C. S. I'RENCH 463 



The enhaiucmeiu spectrum gi\ing the chlorophyll b peak at 650 

 ni/i to our surprise consistently shows a shoulder at about 670 ni/x 

 in tlie data ol Myers and French (48) , of Blinks (9, Figs. 2 and 8), 

 and also here in Fig. 12. \Vc therefore speculate that the 673 m^x 

 form of chlorophyll a may, at least in part, be functionally equivalent 

 to chlorophyll h. Enhancement experiments were therefore made 

 with Botrydiopsis, which contains several forms of chlorophyll a 

 but lacks chlorophyll b, c, and d, as well as the phycobilins. The 

 data of Fig. 14 show that in this alga having only chlorophyll a 

 the "accessory" function can be performed by the 670 m/i, form of 

 chlorophyll a. However, with another chlorophyll a alga, Ochromonas 

 danica, no enhancement was found between 669 and 694 m^. 



At the moment it seems that "accessory" pigments work mainly 

 by activating C.,695. If so, should C,,695 also be called an accessory 

 pigment? In time it will be more clearly obvious what chemical 

 reactions are activated by the different pigment systems, and when 

 that time comes the vague term of accessory pigments will presumably 

 be replaced by a more specific name based on physiological function. 



Action Spectra of Photosynthesis at Saturating Light Intensity 



Myers (unpublished) found that Chlorella exposed to bright 700 m^u 

 light gave a photosynthetic rate at light saturation far below the 

 saturation rate measured at shorter wavelengths. This incidental 

 observation was the starting-point of a thorough study of the rate 

 of photosynthesis at saturating intensities of different wavelengths 

 by McLeod (47). 



As we all know from one of the long-accepted basic facts of photo- 

 synthesis, illustrated in Fig. 15, the rate of photosynthesis rises at low 

 intensity in direct proportion to the intensity, and then more slowly 

 up to a constant rate. Above this region of intensity further increases 

 of light give no greater rates of photosynthesis. 



At low light intensity the rate is dependent on the color as well 

 as the intensity and is very nearly independent of the temperature. 

 The rate is limited by the photochemical first step of the reaction. 

 On the other hand, at high intensity, the rate is limited by a dark 

 reaction, and the intensity or color of the light should have no effect 

 whatever on the measured rate of photosynthesis. 



With this clear and simple picture in mind, all action spectra have 

 been made at low intensity, where the light reaction determines the 

 measured rate. Any attempt to measure an action spectrum of photo- 

 synthesis with saturating intensities of light would at first sight seem 

 to be absurd. 



