PLANT HORMONES AND SEED DORMANCY 169 



Varga and her associates''^ and also Hemberg23-25^ ^^e mainly 

 interested in the mechanism regulating the dormancy of potato 

 tubers, while Wareing and his group^^ are interested mainly in 

 the dormancy of seeds and buds of woody plants. These are the 

 principal groups working on the subject but there are many 

 other investigators working in this field. 



In all these investigations it very soon became clear that it is 

 not possible to draw any definite conclusion, regarding the 

 germination of seeds or the sprouting of buds, while using the 

 extension growth bioassay. Special bioassay tests were, therefore, 

 devised for every case. 



Hemberg-3"-^ ascribed the dormancy of the potato tubers to a 

 growth inhibitor present in the potato peel. Blommaert'^^ and 

 Varga and Ferenczy^^ isolated this inhibitor and showed that 

 its amount decreases as the tubers emerge from dormancy. But 

 when this inhibitor was tested in a potato sprouting test'-^, it 

 was shown to have no effect. 



More convincing results were achieved by Wareing and his 

 group-^. They showed that the dormancy of Xanthium seeds is to 

 a large extent due to the presence of an inhibitor which prevents 

 the growth of the embryo. Germination proceeds only after 

 this inhibitor is either leached out or oxidized inside the seed. 

 They have also shown that the dormancy of Fraxinus seeds^^ is 

 regulated by an interplay between growth inhibitors and germi- 

 nation stimulators. The inhibitors present in the endosperm and 

 the embryo itself, are probably the main cause of the dormancy 

 phenomenon. During stratification there is no change in the 

 inhibitor's content, but a germination stimulator is formed which 

 counteracts the effect of the growth inhibitor and brings about 

 the breaking of dormancy, thus enabling germination (Fig. 2). 



Similar investigations were carried out in our laboratory with 

 lettuce seeds^^. The same light-sensitive variety (Grand Rapids) 

 was used. The seeds were imbibed in water, in the germination 

 inhibitor — coumarin, or in the germination stimulator — 

 thiourea. Germination inhibitors and germination stimulators 

 were followed up in all the three series (Fig. 3). It is evident that 



References p. 172 



