42 PHOTOSYNTHESIS 



confined to the study of the effect of different factors on 

 the rate of photosynthesis in the living plant. Consequently 

 there developed only very general ideas as to the chemical 

 mechanism of the process. 



The over-all equation given in Chapter i represents the 

 processes of photosynthesis and of respiration and summar- 

 izes in a qualitative manner the response of the plant to cer- 

 tain external factors, such as carbon dioxide, light, and water 

 supply. In this chapter we shall consider the further advance 

 which was made when these effects were studied quantita- 

 tively. 



THE FACTORS AFFECTING THE RATE OF PHOTOSYNTHESIS 



Interaction of factors 



The first object in the study of a process is to find the 

 experimental conditions for the attainment of the maximum 

 rate. Thus the intensity of illumination, the concentration 

 of carbon dioxide, and the temperature may all be varied 

 and all will affect the rate of photosynthesis. F. F. Blackman 

 (1905) demonstrated experimentally that the effect of a vari- 

 able factor depended on the level at which different constant 

 factors were held, i.e. that there was an interaction of factors. 

 This he interpreted as the behaviour of a system consisting 

 of a set of linked reactions any one of which might under 

 appropriate conditions become the main determinant of the 

 over-all rate. Thus for a given light intensity there will be a 

 concentration of carbon dioxide, at which all the available 

 light energy is being utilized. Increase in the concentration 

 of carbon dioxide cannot then appreciably increase the rate 

 of photosynthesis. Similarly, for a given concentration of 

 carbon dioxide there will be a light intensity above which a 

 further increase will not appreciably affect the over-all rate of 

 photosynthesis because the rate is being determined largely 

 by the rate of supply of the carbon dioxide reactant. Under 

 the conditions for maximum rate a small variation in any of 

 the factors will not produce an appreciable effect on the rate. 

 With this understanding of the interaction of factors it will 

 be seen that the study of the effect of varying any individual 



