418 PLANT PHOTOPERIODISM 



centrations — concentrations so low as to be undetectable by our physi- 

 cal methods. We have already seen, however, that it is not possible to 

 inhibit formation of the flowering impulse in leaves by amino acid 

 antagonists or by antagonists of nucleic acid synthesis. Our conclusion 

 must be, therefore, that the material produced in the leaf as a result 

 of photoperiodic induction does not behave as though it were a specific 

 new nucleic acid or specific new protein. The material produced in the 

 leaf does not possess the properties of the more familiar self -perpetuat- 

 ing entities. Let us therefore consider a further possibility. This possi- 

 bility is suggested by the experiment of Carr (1953) which has been 

 repeated and extended by Lincoln (1954), Salisbury (1955), and 

 others. The observation is that a plant such as Xanthium becomes 

 photoperiodically induced only if an actively growing bud is present to 

 receive the stimulus sent out from the leaf. If no actively growing bud 

 is present, as for example, if buds are removed a day or two after the 

 end of the inductive dark period so that previously dormant axillary 

 buds must grow out, then these new shoots are not induced but remain 

 vegetative. These results can be interpreted on the basis of the hypothe- 

 sis that the leaf, as a result of photoperiodic induction, sends out a 

 pulse of flowering stimulus. The stimulus must be dissipated within a 

 few days after the end of a photoperiodic treatment since the treated 

 leaf is no longer capable of causing flowering in buds which develop 

 after this period of time. If, on the other hand, an actively growing 

 bud is present during and after photoperiodic induction, then the plant 

 as a whole becomes induced, and axillary buds, as they grow out, be- 

 come reproductive. How can we interpret this phenomenon? It seems 

 to me that one possibility is that the leaf, as the result of appropriate 

 photoperiodic treatment, sends to the growing bud the material which 

 brings about transformation of this bud into a reproductive one. The 

 bud is induced. It is caused to start the production of what we may call 

 "transformation product." From this transformed bud, materials can 

 now go to other buds and bring about more transformation. The self- 

 perpetuative aspect of induction is by this hypothesis referred com- 

 pletely to the actively dividing meristematic tissue. It is suggested that 

 it is the material produced in transformed buds which is transported 

 from one plant to another across graft unions, and so on. 



This hypothesis suggests that there may be a real distinction be- 



