PHOTOPERIODISM IN SEEDS AND BUDS 77 



of red when the latter was followed by blue. The period of blue irradi- 

 ation was not stated, but was presumably short. When we used IVi 

 min of red (at 100 /xw/cm-), followed by various periods of blue (at 

 100 Mw/cm-) ranging from Vi hr to 4 hr, some reversal was obtained 

 with periods of 1-2 hr, but much more effective reversal was obtained 

 with 4 hr of blue. In a further experiment, in which we used 1 V2 min 

 of red and 4 hr of blue, the effects of a succession of irradiations with 

 red and blue were investigated. It was found that repeated photo- 

 I reversal can be obtained (Table I), and that the response of the seed 



Table I. Lettuce, var. Grand Rapids, Photoreversal of 

 Promotion and Inhibition of Germination by Red and Blue 



° R, 1 J^ min red at 100 ;uw/cm- from fluorescent source; B, 4 hr 

 blue at 100 /xw/cm^ from fluorescent source. Treatments were 

 commenced 26 hr after sowing. 



is determined by the nature of the last irradiation, as in the interaction 

 between red and far red (Borthwick et ah, 1954). This would appear 

 to be the first case in which successful reversal of the effects of red by 

 blue have been obtained. 



With the series of interference filters, we attempted to determine the 

 effective spectral region for the inhibition of germination by blue, and 

 it was found that the greatest inhibition was obtained with the filter 

 peaking at 452 m.\x. 



The effects of blue have also been investigated in birch. We have 

 never successfully induced any germination of birch by exposure to 

 blue alone, but there is a marked interaction between blue and red. At 

 a temperature of 21-22°C, birch seed was exposed to 8 hr of red light 

 to induce germination, and this treatment was either preceded or fol- 

 lowed by blue at 80 /^w cm-, with the blue fluorescent source de- 

 scribed above. It was found that 24 hr of blue given after the red 

 markedly inhibited germination (Table II). On the other hand, blue 



