The color of light is so important to photoperiodic plant responses 

 that discoveries about this have been major advances in understanding 

 photoperiodism- -described in following sections. 



SINGLING OUT RED 



Since 1945, scientists have been able to concentrate on red as the 

 color of light --the portion of white light --vital to plant growth changes. 



The importance of red was established by treating plants with waves 

 of different lengths that make up each color band in the spectrum, frona 

 violet at about 4000 A. to the visible limit of red about 7200 A/ 



An Unusual Light-Breaking Instrument 



Treating plants with light of individual rainbow colors was achieved 

 when ARS scientists built an unusual spectrograph at Beltsville in 1944. 

 Since materials were short in World War II, the instrument was made 

 mainly of scrap. Yet its size and effectiveness have rarely been dupli- 

 cated, and it continues in use today. 



Main features combined in the instrument were a carbon arc lamp 

 producing light strong enough to throw a beam far down a windowless 

 room, and two large glass prisms that dispersed the white light into 

 broad bands of separate colors on a screen. 



The experimental advantages gained were these: 



• The rainbow -colored band was so long that a row of 14 small pot 

 plants could be irradiated at once, to induce their growth responses 

 to separate colors of light. The entire color band was over 5 feet 

 long and 3 inches high. 



• The focal length of the instrument --nearly 3 3 feet --gave remark- 

 ably pure light of the various colors of the spectrum. There was 

 so little scatter effect of one color band infringing on another that 

 radiation impurity was a mere five -hundredths of one percent. 



Light of Many Colors on Plants 



For systematic spectrum testing, two short -day plants were chosen- - 

 the soybean and the cocklebur- -because both had been used extensively in 

 photoperiodic research. 



To range a maximum number of pots on a bench within the spectruna 

 rays, all foliage of each seedling was removed except a single leaf. In 

 each 24-hour cycle, all plants received a dark period totaling enough 

 hours to start flowering- -but with a brief interruption around midnight, 

 when each plant received light of a designated color. Figure 1 shows 

 plants ranged for such a test. The single leaf of each plant was fastened 

 to the screen, so that the exact wave lengths of light received could be 

 recorded. Throughout 2 hours around midnight, when a light interruption 



lA. = Angstrom unit. One such wave length unit is a hundred millionth of a centimeter. 



