1044 BIOLOGICAL EFFECTS OF RADIATION 



In Warburg's experiments the light and dark periods were equal. 

 Emerson and Arnold (24, 25), with an ingenious apparatus, by making 

 the periods of illumination much shorter than the dark periods, were 

 able to attain increased photosynthesis of 300 to 400 per cent over the 

 continuous light with only 50 flashes per second. They have shown 

 that the light reaction is not affected by temperature and is capable of 

 proceeding at great speed, in about 0.00001 sec. On the other hand, 

 the dark reaction is dependent on temperature and requires less than 

 0.04 sec. for completion at 25° and about 0.4 sec. at 1.1°. 



It is as yet not knowTi how the dark and light reactions work together 

 in the photosynthetic process nor which of the two reactions may be 

 considered to precede the other. Obviously this phase of the problem is 

 greatly complicated by the fact that, as far as we know, photosynthesis 

 is dependent upon living protoplasm. 



One of the most striking properties of chlorophyll is its red fluores- 

 cence. When a molecule absorbs radiant energy it passes into an acti- 

 vated state. In this condition it may react or it may return to its 

 normal state by loss of this energy through collision, reradiation of the 

 same wave-length of light as absorbed (resonance) or reradiation of the 

 energy as light of longer wave-length than that absorbed (fluorescence). 

 It is well known that certain foreign substances have the ability to 

 quench the fluorescence of compounds and that this process is accom- 

 panied by an activation of the foreign molecule (120). In this manner 

 fluorescent compounds may act as transporters of radiant energy. 



Oxygen strongly quenches the fluorescence of chlorophyll, whereas 

 other gases such as nitrogen, hydrogen, and carbon dioxide do not have 

 this effect (50). In view of the fact that oxygen has this property and 

 has been shown to be excited by light in the same spectral region as the 

 fluorescence of chlorophyll, and to remain in this activated state for as 

 long as 7 sec. (76), it has been proposed that oxygen may play the role of a 

 "collector and transporter" (50) of energy in the photosynthetic process. 

 That chlorophyll and other fluorescent dyes sensitize photochemical 

 oxidations has been amply demonstrated (29, 84). 



Kautsky, Hirsch, and Davidshofer (50) have reported the interesting 

 fact that in living leaves this fluorescence is decidedly quenched during 

 active photosynthesis. Inhibition of photosynthesis through hydro- 

 cyanic acid increases the fluorescence. These investigators conclude that 

 the quenching of the fluorescence of chlorophyll during photosynthesis 

 is caused by oxygen, that the radiant energy absorbed by chlorophyll is 

 transferred to the oxygen, that, in fact, oxygen is the only molecule 

 in the photosynthetic system to which the absorbed radiant energy is 

 transferred. It is, of course, not yet clear what role is to be ascribed to 

 the activated oxygen in the reduction of carbon dioxide. Willstatter 

 (140) has questioned the conclusion of Kautsky that oxygen is the only 



