MONOCHROMATIC LIGHT CURVES 



1163 



ever, no quantitative agreement between action spectrum and absorption 

 spectrum can be expected under these conditions, and conclusions drawn 

 from differences between them, e. g., as to the role of the accessory pig- 

 ments, are in the nature of more or less plausible guesses. 



Because of the coincidence of the absorjjtion bands of the carol enoids in green 

 plants with the blue-violet bands of the two chlorophylls, correct guessing is in this 

 case much more difficult than in the case of brown or red algae. Engelmann (1887) 

 recognized tliis and based his suggestion that the carotenoids of green plants also act as 

 sensitizers in photosynthesis not on direct experiments with these plants, but on analogy 

 with the results obtained with colored algae. He quoted, as an additional argument in 

 favor of this suggestion, the observation that leaves of the aurea varieties have a com- 

 paratively high yield of photosynthesis, despite their deficiency in chlorophyll. How- 

 ever, he did not consider this argument as conclusive, at least not without x'enewed study; 

 and since then Willstatter and Stoll (1918) have shown that aurea leaves possess a high 

 relative efficiency also in light filtered through a yellow filter. Willstatter and Stoll saw 



400 



800 



500 600 700 



WAVE LENGTH, m/i 



Fig. 30.9. Action spectrum of photosynthesis of wheat 

 "quantized" by Burns (1937, 1938) (after Hoover 1937). 



in this proof that leaf carotenoids do not contribute to the sensitization of photosynthesis 

 in aurea leaves. However, this conclusion was not convincing because in their experi- 

 ments not only the relative, but also the absolute, yields in both green and aurea leaves 

 were almost unaffected by the interposition of a yellow filter. In other words, the in- 

 tensity of blue-violet light was negligible; therefore their experiments, while proving 

 that aurea leaves are liighly efficient in the light absorbed by chlorophyll alone, proved 

 nothing as to the efficiency or inefficiency of the carotenoids. 



Wurmser (1921^) found the rate of photosynthesis of Ulva lactuca in the blue-violet 

 to be lower than in the green, but higher than in the red (calculated for absorption by 

 chlorophyll alone), thus indicating a possible active participation of the carotenoids. 

 Schmticker (1930) found, by bubble-counting experiments with Cabomba and Crypto- 

 coryne, that the light intensity required to achieve a certain rate of photosynthesis in- 

 creased from the red to yellow and green inversely proportionately to the wave length — 

 thus indicating a constant quantum yield; in the blue and violet, on the other hand, 

 the increase was about 15% larger than was required by the quantum correction if all 

 pigments were assumed to be active, but somewhat less than could be expected if the 

 carotenoids were entirely inefficient. 



