102 THE FLOWERING PROCESS 



perfect. The absorption of the whole plant is much closer to the 

 photosynthesis action spectrum, but one shoulder in the plant curve 

 at 460 to 500 millimicrons does not match the photosynthesis curve 

 but is accounted for by the carotene curve. The many problems in 

 this approach are well illustrated (e.g. the absorption spectrum of 

 chlorophyll in ether solution is not the same as the absorption 

 spectrum of chlorophyll in the cell). 



2. Method of Studying the Pigment in the Flowering Process (19) 



Properties of the photoperiodism pigment were studied beginning 

 about 1940 by workers at the United States Department of Agriculture 

 Plant Industry Station at Beltsville, Maryland. A piece of equipment 

 essential to the study of this problem was one which would produce 

 light of very specific wavelengths at relatively high intensities. The 

 problem was primarily to obtain a source of white light of an intensity 

 which would provide ample light in relatively narrow wavelength 

 bands after passage through a prism. Such a light source was 

 obtained from the rejected first model of a movie projector which had 

 been constructed for one of the world's largest theatres. Light from 

 the powerful arc could be focused with reflectors and then passed 

 through condensing lenses and the prism to produce an intense 

 spectrum, even when the spectrum was 3 m wide from the blue to the 

 red end. Of course, intensity could be controlled in a number of ways, 

 such as by controlling the aperture (or slit) through which the beam 

 of white light passed or by adjusting the distance of the test plants 

 from the light source. 



The actual measurements were made by exposing plants to one or 

 more long dark periods, which would normally promote flowering 

 of short-day plants or inhibit flowering of long-day plants, and then 

 interrupting each of these dark periods by placing the leaves of the 

 test plants in various regions of the spectrum at different intensities 

 for different lengths of time. The time-intensity combination which 

 allowed about 50% of the plants in a treatment to flower was 

 determined. Obviously a great many experiments were required. 

 Groups of plants had to be treated in such a way that each plant in 

 the group received the same time, intensity, and wavelength of light. 

 These plants then had to be examined a number of days later, and 

 often the experiment had to be repeated, making appropriate 



