172 A. POLJAKOFF-MAYBER 



Thiourea, a germination stimulator, apparently changes the 

 ratio between the various natural substances affecting germi- 

 nation, as it induces the formation of germination stimulators. 

 This apparently occurs during the initial phases of imbibition. 

 This change may initiate a chain of reactions eventually leading 

 to germination. 



To sum up the facts presented here, it seems that the plant 

 hormones do affect germination, perhaps by changing the initial 

 ratio between the natural germination or gi-owth stimulators and 

 inhibitors. This idea is supported by the facts that gibberellin 

 counteracts the inhibitory effect of coumarin^^ ^j^^j ^j^^^ lettuce 

 seeds do contain appreciable amounts of gibberellin-like 

 substances^^. The natural inhibitors and stimulators are as yet 

 not identified and we do not know whether they may be classified 

 as hormones. A more complete study is required for the identi- 

 fication of these substances and for the evaluation of their 

 changes under various treatments affecting germination. Al- 

 though it seems possible that the factors involved in dormancy 

 exercise their effect through the internal equilibrium of such 

 hormones, it is as yet by no means understood which are the 

 basic reactions affected by the hormones and it is as yet not 

 certain that this is the only mechanism involved. 



ACKNOWLEDGEMENTS 



Our thanks are due to Prof. P. F. Wareing and to Nature for 

 their permission for the use of Fig. 2 and to Dr. S. Blumenthal- 

 Goldschmidt for her permission for the use of Fig. 3. 



REFERENCES 



1 L. J. AuDUS, Plant Growth Substances, Leonard Hill, London, 1959. 



2 T. Weevers, Fifty Years of Plant P/i>'5/o/o^v, Chronica Botanica U.S.A., 

 1949. 



^ A. C. Leopold, Auxins and Plant Growth, University of California Press, 

 1955. 



4 P. W. Brian, S.E.B. Symposium, XI, (1) 1957, 166. 



5 B. B. Stowe and T. Yamaki, Ann. Rev. Plant Physiol, 8 (1957) 181. 



