Flower Promotion or Flower Inhibition? • 81 



the Nicotiana sylvestris experiment they are supplied by Maryland 

 Mammoth whether on long or short days. 



Flower initiation in strawberries, Fragaria, requires short days, 

 at least under certain conditions. Hartmann (1947) showed that 

 daughter plants would initiate flowers in long days if the adult 

 plant, to which they were still connected by runners, was exposed 

 to short days; he interpreted these results in the conventional 

 "florigen" manner. Guttridge (1959) has since performed experi- 

 ments suggesting the opposite— that flowering occurs when the 

 level of a flowering inhibitor, which also promotes vegetative 

 growth, is sufficiently reduced. This postulated substance would be 

 produced in long but not in short days, and might even be 

 destroyed in the latter. The evidence is analogous to that on the 

 translocation of flowering hormones. 



Plants kept on long photoperiods (using light-breaks) promote 

 vegetative growth and inhibit flowering in runner-attached plants 

 under short photoperiods. This is favored by earlier daily illumina- 

 tion of the plants on long days, although earlier illumination itself, 

 without light-breaks to create an effective long photoperiod, has no 

 effect. These results of course again suggest translocation of the 

 substance in question— this time the flower-inhibiting, growth- 

 promoting substance— in the predominant direction of carbohydrate 

 movement. Experiments with radioactive phosphorus as a tracer 

 confirmed the postulated direction of assimilate movement.. 

 Guttridge's results are thus more consistent with the "simple 

 inhibitor" hypothesis than with "florigen"; here the "donor" is 

 vegetative, the "receptor" potentially flowering. 



The earliness of flowering in certain pea varieties— by which 

 is meant whether the first flower appears at a lower or higher node 

 —can be influenced in several ways other than (in some varieties) 

 photoperiod or cold treatment. These include removing the 

 cotyledons, making cuttings from the young seedlings, grafting of 

 early onto late varieties or vice versa, or even grafting stock and 

 scion of the same variety. The situation is complicated by the fact 

 that certain treatments, which can be broadly described as in- 

 hibitory, may inhibit vegetative growth more than flowering so 

 that the latter actually occurs at an earlier node, though no sooner 

 in time. Haupt (1958) has concluded on the basis of his own 

 experiments and those of others that transmissible flower-promoting 



