70 PHOTOSYNTHESIS 



fluorescence. Thus if the rates of the processes resulting 

 in dissipation increase, when the rate of photosynthesis 

 decreases, the fluorescence may also decrease. In addition 

 to the effect of intermediates of photosynthesis upon the 

 processes of dissipation, Franck has postulated the forma- 

 tion by the cell metabolism of other substances (called by 

 him 'narcotics') which are supposed to inhibit both energy 

 dissipation and photo-chemical reactions. These substances 

 are considered to be produced by the cell in the dark, especi- 

 ally under anaerobic conditions, and during photo-oxidation. 

 They are thus of particular importance during the induction 

 phase and under conditions of low concentrations of carbon 

 dioxide or very high light intensities. Further studies of 

 fluorescence in the plant may be expected to reveal com- 

 plexities of which there is yet only a vague understanding. 

 Much work remains before a unifying interpretation of all 

 the data can be hoped for. 



PRODUCTION OF LIGHT BY GREEN PLANTS 



Using both algae {Chlorella, Scenedesmus, and Stichococcus) 

 and higher plants, Strehler and Arnold (1951) showed 

 that after illumination had ceased, light was given out by 

 plants for a period which may be as long as two minutes, 

 the half time being of the order of i to 2 seconds. The 

 intensity is of the order of io~^th that of the exciting light 

 and its discovery was made possible by the use of sensitive 

 photomultiplier tubes. Heat irreversibly destroys the ability 

 to luminesce; luminescence is inhibited by compounds 

 which inhibit photosynthesis, e.g. cyanide, azide, and 

 hydroxylamine. Luminescence is 'saturated' by exciting 

 intensities of the same order as those required to saturate 

 photosynthesis. High concentrations of carbon dioxide 

 (7*5%) depress the emission of light but the depression is 

 reversible. Chloroplast suspensions show similar effects to 

 whole cells (Strehler, 195 1). Thus the properties of lumin- 

 escence are different from those to be expected if the light 

 emission were phosphorescent; it is thought probable that 

 light originates from the reversal of some steps of the photo- 

 synthetic mechanism. 



