LIGHT AND VEGETATION 115 



number of flashes, whatever their frequency might be. This is 

 so when the interval between flashes is sufficiently long — more 

 than ^ second in the conditions in which Emerson operated. 

 If the flashes succeed one another more rapidly, the utiUzation 

 of the light is not so good and the efficiency of each flash 

 diminishes. 



It is ordinarily concluded that the dark reaction, stimulated 

 by photochemical excitation, is accompHshed during this 

 interval of /o second. Emerson stated that the minimum 

 duration of this dark interval depended on the temperature 

 and that it was affected by hydrocyanic poisoning. We must 

 suppose, too, that excessive activation by Hght mihtates 

 against a high efficiency of photosynthesis. We have already 

 mentioned several manifestations of this — inhibition in 

 excessively strong Hght arresting photosynthesis and inter- 

 mittent lighting resulting in an increase of efficiency. 



Emerson compared these results with those that would 

 be obtained if the number of chlorophyll cells were limited. 

 When the Hght has acted on one of them, other reactions, 

 which can be accompUshed in darkness, must take place 

 before this cell becomes available for a new action of light. 

 Before these reactions are completed, the light is without 

 action and its energy is wholly unused. In the conditions of 

 the experiment quoted, the duration of the re-entry into a 

 state of receptivity is ^-s second. 



This process must not be taken literally; it is only an 

 hypothesis or a simple way of describing the principal results 

 observed. Other hypotheses are undoubtedly possible and the 

 real process is still unknown to us. 



Emerson's Functional Units 



The experiments with intermittent lighting introduce, 

 according to Emerson, a new conception of very great 

 interest — the idea of functional units, which is Hnked with 

 that of the chlorophyll cells just described. The idea is 

 introduced in this way. 



If the illumination in continuous light is progressively 



