40 • Photoperiodism: Attempts at Analysis 



devoted to the possible nature and metabolic function of such a 

 reversible pigment system, on the assumption, of course, that it 

 existed and was not a misinterpretation of two separate light 

 effects. The assumption has since been justified, and the specula- 

 tions may soon give way to data. Workers at Beltsville (Butler 

 et al., 1959), using relatively sophisticated spectrophotometric 

 techniques, have shown that intact tissues and properly prepared 

 extracts of etiolated (dark-grown) seedlings of various species, such 

 as corn, Zea mays, contain a pigment with the predicted reversible 

 changes in absorption characteristics in the red and far-red. The 

 pigment is present in very low concentrations— the etiolated tissue 

 in which it was observed was nearly white— and is either a protein 

 or closely bound to a protein. The development of better extrac- 

 tion and purification techniques should soon make it possible to 

 characterize the pigment further and aid in establishing its imme- 

 diate biochemical function. The rapid developments which should 

 ensue may make further discussion on these points obsolete when 

 printed. 



Even discovery of the immediate biochemical function of the 

 pigment, no easy matter in itself, will not completely clarify its 

 role in photoperiodism. Much more physiological work is still 

 required on this question. The only generalization that will hold 

 at present is that the red, far-red system mediates the low-intensity 

 light effects and may also be involved in the critical time-require- 

 ments. There is no clear evidence, however, as to the precise way 

 in which the pigment is linked to subsequent events in the induc- 

 tion process, and the relation may well differ from species to species 

 even within a given response class. 



The pigment has been dubbed "phytochrome" by its dis- 

 coverers (see Borthwick and Hendricks, 1960). Though the name 

 is unfortunate both because it is general (Greek for "plant" plus 

 "color" or "pigment") and because it is liable to be confused when 

 spoken with the cytochromes, so significant in the biochemistry of 

 respiration, it will undoubtedly be perpetuated. 



PROLONGED EXPOSURES TO LIGHT OF 

 DIFFERENT COLORS 



In the 1930's and 1940's, Funke (see Funke, 1948) used sunlight 

 filtered through white, red, or blue glass to lengthen photoperiods 



