1070 THE LIGHT FACTOR. I. INTENSITY CHAP. 28 



dants" and "substitute reductants" considered in chapter 8; see, for ex- 

 ample, page 543, Vol. I) or inert, such as various "narcotics." The latter 

 may perhaps displace from the chlorophyll complex not only the reactants, 

 CO2 or R,H, but even their "carriers," A and A'. Franck and co-workers 

 (1941-1950) concluded, mainly from studying induction phenomena (chap. 

 33), that narcotizing substances are formed within the cell, e. g., in the 

 dark, by fermentation, particularly under anaerobic conditions, and in 

 light, by photoxidation, or by the mechanism which we have repeatedly de- 

 scribed previously — the reaction of the accumulated oxygen precursors, 

 "photoperoxides," with oxidizable cellular constituents, a reaction that oc- 

 curs whenever the removal of these peroxides is too slow, e. g., in conse- 

 quence of insufficient concentration of the enzyme Eg. This "self-narcoti- 

 zation" is considered by Franck the main cause of the most striking changes 

 in fluorescence — when <p increases by a factor of two or more (c/. fig. 28.50). 

 Many instances of an increase in the yield of fluorescence by adsorption of 

 "protective" substances have been observed in vitro, although it is by no 

 means a general rule that all adsorption increases fluorescence. In chapter 

 23 (page 776) we have seen that association with such substances as lecithin 

 or oleic acid protects the fluorescence of chlorophyll, while adsorption on 

 starch, alumina or proteins quenches it more or less completely. It seems 

 likely that it is the nature and orientation of the forces between the pigment 

 molecule and the adsorbent that determine whether the effect of these 

 forces is to permit the excitation energy to spread over a larger number of 

 degrees of freedom, including those of the associate molecules, and thus 

 facilitate its conversion into heat, or whether their effect is to orient and 

 stiffen some otherwise freely vibrating or rotating parts in the pigment mole- 

 cule itself, thus making the dissipation of energy within it more difficult. 

 Another way in which complexing or adsorption may delay internal dis- 

 sipation of excitation energy, offers itself if dissipation occurs only after 

 the excited molecule assumes a certain configuration (corresponding to the 

 crossing point of two potential energy curves in the diatomic model). 

 Temporary dissipation of the excitation energy over a larger number of 

 degrees of freedom (which becomes possible when the molecule is com- 

 plexed or adsorbed) can lengthen the average time needed by the molecule 

 to reach the "critical" configuration. 



With chlorophyll fluorescence in vivo, the effect of a typical narcotic 

 ethylurethan was found to consist in an increase in yield of fluorescence in 

 ChloreUa by about 25% (fig. 28.49), in a quenching of fluorescence of 

 Chromatium in the absence of reductants (fig. 28.50A), and a considerable 

 enhancing effect (up to about -f 40%) on the same fluorescence in the pres- 

 ence of a reductant (fig. 28.50B). (The yield of fluorescence in the presence 

 of 3% urethan is the same with and without the reductant — about halfway 



