298 PHOTOSYNTHESIS 



(as is presumably the case), the speed of the process as a whole will 

 depend chiefly on the speed of M -^ P (the velocity of the whole 

 catenary process being regulated by the speed of the slowest member) . 

 It is therefore evident that when the temperature is raised its effect 

 on the process as a whole will depend chiefly on its effect upon M -^ P 

 rather than upon its effect upon S~^ M. Hence if the light reaction 

 S~^M has a low temperature coefficient, while M —^ P (which is 

 not a light reaction) has a high coefficient, the temperature co- 

 efficient of the process as a whole will be high. It is therefore not sur- 

 prising to find that photosynthesis has a temperature coefficient of 

 1.81. Analogous cases exist in photochemistry.^" 



It is to be expected that similar relations will be found in helio- 

 tropic and heliotactic reactions where the stimulus is given by light 

 and the growth or movement which follows is due to a slower process 

 with a coefficient presumably higher than that of the light reaction. ^^ 



SUMMARY. 



The temperature coefficient of photosynthesis in Ulva (between 17° 

 and 27°C.) is 1.81. This may be explained by assuming that the 

 process involves a light reaction with a low coefficient followed by an 

 ordinary reaction with a high coefficient. 



^° Cf. Bovie, W. T., Science, 1913, xxxvii, 373. 



^^ According to T. Nybergh {Ber. hot. Ges., 1912, xxx, 542) this is the case for 

 the oat seedling, but this is disputed by Marie de Vries {ibid., 1913, xxxi, 233; 

 Versl. wis. en natuurk. Akad. Wetensch, te Amsterdam, 1913, xxi, 1056.) In cer- 

 tain cases the whole process may depend on the diffusion of a substance (pro- 

 duced in the light) from the point of origin to another region. The coefficient 

 may then be that of diffusion (usually not over 1.28). 



