XIII. ACTION SPECTRA VXD ABSORPTIOX SPECTRA 



439 



thesis by the leaf of a higher plant, where stirring is not possible 

 (compare Figs. 3H and 6). Here, one meets a situation comparable 

 lo that found in the majority of photobiological processes, where the 

 photochemical reaction goes on in a given part of an organism, whose 

 i-elationship with other parts is fixed, and one cannot assume that 

 the incident energy is a measvu-e of the absorberl enei'gy, nor that the 

 chemical environment is homogeneous. In systems of this type, it is 

 possible when the conditions are appropriate to use an approximation 



100 



80 



O 60 



I- 



a. 



ce 



o 



(f) 



m 40 



< 



20 



o o Total absorption 

 • — ■• Active absorption 



42 mm. 



I 1 



J L 



40 mm. 



39 mm 



400 



440 



480 



640 



680 



720 



520 560 600 

 WAVELENGTH, m^ 



Fig. (i. A comparison of absorption by cells of Chlorella to "active absorp- 

 tion" (action spectrum) calculated on the basis of quantum yield 0.084 {17). 



in estimating the effectiveness from the incident energy, or intensity 

 required to induce a given biological end ]ioint. Let us assume an 

 ideal photobiological process in which the reciprocity law is obeyed, 

 so that the end point corresponds to the absorption of a given number 

 of quanta. Let us write Beer's law in a slightly different form, by 

 assmning that I and /o are measured over the same given interval, so 

 that we deal with corresponding numbers of quanta instead of intensi- 

 ties: 



.V/iVo = e-<"^' 



(15) 



where iVo is the incident, A^ is the emergent number of quanta pei' 

 unit area for a given interval of time, ^ is a constant corresponding 



