CHEMICAL NATURE OF THE INDUCTIVE PROCESSES 



JAMES BONNER 

 Division of Biology, California Institute of Technology, Pasadena 



This discussion will concern photoperiodic induction. By such induc- 

 tion we mean the changes which occur in a plant that enable it to 

 continue to respond to an earher photoperiodic treatment. Let us 

 consider a specific case, photoperiodic treatment of the cocklebur, 

 Xanthium, a short-day plant. We give to the cocklebur a photo- 

 periodic treatment which we know to be effective in causing flower- 

 ing. Such a treatment may consist, for example, of a single long dark 

 period. The dark period may be S^/i to 16 hr or more in length. Ap- 

 proximately 48 to 60 hr after the beginning of the dark period, differ- 

 entiation of the growing point begins. Obviously, chemical processes 

 have gone on in the plant during this 48- to 60-hr period — chemical 

 processes which result in a new and specific kind of differentiation. 

 What are these chemical processes? 



Our Xanthium plant, once given an inductive photoperiodic treat- 

 ment, may continue to make flower primordia for many weeks or 

 months. It "remembers" that it has once had a long dark period. This 

 is induction. In a formal way induction resembles the transformation 

 which has been studied so extensively by microbiologists. In both 

 cases we give to our organism an agent. This agent permanently alters 

 the receptor organism. In the case of transformation, we know what 

 the transforming agent is, namely, a piece of DNA. This DNA is 

 incorporated into the genetic complement of the cells of the receptor. 

 But what are the chemical changes which result in induction in the 

 case of photoperiodic treatment? We do not know. We can only 

 discuss possibilities. 



This discussion will be limited to photoperiodic induction of flower- 

 ing phenomena. The limitation is necessary because {a) we know 

 more about induction in the case of flowering than in any other case, 

 and {b) vegetative responses are not in general inductive. They are 



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