SPECTRAL PROPERTIES OF CELLULAR PIGMENTS 35 



curve of this type as the discovery of a new fluorescent pigment. 

 Nevertheless, when the danger of making such mistakes by improper 

 interpretation of fluorescence spectra is realized, it is possible to use 

 fluorescence spectroscopy for diflicult problems of pigment identifica- 

 tion. 



Goodwin, Koski, and Owens (1951) discovered an orange-fluo- 

 rescing pigment in the cells surrounding the guard cells of the epi- 

 dermis of vetch leaves. By comparing its fluorescence spectrum with 

 those of known porphyrins, they established that this pigment, insolu- 

 ble both in water and in alcohol, is similar to or identical with uro- 

 porphyrin I methyl ester. In view of its very specific distribution, the 

 function of this pigment may eventually be found to have some con- 

 nection with the action of guard cells. 



If several fluorescent pigments occur together in living cells, it is 

 possible to excite fluorescence of one or another by using incident 

 wavelengths which are absorbed by the different pigments. Only 

 chlorophyll fluorescence appears in a red alga illuminated by blue 

 light (French and Young, 1952). However, green light, which is 

 absorbed almost entirely by phycoerythrin, shows phycoerythrin, 

 phycocyanin, and also chlorophyll fluorescence. The action spectrum 

 was determined for the excitation of chlorophyll fluorescence in the 

 red alga Porphyridium cruentum. The action spectrum matched the 

 absorption spectrum of the red phycoerythrin. Therefore, it is pre- 

 sumed that the accessory phycobilin pigments are arranged within 

 the chloroplasts in such a way that they can transfer energy to chlo- 

 rophyll. Experiments like this have led to the conclusion that the 

 participation of these accessory pigments in photosynthesis is simply 

 by extra light absorption. Chlorophyll seems to have some peculiar 

 chemical property necessary for the photosynthetic reaction in addition 

 to its ability to absorb light. 



INVESTIGATION OF PHOTOBIOLOGICAL EFFECTS 

 WITH CROSSED GRADIENTS 



We have already seen that it is possible to measure the action spec- 

 trum of certain photobiological effects by exposing the material to a 

 field in which the intensity of light varies along one axis and wave- 



