LIGHT AND THE PIGMENT 



115 



intensity is used, and exposure time is usually held constant. This 

 varies from a few seconds in some experiments to two or three hours 

 in others, and the plant which is used makes quite a difference in the 

 response. Chrysanthemums, for example, are notoriously insensitive 

 to light interruption unless a number of short flashes are given rather 

 than a single long continuous exposure (see Section 5 below). With 

 most plants the experiment is complicated by the fact that a number 

 of inductive cycles are required to bring about flowering, so the 

 experimenter must resign himself to one or two weeks of sleepless 

 nights while he arises at regular intervals to flash lights at his plants 

 (unless he has an automatic set-up such as Hamner's at the University 

 of California in Los Angeles — see Chapter 5). With the cocklebur 

 a single dark period can be used, but if the investigator does many 

 such experiments he may have to shift his metabolism in the direction 

 of night work anyway. 



The results of a typical experiment with cockleburs are shown in 

 Fig. 7-6. The light interruptions are most effective near the middle 

 of the dark period. The curve on the left, which indicates increasing 



2 4 6 8 10 12 14 16 



HOURS AFTER BEGINNING OF THE DARK PERIOD 



Figure 7-6 

 The effects of a 60-sec red light interruption given at various times 

 during a 16- or a 12-hr dark period upon subsequent flowering. Data 

 from Frank Salisbury and J. Bonner, 1956, Plant Physiol. 310, 141-147. 



