Borthwick et al. 



-75- 



Wave Length Dependence 



Stance escapes photosensitized destruction, even in continuously illuminated 

 leaves, to cause floral initiation ; and an inhibiting concentration is reached 

 after an adequate dark period. Light, accordingly, acts through the same 

 mechanism in both cases, that is, possibly to destroy the same compound 

 by a photosensitized reaction. It is of interest in this connection that 

 Melchers and Lang (12) postulated a reaction in the leaves of Hyoscya- 

 mus that inhibits floral initiation. 



Intermediate type plants that flower only when the day is adequately 

 long but not too long, have chiefly been described by Allard (1). Some 



3500 -WOO 4500 5000 5500 6000 6500 7000 7500 

 WAVE LENGTH IN ANGSTROM UNITS 



3S00 4000 4600 5000 5500 6000 6500 7000 7500 

 WAVE LENGTH IN ANGSTROM UNITS 



Composite action spectrum curves showing the energy required at various wave 

 lengths for the suppression of floral initiation in soybean and cocklebur (18). 



examples are Mikania scandens, Phaseolus polystachios, Eupatoriimi tor- 

 reyamtm, and a New Guinea form of Saccharum spontaneum. The last 

 of these is most striking in flowering only for day lengths within the range 

 of 12 to 14 hours. These plants, described in terms of the above hypothesis, 

 are ones in which concentrations of the active substance produced under 

 continuous light are insufficient to cause floral initiation and in which in- 

 hibiting concentrations are reached after an adequate dark period. 



Indeterminate plants can be considered as those in which adequate 

 concentrations of the active substance are produced in continuous light and 

 limiting values are not exceeded during dark. Floral initiation by plants 

 of this class might be most effective for continuous light, intermediate 

 periods, or for short days in which case similarity to one of the above 

 three classes would be indicated. 



Only one other type of photoperiodic response in plants is suggested 

 by the hypothesis, namely, failure of floral initiation under any condition. 



