442 HAROLD F. BLUM 



that of the incident. These factors, particularly the latter, are prob- 

 ably small in many cases, but they are possible sources of error in 

 some. 



For any photochemical process there must be a limiting size of 

 quantum that supplies the necessary energy of activation or — in the 

 case of the endergonic photosjaithetic process — the energy of reaction 

 as well. This may limit effectiveness at the long wavelength end of 

 the spectrum, causing failure of agreement between action and ab- 

 sorption spectra. It has already been pointed out that no photochem- 

 ical processes are to be expected in the pure rotation-vibration region 

 of the infrared. In the case of inactivation of enzymes by ultraviolet 

 radiation, the quantum yield is not independent of the wavelength. 

 This may also be true in other cases, resulting in discrepancies be- 

 tween action and absorption spectra. 



In addition to the "physical" sources of discrepancy just de- 

 scribed, there are "biological" sources. One finds frequently, par- 

 ticularly in the older literature, descriptions of the "antagonistic" 

 effect of radiation from different regions of the spectrum. While it is 

 possible that in a simple nonliving system certain wavelengths may 

 have a modifying action on that of other wavelengths — since different 

 parts of the light absorber molecule may be involved in different reac- 

 tions — it is probable that most if not all cases of "antagonism" in 

 living systems result from the complexity of these systems. We may 

 cite the case of the germination of lettuce seeds studied by Flint and 

 McAlister (S, Ch. 3), in which the over-all effect was found to be 

 resolvable into three separate action spectra. A somewhat different 

 example is that of inhibition of the er3^thema of sunburn by massive 

 doses of ultraviolet radiation. The erythema is the manifestation of 

 the dilation of minute vessels in the dermis, in response to a dilator 

 substance elaborated in the more superficial ]\Ialpighian laye)' as a 

 result of the action of radiation of wavelengths shorter than about 

 320 mn. The Malpighian layer itself contains no vessels ; the shorter 

 of the erythema-producing wavelengths fail ^'irtualIy to pass through 

 this layer, but a certain portion of the longer wavelengths penetrate 

 more deeply and reach the vessels of the dermis, which they appar- 

 ently inhibit from responding to the dilator substance (13,14). 



Another instance of participation of more than one light absorber 

 is that in the blue-green alga Chroococcus, where Emerson and Lewis 

 {16) found that in addition to chlorophyll the pigment phycocj^inine 

 acts as a light absorber in photos j^nthesis. 



