178 S. KLEIN 



Within certain temperature ranges the seeds have a high germi 

 nation percentage only in darkness or after exposure to a certain 

 amount of light, both qualitatively and quantitatively definable. 

 In a number of seeds, furthermore, a photoperiodic regime can 

 increase germination. 



What makes matters complicated is that frequently, in a given 

 dispersal unit, not one mechanism only is working but many 

 are coupled together, so that very restricted conditions have to 

 pertain to allow the seeds to germinate. To gain an under- 

 standing of this aspect the problem is best approached ecolo- 

 gically and we shall hear more about this from Dr. Koller. 



Another complication is that although the embryo as a whole 

 is a biological unit, various parts of it may respond differently. 

 A certain treatment may allow growth of one organ, let us say 

 the radicle, but may not be sufficient to allow further devel- 

 opment of the hypocotyl. Again, this requires the existence of 

 different and specified conditions at various times. 



All this shows that the state of dormancy is a question of 

 equilibrium between various mechanisms and not a case of 

 one mechanism alone being blocked, or of the presence or 

 absence of a certain substance. One should study, therefore, the 

 interaction of various phenomena previous to, and after, 

 dormancy-breakage, in order to get an overall picture of what is 

 going on in a certain organism; the drawback being that a 

 particular organism may be well fitted to give information on 

 one aspect but not on another. 



In order to examine details of a single factor, for example, 

 light-mechanism, it has been worthwhile to study this mechanism 

 in different organisms, and not only in regard to germination 

 alone. On the other hand, when nitrogen-metabolism is studied 

 in seeds rich in protein, and carbohydrate-metabolism in those 

 rich in starch, there is a certain danger in transferring con- 

 clusions from one type of seeds to the other. 



We have concentrated our efforts on a single object, the 

 lettuce seed, where all the aspects are equally difficult to study. 

 Instead of giving examples of differences between states 



