SEED DORMANCY AND DORMANCY BREAKING 1 79 



before and after germination from various plants, let me tell 

 you something of the changes known to us to occur in one single 

 organism, the light-sensitive lettuce seed. 



The dispersal unit of the lettuce seed, an achene, is composed 

 of an embryo, the cotelydons of which serve as storage organs, a 

 two-layered endosperm, a seedcoat, and a thick-walled fruit 

 coat which, in case of the light-sensitive variety which will be 

 discussed here, contains a brownish-black pigmental. 



When the seed has been imbibed in water, germination 

 depends both on light and temiperature conditions. Below 

 approximately 18% seeds germinate to a high degree in darkness, 

 and light, therefore, increases the germination percentage only 

 to a small extent. 



Between approximately 20-28'' the seeds require light for a 

 high percentage of germination and remain dormant in the dark. 

 The light requirement is strongest in freshly harvested seeds, and 

 decreases with age of the seed. A mechanical treatment, such as 

 removing or pricking the coats and the endosperm, or chemical 

 treatment with e.g., thiourea, will cause germination even in 

 the dark. 



Above approximately 30°, dormancy cannot easily be broken 

 by light, but germination will result after various other treat- 

 ments such as stratification for a few days previous to imbibition, 

 high oxygen pressure or, again, removal of the endosperm. 



One of the most effective ways of overcoming the thermo- 

 dormancy of lettuce seeds was found by Thornton-^ who states 

 that lettuce seeds at 35° will germinate normally in an atmos- 

 phere of 40-80 %C02. 



We can already see here the interaction of various mecha- 

 nisms^o. At low temperatures, a photomechanism is taking place 

 indicated by the slightly higher germination of illuminated seeds. 

 At the same time other factors are at work which allow the seed 

 to germinate in darkness. At higher temperatures, these factors 

 cannot any longer break the dormancy, which however can still 

 be overcome by the light mechanism. At still higher temperatures 

 even this mechanism, is inhibited. Furthermore, the fact that 



References p. 190 



