PHOTOPERIODISM IN SEEDS AND BUDS 79 



higher intensities than that used (100 /xw/cnr) in the present experi- 

 ments. 



There would no longer seem to be any doubt that the inhibitory 

 effects of far red and blue are operating through the same photo- 

 receptor. This conclusion is in full agreement with the fact that blue 

 and far red are frequently found to act similarly in internode elonga- 

 tion and in the flowering of certain species (see Wassink and Stolwijk, 

 1956). The promotive effects of blue on lettuce seed germination are 

 best explained on the hypothesis that the photoreceptor for red also 

 has an absorption in the blue, as suggested by Borthwick et al. 



The effects of blue clearly merit further attention not only because 

 of their importance for the study of dormancy, but also because eluci- 

 dation of the phenomena in seeds may have an important bearing on 

 the interpretation of the effects of blue on flowering. 



POSSIBLE ROLE OF INHIBITORS IN DORMANCY 



We turn now from a consideration of the photoreactions involved 

 in seed photoperiodism to what would seem to be another important 

 factor in determining the overall response. The first piece of work to 

 be described was carried out on birch seed by Dr. M. Black at 

 Manchester (1957). 



Now, the light requirement of birch seed is a property only of the 

 intact seed, since embryos from which the pericarp and endosperm 

 have been dissected will germinate equally well in both light and dark 

 and show no apparent photoperiodic effects at all. This indicates that 

 the presence of the pericarp or endosperm must have an inhibitory 

 effect on the growth of the embryo, and that light is necessary to 

 enable the embryo to overcome this inhibitory effect. 



Two possible ways in which this inhibitory effect might arise would 

 appear to be that either ( 1 ) the pericarp or endosperm contains an 

 inhibitory substance which holds the embryo dormant, or (2) these 

 seed coverings might interfere with gaseous exchange, particularly 

 with oxygen uptake by the embryo. Both these types of mechanism are 

 known to play a role in the dormancy of certain seeds. In order to 

 test the first possibility, birch seeds were extracted with 80% aqueous 

 methanol, the extract was concentrated and then chromatographed on 



