PHOTOPERIODISM IN SEEDS AND BUDS 81 



light is primarily on the embryo, which is thereby stimulated to over- 

 come the effect of the inhibitor. 



On the other hand, it was found that it is not necessary to remove 

 the pericarp from the embryo completely in order to abolish its light 

 requirement. Slitting the pericarp and endosperm on one side, or 

 even simply pricking, is sufficient to bring about an appreciable 

 germination in the dark. This observation is difficult to interpret on 

 the "inhibitor hypothesis" and suggests rather that the inhibitory effect 

 of the pericarp is due to interference with gaseous exchange. Further 

 evidence in support of this view is seen in the fact that pricking the 

 pericarp is even more effective in stimulating germination if the seeds 

 are subsequently maintained in an atmosphere of high oxygen content 

 instead of in air. However, a high oxygen tension is ineffective if the 

 pericarp is maintained intact. These results strongly suggest that inter- 

 ference with oxygen uptake also constitutes an important part of the 

 inhibitory effect of the pericarp. Experiments to determine the mini- 

 mum oxygen requirements for the growth of birch embryos indicated 

 that they will germinate even in commercial nitrogen which has been 

 passed through alkaline pyrogallol, and which must have had an ex- 

 tremely low oxygen content. Therefore, it seems unlikely that the 

 oxygen requirement would not be met in an intact seed maintained in 

 an atmosphere of 70% oxygen. Thus interference with oxygen uptake 

 by the pericarp does not seem adequate to explain all the phenomena. 

 Now, from a study of dormancy in Xanthium seeds it appears that 

 both interference with oxygen uptake by the testa and the occurrence 

 of a growth inhibitor in the embryo play important roles, and that 

 oxygen is necessary for the breakdown of the inhibitor, before germi- 

 nation can occur (Wareing and Foda, 1957). A similar hypothesis 

 would seem to be best adapted to explain the dormancy effects in 

 birch seed. Some evidence in support of this hypothesis is seen from 

 the results of an experiment in which leached and unleached seeds 

 were first scratched and then exposed to various oxygen concentra- 

 tions. It was found that leached seeds gave an appreciably higher 

 germination at low oxygen tensions than did unleached seeds. 



We have found that gibberellic acid is effective in breaking the 

 dormancy of birch seed, as with lettuce seed (Lona, 1956; Kahn et 



