246 CONTROL OF REPRODUCTION 



1938; Lang and Melchers, 1943). Generally speaking, short-day 

 plants flower when exposed to light-dark cycles containing dark periods 

 longer than the critical value. Long-day plants, on the contrary, flower 

 only when exposed to light-dark cycles containing dark periods shorter 

 than the critical value. Or we may define short-day and long-day plants 

 roughly in still another way based upon their responses to interruptions 

 of a long dark period by a light flash. Short-day plants produce flowers 

 when grown on a regime of long nights, and this flowering is inhibited 

 by a light flash given during the night (Hamner and Bonner, 1938). 

 Long-day plants, on the contrary, do not flower when grown on a 

 regime of long nights, but do flower if this long night is interrupted by 

 a light flash (Katunskij, 1936). 



There are special cases, however, for which these generalizations do 

 not hold true. Harder and Giimmer (1947) and Lang (1954), for 

 example, grew short-day, and therefore long-night plants, under con- 

 ditions in which flowering occurred only if flashes of light were sup- 

 phed at periodic intervals. This is, however, a special variant of the 

 more general propositions stated above and can be understood in terms 

 of further responses to be described below. 



The photoperiodic effectiveness of a dark period, then, may be 

 negated by light, even of very low intensities, and the photochemical 

 characteristics of the process by which this takes place can be de- 

 termined. A great deal of information about photoperiodism has re- 

 sulted from such studies (Parker et ah, 1946, 1950). This information 

 may be summarized by saying that plants behave as though they con- 

 tain in the dark a pigment which absorbs red light, thereby negating 

 the photoperiodic effectiveness of a long dark period. By absorption of 

 red light the pigment appears to be converted to a different pigment 

 which absorbs at longer wavelengths, with a peak in the near infrared 

 (Borthwick et ai, 1952; Downs, 1956). Thus if a red light treatment 

 is immediately followed by a near infrared treatment, the plant behaves 

 as though it had been maintained in darkness. Photoperiodic behavior 

 is thus mediated in part by what we refer to as the pigment system. 



It is of course well known that the leaves of the plant are the per- 

 ceptors of the radiant energy. Thus if we subject one leaf of the short- 

 day plant Xanthium to long nights, the remainder of the leaves being 

 maintained in long-day conditions, the plant flowers in response to the 



