42 • Photoperiodism: Attempts at Analysis 



although it flowers rapidly in continuous white light. However, 

 small amounts of far-red given with the continuous red brought 

 about flowering, as did also blue light. Nine hours of blue once 

 every third day would permit flowering under otherwise continuous 

 red light. There is some question as to whether the slight far-red 

 contamination in the blue might be responsible for the original 

 effect reported, but it has since been repeated with much purer 

 sources (Wassink et al, 1959). Thus, in Hyoscyamus, blue and 

 far-red may be physiologically equivalent for flower initiation. 



Meijer (1959) has reported a number of complex experiments 

 on flower initiation in the SDP Salvia occidentalis. One of the 

 most interesting results is that a standard 15-minute red light- 

 break during an inductive dark period does not inhibit flowering 

 if the main (8-hour) light period is of red or green light. It does 

 inhibit, however, if the main light period is of blue (all main 

 light periods being of the same energy) or if the red or green 

 periods are supplemented with far-red. It should also be noted 

 that Salvia occidentalis, like Perilla crispa (Chapter Two) will 

 flower even in continuous white light of sufficiently low intensities; 

 at higher or even lower intensities, it again behaves like a proper 

 SDP by failing to flower. Even more complex work on Hyoscyamus 

 has been recently reported by De Lint (1960), to whose extensive 

 work the reader should go for further details. 



Work of this kind has certainly indicated that light quality 

 and intensity have more effects on flower initiation and other 

 aspects of development than can readily be explained through 

 what is known of the red, far-red system at present. Unfortunately, 

 even the effects of blue on this particular system are not under- 

 stood; there is evidence that, in various organisms, blue (at high 

 energies) may act like either red or far-red. Whether this is a 

 direct action on the red, far-red reversible pigment itself or an 

 indirect one, through other pigments or metabolic systems, is un- 

 certain. Due to the difficulties, already mentioned, of interpreting 

 such studies, the only suggestions at present are purely speculative. 



B. TIME RELATIONS AND ENDOGENOUS 

 RHYTHMS IN PHOTOPERIODISM 



The characteristic defining aspect of photoperiodism is the 

 importance of the time relations of light and dark conditions. The 



