THE PHOTOPERIODIC PROCESS 251 



persistent change has taken place in the photoperiodically induced 

 plant — some change which causes the plant to continue the production 

 of those materials which result in flowering. And this changed state, 

 the state which results in persistent flowering after photoperiodic treat- 

 ment, photoperiodic induction, is known as the induced state. 



We can think of photoperiodic induction as a catenary sequence of 

 processes. This catenary sequence has been most studied and most 

 ambitiously (if tentatively) separated into its individual steps in the 

 case of the short-day, long-night plants. This separation has been 

 formulated by the work of many investigators, among whom the 

 names of Hamner, Borthwick, Hendricks, Liverman, Salisbury, Lock- 

 hart, and other attendants at this symposium are prominent. And I 

 think that all would agree that we can, for the time being, describe the 

 catenary sequence by the mnemonic HPTHSTI. The letters of this 

 mnemonic stand respectively for: initial high-intensity light reaction, 

 pigment decay, time measuring, hormone synthesis, stabilization, 

 translocation, and induction. 



Let us summarize in a few words what is known of each of these 

 factors. The short-day plant, in order to be responsive to a long dark 

 period, must have been subjected to an immediately previous period 

 of high-intensity light, as has been so elegantly shown by Hamner 

 (1940). During this period of high-intensity light, sugars or other 

 photosynthetic products are made which are required for effectiveness 

 of the following dark period (Liverman and Bonner, 1953). And, as 

 appears in the work of Biinning, reported to this symposium, the 

 light during the high-intensity light period must also be such as to 

 maintain the pigment system in the far-red-absorbing form. At the 

 end of the high-intensity light period our plant is placed in darkness. 

 During the first 2 to 3 hr of this dark period something happens 

 which we may call "pigment decay." The pigment system is in the 

 far-red-absorbing form at the end of the light period. In the dark it 

 transforms thermally and spontaneously to the red-absorbing form, 

 as is indicated by experiments of Salisbury and Bonner (1956) and 

 of Lockhart (1956). After the completion of pigment decay the 

 time-measuring process starts measuring time and ticks off a further 

 6 hr or so — measures the length of time required to attain the total 

 critical dark length of 8.5 hr. The time-measuring process may very 



