82 PHOTOCONTROL OF GROWTH 



ai, 1957), Moreover, the dormancy of birch and Xanthiiim seed has 

 several other features in common with that of lettuce seed. For 

 example, light-requiring lettuce seed may be induced to germinate if 

 maintained in an atmosphere of pure oxygen (Borthwick and Robbins, 

 1928), or if the pericarp is split or pricked (Evenari and Neumann, 

 1952); this suggests that oxygen effects are important also in this 

 seed. 



These observations raise the question as to whether inhibitors also 

 play a role in the dormancy of lettuce seed. The presence of inhibitors 

 in lettuce seed has been demonstrated (Shuck, 1935; Wareing and 

 Foda, 1957; Poljakoff-Mayber et al., 1956), and one of these is a 

 water-soluble substance occurring at about the same position on 

 chromatograms as the main Xanthiiim inhibitor (Wareing and Foda, 

 1957), It has not been possible, however, to determine whether the 

 inhibitors play any role in the dormancy of lettuce seed, which is, for 

 this purpose, more difficult material than Xanthium seed. Nevertheless, 

 the close parallel between the dormancy phenomena in the seeds of 

 these two species (both members of the family Compositeae) strongly 

 suggests that the underlying mechanism is the same in both cases. It is 

 tempting, therefore, to postulate that the dormancy of lettuce seed 

 involves a growth inhibitor, the effect of which is in some way over- 

 come by light, as in birch seed. One difficulty for this hypothesis is 

 that we have recently found that certain light-requiring varieties of 

 lettuce seed contain no detectable amounts of the water-soluble 

 inhibitor found in Grand Rapids. It must be remembered, however, 

 that the light requirement of lettuce seed is very small, and this may 

 arise from the fact that the level of inhibitor present is also very low. 



The responses of birch seeds also have certain features in common 

 with those of birch buds (Wareing, 1957). It seems unhkely that 

 dormancy in the buds is due primarily to interference with oxygen 

 exchange, since frequently it is observed that the terminal bud formed 

 in response to short days is lax and by no means tightly enclosed by 

 bud scales. Moreover, it is clear that interference with oxygen ex- 

 change by the bud scales cannot be the primary factor inducing the 

 formation of resting buds, since until such a bud is formed there is no 

 interference with oxygen exchange by the shoot apical region. On the 

 other hand, some evidence that bud dormancy may be due to the 



