THE NATURE OF PHOTOSYNTHESIS 161 



h. Internal Factors. 



The external conditions which influence photosynthetic activity, namely, 

 temperature, light intensity, and partial pressure of carbon dioxide can be 

 easily determined and the intensity of each of these influences can also 

 be controlled experimentally. As a consequence the efifect of these factors 

 on the photosynthetic process can be studied experimentally, and although 

 the analysis of the reactions involved has not proved to be a simple task 

 nor is as yet complete, it has been possible to gain considerable informa- 

 tion. The study of the influence of these external factors has served to 

 emphasize the complexity of photosynthesis and to bring to light the 

 fact that there are conditions or influences within the cell which are of 

 equal or greater significance in determining the rate of photosynthesis. 

 The most evident of these is probably the chlorophyll-content of the plant 

 and the relation of this to photosynthetic activity has already been dis- 

 cussed. The existence of these internal factors becomes evident from 

 the fact that under circumstances the photosynthetic rate varies inde- 

 pendently of the external factors such as temperature, light intensity and 

 carbon dioxide-concentration. Moreover, with constant external condi- 

 tions, the rate of photosynthesis does not run parallel with the chlorophyll- 

 content. So that, besides the latter factor, there must be some other, 

 internal factor which is determining the rate of the reactions. 



Conclusions regarding the existence of an internal factor have been 

 arrived at through inference rather than by direct experimental demon- 

 stration. Our knowledge regarding internal factors has not progressed 

 very iar ; in fact, a good deal of the subject is still purely hypothetical. 

 It is an exceedingly difficult experimental task ; external conditions can 

 be altered at will, but to deal with material or conditions inside of the cell, 

 material about which we know virtually nothing, presents unusual ob- 

 stacles. Moreover, it is uncertain whether there are several internal 

 factors, or whether different properties of the same thing have been 

 described. 



Pantanelli ^^^ came to the conclusion that a very important role in the 

 photosynthetic process is played by the protoplasmic activity of the color- 

 less components of the chloroplasts. This was based upon the observa- 

 tion that the photosynthetic activity becomes inhibited in time and after a 

 period of darkness again attains its original rate. The inhibition in Pan- 

 tanelli's experiments was probably not due to an accumulation of the 

 products of photosynthesis, for the duration of illumination was short, nor 

 was apparently the chlorophyll-content decreased though the illumination 

 intensity was high. The photosynthetic activity shows the phenomenon 

 of fatigue and recovery very similar to that of the animal muscle. With 

 this inhibition of photosynthetic activity there occurs a retardation of 

 the protoplasmic streaming. A disturbance in protoplasmic streaming con- 



"* Pantanelli, Jahrh. zi'iss. Bol., 39, 184 (1903). 



