Some animal life changes are under photoperiodic control, as Garner 

 and Allard indicated. Photoreactions have helped to explain some of the 

 reproduction, dornnancy, and migration habits in animal life. 



Plant forms from the highest down to the tiny algae are believed to 

 time growth changes by day length. But the system has been studied first 

 and foreinost in seed plants because of their major importance to agri- 

 culture. 



Although an indeterminate plant, such as the tomato, can fruit from 

 the tropics to the Arctic, such plants contain the same light -responding 

 mechanism as short -day and long -day plants, though operating less 

 dominantly. The tomato at nnaturity can develop skin color only in close 

 compliance with photoperiodic behavior. 



CLARIFYING THE ROLES OF NIGHT AND LIGHT 



Giving Darkness its Due 



For nearly 20 years after photoperiodism was discovered, plant 

 scientists assumed that plants timed growth changes by some means 

 of measuring daylight. But experiments devised in 1937 showed that 

 plants measure the dark period in a 24-hour cycle, not the light. That 

 is, a plant starts a new growth stage on a signal developed by clocking 

 hours of darkness. 



Remarkable features of this timing of darkness were demonstrated 

 by soybean seedlings, which served as convenient and highly responsive 

 test plants. 



The soybean's dark-timer proved itself a precision instriiment that 

 could recognize a few seconds of light as an interference. A minute of 

 artificial light at midnight each night ■was enough to keep young soybean 

 plants from flowering. The dark -timer inside the plant disregarded the 

 insufficient dark period prior to the light interruption and started clocking 

 all over the instant darkness resumed. 



When a light interruption was shifted toward the beginning or end 

 of the dark period, the test plants responded less consistently or even 

 ignored the light break entirely. Their nnaximum response came about 

 midway in a long dark period. The exact reason for this difference has 

 not yet been explained. 



The dark-timer was sensitive to interruption from even faint light. 

 This accounts for some bloom failures such as those cited on page 1. 



In one test, soybeans were induced to bloom by keeping a single leaf 

 on a plant in total darkness for 16 -hour nights, while all the rest of the 

 plant had inadequate darkness of 8 -hour nights. Such a spread effect 

 from a small control center indicated an intricate channeling inside the 

 growing plant. 



Tests of many plants besides soybeans confirmed the clocking of 

 darkness as a prevailing plant characteristic. A long -day plant like barley 

 has been brought to flower in winter greenhouse conditions by a little 

 midnight lighting to keep its dark periods short. Barley's response to 

 interrupted darkness is not so fast as the soybean's, but as little as an 

 hour of light separating parts of a dark period is enough to promote 

 flowering in most barley varieties. 



