THE PERIODIC ASPECT 



553 



by experiments in which the daily 8-hr hght exposure is interrupted 

 by dark periods (see Fig. 1). When in this way the second exposure 

 extends into the scotophil phase of Biinning, i.e., the sum of the hght 

 and dark periods is more than 12 hr, growth is strongly inhibited, and 



10 



8- 



o 



•o 



e6H 



E 



5 

 o 



o 



T 



2 



1500 fl -c. 

 1000 ft.-c 



+ 500 ft.-c. 



8 



2 4 6 



INTERVENING DARK PERIOD (hr) 



8- 



o 



6- 



E 



x4H 



UJ 



3:2-1 



UJ 



"1 1 1 



2 4 



INTERVENING DARK 



6 



-T- 



8 



PERIOD (hr) 



Fig. 1. Growth in millimeters per day (ordinate, upper curve) and 

 fresh weight production in grams per plant (ordinate, lower curve) for 

 tomato plants grown at three light intensities with 8 hr of light per 24 hr. 

 The 8 hr of light was given in two 4-hr periods with zero, 2-, 4-, 6-, or 

 8-hr dark periods, intervening (abscissa). 



