1164 THE LIGHT FACTOR. III. COLOR CHAP. 30 



After the problem of the role of carotenoids in photosynthesis had been brought 

 to the foreground by experiments with brown algae (cf. section 5), Montfort (1940) 

 made a new attempt to determine whether light absorption by the carotenoids of green 

 plants also contributes to photosynthesis. He compared the rates of photosynthesis of 

 the green alga Ulva lactuca in red and orange light (\ > 550 m/x) with that in blue-green 

 light (X 350-625 ran, maximum at 450-500 m/x) of equal incident intensity. Table 

 30.IV shows the results. Comparison of the last three figures in the table shows that the 



Table 30. IV 

 Photosynthesis of Ulva lactuca in Colored Light (after Montfort 1940) 

 P = Rate of Photosynthesis; A = Rate of Absorption 



P (blue-green)/P (orange-red) . 89 



A (blue-green)/A (orange-red)° 1 . 19 



A (blue-green)/A (orange-red)'' 1 .00" 



iP/A) (blue-green)/(PM) (orange-red)" 0.75 



(P/A) (blue-green)/(P/A) (orange-red)'' 0.89" 



Quantum correction: X ( orange-red )/X (blue-green) O.??** 



" All pigments. 



'' Chlorophyll alone. 



" Calculated from data on extracts. 



'' For wave lengths 625 and 480 mju. 



quantum yield in the blue-green was only slightly lower than in the red, if referred to the 

 absorption by all pigments, but much higher if referred to chlorophyll alone. This 

 speaks in favor of an almost equal efficiency of chlorophyll and the carotenoids. Thus, 

 the earlier observations of Wurmser, Schmlicker and Montfort can all be quoted in 

 support of the conclusions derived by Emerson and Lewis from the much more con- 

 vincing measurements in weaker and truly monochromatic light, that the carotenoids of 

 green plants do contribute actively, but less efficiently than chlorophyll, to the sensitiza- 

 tion of photosynthesis. 



In ChlorcUa, the quantum yield deficiency in blue and violet light is not 

 likely to be caused by the presence of a yellow pigment other than the carot- 

 enoids. (The comparison of the absorption spectrum of live cells with 

 that of the extracted pigments, cf. fig. 22.44, does not indicate the presence 

 of such a pigment.) In some higher plants, on the other hand, pigments of 

 the flavone or anthocyanine class often are present in the cell sap or cell 

 walls and compete with the photosynthetically active pigments for blue- 

 violet ({uanta, or even serve es "color screens," particularly when they are 

 located in the* epidermis, or in the cell walls between the chloroplasts and 

 the external light source. The presence of these pigments should leave the 

 saturation yield unaffected, but should depress the quantum yield in the 

 linear range and in the region of partial saturation. Burns (1933, 1942) 

 noted that the quantum yield of photosynthesis of spruce and pine seedlings 

 in the blue-violet (390-470 m/x) was only half as large as in the red (630- 

 720 mn), or red plus orange (560-720 ran). This can be attributed to the 



