BUILDING UP OF INTERMEDIATES 1409 



in itself cause (similarly to diffusion) only a gradual approach of the gas 

 exchange to the steady state. (Shiau and Franck, 1947, suggested that a 

 combination of catalyst inactivation with a shift in the distribution of inter- 

 mediates is a possible cause of the "second wave" of induction; cf. sections 

 2(a) and 4 below.) 



The simplest example of induction based on the building up of intermediates is 

 found in radioactive decay. For example, if uranium is freed of all its daughter ele- 

 ments, the production of emanation will at first be zero; it will increase with time, and 

 reach a steady rate after an "induction period" of several thousand years (this being 

 the time required to build up to a stationary level the intermediate elements between 

 uranium and emanation, notably radium). Similarly, if the dark metabolism of plants 

 ehminates all intermediates of photosynthesis left over from a preceding illumination 

 period, the concentration of these intermediates must be built up to a stationary level 

 before the formation of the final products will attain its full speed. 



Two types of intermediates have to be considered — those that require 

 light for further transformation and those that can complete their trans- 

 formations in the dark ("photochemical" and "thermal" intermediates). 

 Reaction schemes that assume only one primary photochemical process 

 (e. g., scheme 7. VI, 9. Ill or 24.1) contain thermal intermediates only. 

 In reaction schemes based on two or several consecutive photochemical 

 steps (e. g., 7.V or 7.VA), the photochemical intermediates evoke the great- 

 est interest ; but these schemes usually also contain thermal intermediates 

 (e. g., the intermediary reductant, ROH, and the peroxide, ROOH, in 

 scheme 7.VA). 



(a) Photochemical Intermediates 



The role of "photochemical" intermediates in induction has been dis- 

 cussed by Franck and co-workers (1941, 1945), whose scheme of photosyn- 

 thesis (7.VA) contains five of them. Since these intermediates are in- 

 capable of completing their transformation without the aid of light, they 

 must, at the end of illumination, either persist in the dark or disappear by 

 back or side reactions, without contributing to the yield of photosynthesis. 



Franck favored the first alternative. He argued that if photochemical 

 intermediates were to be built up anew, at the beginning of each illumina- 

 tion period, the duration of the resulting induction period should increase 

 with decreasing light intensity (which is not observed in nature) ; in moder- 

 ate light, the induction should last much longer than is actually observed. 



This relation between the light intensity and the length of the induc- 

 tion period follows from the consideration that in the linear part of the light 

 curve the steady state of photosynthesis requires all photochemical reac- 

 tions to run at the same rate, and that to achieve this "equipartition of 

 light energy" an excited chlorophyll molecule must have the same chance 



