1032 BIOLOGICAL EFFECTS OF RADIATION 



Definite proof of the photosynthetic activity of the purple sulfur 

 bacteria (129) and the red sulfur bacteria (31a) has now been found. 



Although the sulfur-free purple bacteria have not been shown to be 

 autotrophic, they can grow in the dark on organic media only in the 

 presence of oxygen (129). 



The pigment complex of these bacteria has been found to contain 

 two principal pigments, one green corresponding to, but not identical 

 with, the chlorophyll of higher plants, bacteriochlorophyll (102), and 

 a purple pigment, bacterioerythrin (129a). 



The bacteriochlorophyll is similar to chlorophyll in composition. 

 It appears to be made up of two components corresponding to chloro- 

 phylls a and h. The ratio of the two pigments is, however, reversed. 

 In ether the following bands have been reported: 



E.A 6810 A; 5920-5640 A; 4240 A E.A. 



The purple component is of carotenoid nature. Its formula is C48H66O3 



(129a). The absorption maxima are: 



In carbon bisulfide: X5690, 5300, and 4990 A. 



In ethanol: X5280, 4950 A. 



It is worthy of note that in this photosynthetic organism also, the 

 pigment complex is composed of both a chlorophyll and carotenoid 

 compound, absorption bands of which are nearly complementary. 



EFFECT OF LIGHT INTENSITY IN PHOTOSYNTHESIS 



In recent years the effect of light intensity on the photosynthetic 

 })rocess has been extensively investigated. The experiments show that 

 the "limiting factor" or Blackman hypothesis can never be fully realized 

 and can be approached in only a few cases. The "relativity law" (72), 

 on the other hand, cannot be examined critically because of the difficulties 

 in discerning and in handling quantitatively the large number of variables 

 simultaneously operative in photosynthesis. 



Hoover, Johnston, and Brackett (44) conclude that there is "linear 

 variation of carbon assimilation as a function of light intensity . . . over 

 a limited range." The results indicate that the Blackman principle 

 is closely approached, notwithstanding the lack of ideal experimental 

 conditions for "not all the chloroplasts can be maintained in the same 

 light intensity, nor can all the surfaces of the leaves be brought in con- 

 tact with exactly the same concentration of carbon dioxide. . . . On 

 this account there is a transition region about the point of inflection." 

 Van den Honcrt (125), using the filamentous alga Hormidium flaccidum, 

 found that when light is the limiting factor the properties of the assimila- 

 tion process agree fairly well with Blackman's formulation. The devia- 

 tions found, however, amounted to 25 or 30 per cent at the transition 

 })oints. 



