INTRODUCTION 1145 



even for plants with identical contents of all pigments (or suspensions of 

 identical cells) the action spectrum still depends on two individual factors. 

 The importance of one of them — the optical density of the sample — was 

 recognized by Engelmann : In a thick leaf or thallus, or a dense cell sus- 

 pension, the absorption spectrum and the action spectrum both are blurred; 

 in the limiting case of complete absorption (approximately realized in 

 Warburg and Negelein's quantum yield experiments; cf. chapter 25, page 

 844), the action spectrum, too, may lose all structure. 



The second factor that affects the action spectrum of an individual plant 

 or cell suspension (without affecting its absorption spectrum) is the in- 

 tensity of illumination. If one would use monochromatic light of such high 

 intensity as to obtain full light saturation at all wave lengths, the rate of 

 photosynthesis (which, in the hght-saturated state, is determined only by 

 the velocity of a dark process) will become identical in all parts of the 

 spectrum. The structure of the action spectrum will appear as soon as the 

 intensity of illumination is reduced below the saturating value. Since the 

 saturating intensity depends on wave length {cf. sect. 4), the shape of the 

 action spectrum will change ^vith decreasing light intensity, until the latter 

 will fall within the practically linear range for all wave lengths. The 

 initial divergence and ultimate convergence of light curves in light of dif- 

 ferent color is illustrated by figure 30.8B obtained by Gabrielsen (1940) 

 with green leaves of Sinapis alba. 



In the low intensity range, the shape of the action spectrum becomes 

 constant, and the spectrum thus acquires a definite significance. Here — 

 and only here— can the action spectrum be compared quantitatively with the 

 absorption spectrum of the specimen, and the question asked whether the 

 specific photochemical efficiency depends on wave length. 



First, however, a "quantum correction" must be applied. According 

 to Einstein's law of photochemical equivalency, one has reason to expect 

 equal numbers of absorbed quanta of different wave length to produce the 

 same photochemical effect, but not equal quantities of absorbed energy. 

 Thus, action spectra have to be "quantized," i.e. expressed in moles per 

 einstein, rather than in moles per erg or calorie. In the linear region, it is 

 legitimate to convert the "equienergetic" action spectrum into the "quan- 

 tized" spectrum simply by dividing all ordinates by the corresponding 

 wave lengths. In the saturation range, this is impossible, and the quantized 

 action spectrum can be obtained only by direct experiment (i. e., by measur- 

 ing the rate in bands of equal intensity expressed in einsteins per square 

 centimeter per second), because here the shape of the action spectrum de- 

 pends on intensity, and spectral bands that have equal intensity if meas- 

 ured in energy units have different intensities if measured in einsteins. 



If the maximum quantum yield of photosynthesis is the same for all 



