SEED DORMANCY AND DORMANCY BREAKING 



189 



EM (A) 

 No effect 



(B) 



Medium increase 

 in dark germination 

 normal root length 



Strong increase 

 in dark germination 

 normal root length 



Fig. 4. The effects of deuteron irradiation on lettuce seed germination and 

 subsequent root growth when applied to different depth levels. A to C: 

 deuterons penetrating into endosperm only. D to F: deuterons penetrating 

 deep into embryo. A and D: Weak dose: B and E: medium dose; C and 

 F: strong dose. FC: Fruit coat; SC — EN: seed coat and endosperm; 

 EM: embryo. The figures are not drawn to scale-". 



by Far Red irradiation, which means that a different mechanism 

 is involved. 



This, however, does not mean that all the treatments we have 

 mentioned affect the endosperm itself. For example, Koller^^ 

 has shown that in certain plants the radicle itself is the light 

 receptor, and recently good evidence in this respect has been 

 brought forward for lettuce seeds by Ikuma and Thiman-^. 



Most probably, therefore, the chain of events starts in the 

 embryo, possibly in the same cells which later on will be the first 

 to grow and divide, but leads in the end to an effect on the 

 endosperm, thereby resulting in increased gas exchange and 

 perhaps reduced mechanical pressure on the embryo. As to what 

 goes on in between these stages, our knowledge is less than 

 scanty. It involves enzymatic activities which will be the subject 

 of another lecture. Of course, I would be leaving this picture 

 still more incomplete, were I not to mention that resumption of 

 growth after a dormant stage certainly also involves changes in 



References p. 190 



