SEED DORMANCY AND DORMANCY BREAKING 177 



In another group, the coats of the dispersal unit exert their 

 influence through substances which inhibit the growth and 

 development of the embryo^' ^. These inhibitory substances 

 have actually been found everywhere in the dispersal unit, 

 including the embryo itself, but on this latter point we shall hear 

 in more detail from Dr. PoljakolT. However, in many cases, the 

 embryo germinates readily after excision though it fails to 

 germinate while enclosed in its coats because of inhibitory 

 substances present in the endosperm or other layers. In such 

 a case, even a small piece of endosperm adherent to the embryo 

 may prevent it from growing. The natural substances cited as 

 inhibitory include alkaloids, unsaturated lactones, unsaturated 

 acids, ammonia, KCN and many others. It is, however, not 

 always clear whether the inhibitory substance has a specific 

 activity or whether the resulting inhibition is an osmotic effect. 

 Lerner, Mayer and Evenari^ have shown that both effects may 

 occur simultaneously, the main effect being either osmotic or 

 specific. 



In a large variety of seeds, the inability to germinate cannot 

 be ascribed directly to the influence of the seed coats, and even 

 if the coats may be involved in the chain of events which 

 eventually lead to germination, a treatment has to be given 

 which will affect the embryo itself. This treatment may be 

 prolonged dry storage, at the end of which a seed which did not 

 germinate when imbibed immediately after harvesting will now 

 do so. In order to break the dormancy during this process of 

 after-ripening, different temperatures may be needed at different 

 times^. 



In still other cases, a period of moisture at low temperature 

 may be needed to awaken the embryo, a process which is known 

 as stratification. With other seeds, alternating temperatures may 

 be needed. The similarity between these temperature require- 

 ments and those for breaking the diapause in insects are 

 obvious^. 



In many cases, seeds have a strict light requirement which is 

 influenced by temperature (for review on this aspect, see^- ^' i^). 



References p. 190 



