Photoperiodism and Light Quality • 41 



for both LDP and SDP. Red and white were the only effective 

 photoperiod-lengthening conditions for many, with blue equivalent 

 to darkness. For a second large class, both red and blue were effec- 

 tive, as well as white. For a third very small class, only white was 

 effective, but neither red nor blue. Funke's "Class IV" has attracted 

 the most interest; these were all of the Cruciferae (Mustard family) 

 and almost all LDP, in which the blue and white, but not the red, 

 were effective in lengthening photoperiod. 



Since Funke, there has been a great deal of work, most of it in 

 the Netherlands, on the vegetative development and flowering of 

 plants grown with relatively high energies (high intensities, long 

 exposures, or both) of various colors of light. For reviews of this 

 work, see Wassink and Stolwijk (1956), Wassink et al. (1959), Meijer 

 (1959), and Van der Veen and Meijer (1959). Although many inter- 

 esting phenomena have been observed, such work is, almost without 

 exception, extremely difficult to evaluate for at least two reasons. 

 First is the immense technical difficulty of obtaining high energies 

 of light in pure spectral bands and over large enough areas to grow 

 groups of whole plants. Often the sources have been more or less 

 impure, as Funke's must have been, so that what appear to be 

 high-energy effects of the main wavelength region may include 

 low-energy effects of other wavelengths. Such contaminations have 

 been gradually reduced (see Wassink et al., 1959) but may still be 

 present. The second problem is, if anything, worse. Consider, for 

 example, the effects of long exposure to high-intensity blue light, 

 no matter how pure. The light may be affecting at least three sys- 

 tems simultaneously. The red, far-red system itself and photosyn- 

 thesis are already obvious, but one must also consider whatever 

 pigments mediate phototropism— the orientation of plant parts with 

 respect to the direction of light— since blue light is the most effec- 

 tive in this process. In addition, fluorescence of chlorophyll and 

 other compounds caused by the blue may expose the cells internally 

 to longer-wave radiations. The difficulties of disentangling such 

 effects and reaching satisfactory interpretations can hardly be over- 

 estimated. Nevertheless, some of this work is of considerable 

 interest. 



The unexpected promotion of Hyoscyamus flowering by far- 

 red at the start of the dark period, mentioned above, was first 

 reported by Stolwijk and Zeevaart (1955) who also observed that this 

 LDP entirely failed to flower when grown in continuous red light, 



