448 LIGHT AND LIFE 



tion different from and accessory to tliat performed by clilorophyll a. 



Tlie accessory function looks like a specific cliemical effect ratlier 

 than a simple increase in the light-absorbing capacity of plants. 

 Furthermore, it appears that chlorophyll b, which is in all ordinary 

 land plants, plays the same accessory role as do the more spectacular 

 coloring materials of the blue-green, red, and brown algae. 



The two exj^eriments forcing a revision in the concept of the 

 function of accessory pigments are the chromatic transient experi- 

 ments and the enhancement of photosynthesis by two wavelengths 

 given together. Furthermore, the comparative inactivity of chloro- 

 phyll in red algae (33, 61), as well as the expanding realization of 

 the simultaneous occurrence of several different forms of chlorophyll 

 a in plants, has contributed to the present picture of the interrelated 

 function of plant pigments in photosynthesis. 



We will first review the various photosynthetic pigments, together 

 with the typical spectra by which they may be identified in living 

 plants. Then we will consider the action spectrum measurements 

 that in comparison with absorption spectra have shown these accessory 

 pigments to be active in photosynthesis. An improved method for 

 measuring action spectra and its use in studying the enhancement 

 spectra will be mentioned. An unusual new fact, the variation of 

 photosynthesis rate with wavelength at saturating intensity, discovered 

 by McLeod (47) , will be described. In conclusion, some relations 

 between several other effects related to the separate function of in- 

 dividual photosynthetic pigments will be postulated. 



All of these phenomena are believed to be due to the interaction of 

 different photochemical products formed by the action of light on 

 different groups of pigments. In short, it appears that tliere are 

 qualitative differences in the chemistry of photosynthesis dependent 

 upon the pigments by which incoming light quanta are collected and 

 channeled. 



Photosynthetic Pigments in Plants 



Chlorophyll a 



The main red absorption band at 670-680 m^ of all photosynthetic 

 plants except the jihotosynthetic bacteria is due to chlorophyll a. 

 In ether solution, chlorophyll a has absorption bands at 662, 615, 

 578, 534, 430 and 410 u\fx as well as other minor bands (55) . (The 

 major bands are italicized.) In absorption spectra of live plants the 

 chlorophyll a bands may be identified but are found at longer wave- 



