250 CONTROL OF REPRODUCTION 



new pathway of differentiation. By the photoperiodic treatment a 

 chain of events has been set in motion which causes the Xanthium 

 plant, say, to remember that it once had one long night. We say then 

 that the plant has been induced. 



With many plants induction does not persist for long, and the plant 

 may ultimately revert to a vegetative condition. But with some species 

 and with Xanthium in particular induction is very persistent indeed. 

 A classical example is that carried out by Hamner (1938) with 

 Xanthium. Young plants were subjected to a few long nights and 

 returned to continuous light, under which they grew, within a year, 

 into small trees. But during the entire year, during this great multipli- 

 cation of cells and tissues, the cocklebur plants flowered continuously. 

 They "remembered" over an extended period of time that they had 

 once had a few short days. Or again, we may take our induced 

 Xanthium plant, return it to long days, and graft it as a graft partner 

 to a vegetative Xanthium plant which has been maintained continu- 

 ously on a long-day regime. The receptor is induced to flower. Some- 

 thing has passed from the induced donor to the vegetative receptor to 

 bring about flowering in the latter. We may now sever the graft union, 

 throw away the donor, and use our indirectly "induced-to-flower" 

 receptor in turn as a graft partner for a second vegetative plant. Our 

 indirectly induced-to-flower donor acts in turn as a donor and brings 

 about flowering in its graft partner. And it has been possible in this 

 way to transmit the flowering impulse with apparently undiminished 

 vigor through as many as five to seven graft transfers (Thurlow, 

 1948). 



How are we to interpret induction — the persisting after effect of 

 photoperiodic treatment? One possibility would be that during the 

 photoperiodic treatment a great deal of flowering stimulus is produced 

 in the induced plant and that this is stored, to leak slowly out over a 

 long period of time. But this interpretation appears an unlikely one in 

 view of the fact that the induced leaves may be cut from the donor 

 plant a few days after induction without apparent diminution in the 

 vigor of induction of the buds of this plant. The hypothesis is made 

 unlikely also by consideration of the great dilution of any material 

 originally produced in the induced leaves in the experiments of Hamner 

 and of Thurlow alluded to above. It appears more probable that some 



