100 p. F. Wareing and T. A. Villiers 



that the failure to grow of unleached, unchilled embryos was due to 

 an inadequate water content or unfavorable moisture conditions dur- 

 ing the germination test. 



It seemed probable that the leaching treatment would result in 

 a reduction of the level of inhibitor in the embryos, as was found for 

 Xanthium pennsylvanicum (23), but no consistent differences could 

 be detected between the inhibitor contents of leached and unleached 

 embryos. The mechanism of the leaching effect therefore remains ob- 

 scure, but it is difficult to see how leaching can be effective in over- 

 coming the dormancy of unchilled embryos except by removing some 

 block to growth. It was observed that the subsequent growth of the 

 seedlings from leached, unchilled embryos is stunted, whereas chilled 

 embryos give rise to normal seedlings even if leached. 



Experiments were also carried out to determine whether the ap- 

 plication of the inhibitor to nondormant embryos prevented their 

 growth. For this purpose, unchilled embryos were first leached for 

 48 hrs., and half were then planted on filter paper moistened with the 

 inhibitor, and the remainder on filter paper moistened with water. 

 The latter readily germinated, whereas very few of those planted on 

 inhibitor solution showed any growth. Thus, the application of the 

 inliibitor to excised leached embryos restores their dormancy. It seems 

 probable, therefore, that the inhibitor must play an important role 

 in controlling the dormancy of the embryos in vivo. 



The question then arises as to whether chilling treatment is ef- 

 fective in breaking dormancy because it results in a reduction of the 

 inhibitor level. To test this possibility, extracts were made of seeds 

 which had been chilled at to 1° C. for varying periods up to 6 

 months. It was found that there was very little change in the inhib- 

 itor content of the seeds (endosperm and embryo) over this period, 

 despite the fact that seeds which had been chilled for 6 months read- 

 ily germinated when transferred to warm conditions. No evidence 

 could thus be obtained of an appreciable redtiction of inhibitor level 

 as a result of chilling. 



Thiourea is also effective in stimulating the germination of dor- 

 mant, unleached embryos; if the embryos are placed on filter paper 

 moistened with 0.5 per cent thiourea, they germinate rajjidly. In 

 order to determine whether thiourea affects the level of inhibitor, 

 embryos treated with 0.5 per cent thiourea for 48 hrs. were extracted 

 and the inhibitor content compared with that of imtreated embryos; 

 no appreciable difference in inhibitor content could be detected. 

 Neither chilling nor thiourea, therefore, brings about a significant re- 

 duction in the inhibitor content. It remains possible that chilling re- 

 sults in ;in increase in an endogenous promoter. No evidence could be 



