488 RADIATION BIOLOGY 



energy values on the ordinate of Fig. 10-1 are given in absolute units, 

 but relative values are shown in Fig. 10-2 since the choice of response 

 level is arbitrary. Relative responses for some other plants are also 

 shown in Fig. 10-2. Cocklebur, Xanthium saccharatum,'^ is similar to the 

 soybean in requiring an adequately long dark period for flowering. Its 

 floral-initiation response in the region of maximum effectiveness in the 

 red is very similar to that of the soybean on an absolute energy scale. 

 In both instances a total energy of about 50 X 10» ergs/cm^ in the region 

 near 6500 A, which can readily be given in less than 1 sec, is adequate to 

 prevent floral induction. 



Barley, Hordeum vulgare var. Wintex, and an inbred annual form of 

 Hyoscyamus niger are representative of plants in which flowering is 

 inhibited by an adequate dark period. This form of Hyoscyamus does 

 not flower in response to 10 successive dark periods of 12 hr each. Inter- 

 ruption of long dark periods in a series of alternating light and dark 

 periods causes Hyoscyamus and Wintex barley to flower. This is opposite 

 to the response of soybean and cocklebur on interrupting the dark period. 

 The long- and short-dark-period types of plants, nevertheless, have closely 

 similar action spectra for floral initiation (Borthwick et al., 1951; Parker, 

 Hendricks, and Borthwick, 1950). 



Qualitative results for formation of visible buds and open flowers of 

 several plants have been obtained by a number of workers using wave- 

 length bands separated by means of filters. The filters used were appar- 

 ently adequate, but the energy required for a given response was not 

 determined. Instead the response for a single energy level in several 

 wave-length regions has often been measured. Such results cannot be 

 completely evaluated unless the response is also measured as a function 

 of energy. A relatively ineffective wave length can appear as effective 

 as the most active region if the biological response approaches complete 

 expression. 



Razumov (1933) and Withrow and coworkers (Withrow and Benedict, 

 1936; Withrow and Biebel, 1936; Withrow and Withrow, 1940) extended 

 the natural photoperiod with radiation obtained by use of filters. They 

 reported that red radiation very effectively inhibited the flowering of 

 short-day plants and promoted the flowering of long-day plants. Blue 

 radiation was ineffective except for China aster, Callistephus chinensis 

 var. Heart of France, which Withrow and Benedict (1936) found respon- 

 sive to all wave lengths in the visible part of the spectrum at the energy 

 level used. Kleshnin (1943, 1946), using PeriUa ocymoides (P. frutescens) 

 and oats var. Pobeda, also found that all parts of the visible spectrum 

 were effective provided that sufficient energy was applied. Katunskij 

 (1937), working with millet, hemp, green beans, and peas, observed that 



2 Plants of X. saccharatum (Parker et al., 1946) were from a seed source used by 

 Hamner and Bonner (1938) and referred to by them as X. pensylvanicum. 



