CHEMICAL NATURE OF INDUCTIVE PROCESSES 417 



leaves of a cocklebur plant to become labeled with C^^02 during the 

 course of the high-intensity light period preceding the photoinductive 

 dark period. We will cause the leaves to photosynthesize C^O^ for a 

 short time only, say 10 or 20 min, in order to assure minimum labeling 

 of cellulose and other structural components. We will now place our 

 plant in a photoinductive dark period. During this dark period flower- 

 ing stimulus will presumably be made. One may imagine that it will be 

 made in part from C^^-labeled material. At the expiration of the photo- 

 inductive dark period we return the plant to light. The high-light in- 

 tensity stabilization process now takes place and in addition photo- 

 synthesis, now with unlabeled CO2, provides sugars which sweep all 

 previously labeled sugars from the leaf and to the various receptor 

 depots. Only after 4 or more hr in light will transport of the flowering 

 stimulus from the leaf begin. We should now be able to detect in the 

 petiole of the leaf, for example, the appearance of any new labeled 

 compound — any compound which appears in the petiole at the ap- 

 propriate time and which might be expected to be the flowering stimu- 

 lus itself. This would appear to be a good and a straightforward ex- 

 periment. Variations upon it are easy to envisage. One might, for 

 example, label a leaf not with C^^02, but with more specific precursors. 

 Experimental difficulties can be envisaged too. Nonetheless, this experi- 

 ment has all the earmarks of a profitable one. 



Let us now consider the events which occur in the bud itself after it 

 receives the flowering impulse. I have drawn attention from time to 

 time to the fact that the induced state is self -perpetuating and thus re- 

 sembles a state of virus infection (Bonner and Liverman, 1953). The 

 plant behaves as though by induction it catches the disease of flower- 

 ing. Transmission of induction by grafting, lack of mechanical trans- 

 mission of the induced state, self-perpetuation of the induced state, are 

 all characteristics which would be expected if the flowering stimulus 

 were a virus-like entity. We and others have therefore looked in leaves 

 for any new macromolecule which might appear during induction. 

 Such a material has been sought by electrophoresis, ultracentrifuga- 

 tion, and serology, and in all cases the answer has been a negative one 

 (Campbell, 1951 ). It has been impossible to find a new and character- 

 istic macromolecular entity in induced leaves. It might be, of course, 

 that such an entity exists but that it is present merely in very low con- 



