Permanence and Location of the Induced State • 85 



then removed and grafted onto another plant, with the same result; 

 this can be repeated as long as the leaf remains healthy, which may 

 be for several months (see Fig. 5-3). There is no evidence that any 

 other part of the plant has a role in the maintenance of the 

 induced state; detached leaves are easily induced by the appropriate 

 photoperiod, as can be demonstrated by subsequently grafting them 

 onto plants on long days. Experiments of this kind are rarely 

 successful with Xanthium. The clearest difference between Perilla 

 and Xanthium lies in the lack of any indirect induction in the 

 former. When Perilla in long days is brought to flower by grafting 

 an induced leaf to it, the leaves it subsequently produces remain 

 noninduced, incapable of causing flowering in another plant on 

 long days. 



On the basis of these observations, the relationship between 

 florigen and the induced state in Perilla and Xanthium. appears to 

 differ considerably. In the former, the induced state is localized 

 in the leaf, produced only by photoperiodic treatment and obviously 

 separable from the transmitted florigen. In Xanthium, indirect in- 

 duction of the developing leaves goes on continually, either as a 

 result of the transmission of florigen itself— in which case the pro- 

 duction of floral stimulus in Xanthium is autocatalytic— or brought 

 about by a second unknown substance moving with it. Without 

 further evidence, the first possibility clearly requires the fewest 

 assumptions, although it raises problems which will be considered 

 later. 



As the induced states in Xanthium and Perilla are maintained 

 in different ways, their permanence also differs. Implicit in much 

 of the Xanthium literature is the idea that, once induced, a plant 

 remains induced throughout its lifetime. In a sense this is not true, 

 since Lam and Leopold (1960) showed that reversion can be brought 

 about by constantly removing the flowering shoots and forcing new 

 ones to grow out, until finally vegetative shoots appear. Several 

 interpretations of these results have been suggested, none preferable 

 to others on the basis of available evidence; but it is nevertheless 

 clear that without such drastic treatment, Xanthium seldom or 

 never reverts even after induction by a single short-day cycle. The 

 Perilla plant, unlike Xanthium, reverts easily to the vegetative 

 state under long days, since the induced older leaves die and there- 

 is no indirect induction to reinduce the younger. It is thus some- 



