176 CHEMICAL AGENTS AND GROWTH 



On the basis of our own earlier experiments (Liverman and Bonner, 

 1953) it still appears that there may be a cyclic mechanism involved 

 in the "light reversible" reaction controlling photoperiodic responses. 

 Certainly no evidence thus far presented would compel one to discard 

 this possibility. Whether auxin or some physiologically related com- 

 pound is involved in the primary light reaction is still not definitely 

 known. Our own experiments with leaf discs and Avena suggest that 

 auxin-like compounds are involved early in the reaction sequence 

 indicated in Fig. 2. Additional evidence favoring this view is that 

 auxins affect nearly all the final responses in some manner or other. 



Experiments from many laboratories suggest that cobalt or a similar 

 substance is rather intimately connected with the primary light action 

 in some manner, possibly by preserving an "activated complex" result- 

 ing from the light exposure. The only really suggestive experiments as 

 to the role of cobalt are those of Galston and Siegel (1954) showing 

 its participation in antiperoxigenic type reactions. Our experiments 

 (Table II) showing a reinforced far-red response in the presence of 

 cobalt suggest that it or a physiologically similar factor may be com- 

 plexed with the far-red receptive form of the "pigment" system. A 

 further, more detailed analysis of the mechanism of action of cobalt 

 may lead to a better understanding of the reactions which it affects 

 and thus to a better understanding of light action in the photoperiodic 

 response. 



Gibberellic acid, too, appears to be effective in the early part of 

 the reaction sequence, since a portion of its effect in causing leaf 

 expansion is overcome by exposure to far-red light. It has not been 

 determined whether this portion of the response is in turn reversed 

 by subsequent red exposure. Thus it would appear particularly 

 important to study the action of this substance in detail with a 

 view to learning more of the biochemistry of the red-far red reaction 

 mechanism. It is not possible to say at this time whether the action of 

 gibberellic acid in overcoming far-red-induced summer dormancy in 

 tomato (Johnson et al, 1956; Liverman and Johnson, 1957) is 

 directly comparable to the reaction cited above, but the implication is 

 that the two reactions may be identical. The control of dormancy in 

 other responses may be related to this same phenomenon. All these 

 indications lead one to suspect that gibberellic acid or a physiologically 



