LIGHT AND THE PIGMENT 105 



Stem, and other plant responses. The immediately obvious conclu- 

 sion has fairly well withstood the test of time during the past 15 or 

 20 years: the pigment absorbing the light which is effective in the 

 flowering process is the same pigment which is effective in germina- 

 tion, halting of etiolation, plumular hook straightening, and many 

 other responses. This is the kind of generalization which delights the 

 heart of any scientist, the kind which is always hoped for, but seldom 

 encountered, especially in biology. 



4. The Reversible Nature of Phytochrome 



At this point in the unfolding of things (about 1951) the group at 

 Beltsville happened to have one of the truly lucky breaks which 

 sometimes reorient an entire research program — or even the 

 research in a major area of study such as plant physiology. It was 

 known that lettuce seeds placed in the spectrum were not only 

 promoted in their germination by orange-red light, but that they were 

 somewhat inhibited by light of still longer wavelengths, so-called 

 far-red. Someone then had a rare insight: the effects of red light 

 might be reversed by a subsequent illumination with far-red light. 

 This guess turned out to be correct! If lettuce seeds were illuminated 

 with red light, nearly all of them germinated, but if seeds which had 

 been so illuminated were then placed in the far-red part of the 

 spectrum, the promoting effects of the red light were overcome, and 

 actually fewer seeds germinated than in a control treatment left in 

 the dark. Furthermore, if they were exposed to red and then far-red 

 and then red, they would germinate. If red and far-red were 

 alternated many times, seeds would germinate if the last illumina- 

 tion period had been red, or they would not germinate if it had been 

 far-red, as shown in Fig. 7-4. The action spectrum for far-red 

 reversal of the red effect is shown in Fig. 7-2 and 7-3 along with 

 the red action spectrum. 



A theory developed immediately : the plant must contain a pigment 

 which changes its form by absorbing red light, and the form it 

 becomes is one which will absorb far-red light. When the far-red 

 absorbing form is illuminated with far-red, it changes back to the 

 form which absorbs red. Of course the crux of the theory was that 

 the biological response of the plant depended upon the form of the 

 pigments. After the pigment's extraction (see below), it was called 



