BENTLEY GLASS 867 



sorbed only by diloiophyll n and not by any accessory pigments, was 

 really very ineHuient in photosynthesis. Robert Emerson and his 

 coworkers first discovered, as mentioned above, that 650 mfx light, 

 over and above the photosynthesis it would induce by itself, acted to 

 make sinuiltaneously applied light at 695 ny more eilective. By 

 using a background light of 695-700 ni/x, Emerson obtained an action 

 spectrum for the "enhancement" of the effect of the longer wave- 

 length. In all cases this action spectrum corresponded to the frac- 

 tion of light absorbed by the accessory pigment present, among those 

 named above (for green plants this means a peak of eflfectiveness at 

 650 nifi, where chlorophyll b absorbs most strongly). This fact meant 

 that chloropyhll /; and other accessory pigments possess some more 

 significant function than that of merely supplying extra energy to 

 chlorophyll a. 



French no\\' points out that the same data can be interpreted in 

 a broader sense, as a mutual enhancement between the pair of wave- 

 lengths — for it is equally logical to use a background light of 650 

 ni/x and determine the efficiency of longer wavelengths in enhancing 

 the effects of the shorter ones. These complex action spectra were 

 directly plotted on the recording action spectrophotometer, the 

 principle of which is to use monochromatic light slowly changing in 

 wavelength and continuously adjusted in intensity so as to maintain 

 a constant rate of photosynthesis as the spectrum is traversed. The 

 results show that not only chlorophyll b but also "forms" of chloro- 

 phyll o with absorption peaks at about 673 m^i, and 684 m/A can 

 enhance the efficiency of chlorophyll a absorbing maximally at 695 

 mfj. (Ca695) . This conclusion, first derived from experiments with 

 Chlorella, was confirmed by work with Botrydiopsis, which has sev- 

 eral forms of chlorophyll a but lacks chlorophyll b. At present it 

 therefore appears that the accessory pigments function principally to 

 "activate" the Ca695 form of chlorophyll a. 



Use of the recording action spectrophotometer also confirmed the 

 existence of the "chromatic transients" discovered by Blinks. These 

 are consistent spikes or dips in the rate of photosynthesis caused by 

 merely changing the color of the illuminating light, even when the 

 steady-state rate is identical at the two different wavelengths. In 

 Chlorella the action spectrum of the chromatic transient is exactly 

 similar to the action spectrum of the Emerson enhancement effect, so 

 that it too reflects the activation of Ca695 by the accessory pigments. 



Experiments done by Myers at saturating levels of light intensity 

 surprisingly showed that at saturation with light of 700 m^ the photo- 



