PHOTOPERIODISM IN SEEDS AND BUDS 85 



mancy. It is possible that the greater production of inhibitor under 

 short days is the result of reduced growth. The observation that dif- 

 ferences in inhibitor content can be detected after 2-5 days of treat- 

 ment, ahhough the short-day plants continued to expand leaves for 

 a further 10 days, would seem to indicate, however, that the inhibitor 

 differences are not primarily due to differences in growth. If the 

 inhibitor hypothesis is substantiated, we shall have made an important 

 step forward in the elucidation of the mechanism of photoperiodism in 

 buds and seeds. It is not suggested that photoperiodic control of 

 flowering necessarily involves growth inhibitors. Considerable evidence 

 suggests that there is much in common between photoperiodism in 

 dormancy phenomena and in the control of flowering (Wareing, 

 1956), and this would seem to imply that the basic light and dark 

 processes are identical in both types of response. Nevertheless, the 

 induction of dormancy is clearly different from the induction of 

 flowering, and it is possible that the changes in inhibitor content ob- 

 served in woody plants are secondary effects arising from earlier steps 

 in the basic photoperiodic processes. 



SUMMARY 



Photoperiodic control of dormancy is well established for both 

 buds and seeds. Among lioht-sensitive seeds the germination of some 

 Species is promoted by long days, whereas in others germination is 

 inhibited by long days and promoted by short days. The seed of 

 Nemophila insignis behaves as short-day seed at temperatures of 21- 

 22 °C. The seed of this species is inhibited by both far red and by blue. 



A reexamination of the effects of blue in Grand Rapids lettuce seed 

 has shown that blue may be promotive or inhibitory to germination 

 depending upon the duration of exposure and the imbibition period. 

 With 4-hr periods of blue, it is possible to reverse the effects of a 

 preceding exposure to red. The photoreceptors for the well-known 

 effects of red and far red in lettuce seed appear to have absorption 

 bands in the blue which overlap. The inhibition of germination by 

 blue and far red appears to involve the same photoreceptor. 



Studies of dormancy in birch seed seem to indicate that the inhibi- 

 tory effect of the pericarp and endosperm involve both the presence of 



