the photosynthetic process, 

 that of light too was somewhat clarified. Also, the recent development in 

 our conceptions of the nature of radiant energy, the quantum theory, the 

 laws of photochemical action have contributed much to better experimental 

 procedure. It may be expected that the mastery of these newer concep- 

 tions of the nature and action of light by experimenters in this field will 

 be of great service in gaining a better understanding of the role which 

 light plays in photosynthesis. Already the two lines of investigation men- 

 tioned, that of the effect of different intensities and that of the influence 

 of different frequencies have been brought closer by the approach from 

 the viewpoint of the quantum theory. 



In the dark a chlorophyllous plant absorbs oxygen and gives off car- 

 bon dioxide in the process of respiration. Even illumination of very low 

 intensity is apparently capable of inducing a reduction of carbon dioxide 



