206 R. M. SAMISH 



days prolong growth. During early rest the effect of short days 

 can be reversed by long light periods, but later on, when rest 

 becomes deeper, it cannot be reversed any more by this treat- 

 ment. At this stage chiUing is required in order to break rest. 



The perceptor of the photoperiodic stimulus is the leaf. As a 

 matter of fact, we can prolong growth even into winter by 

 defoliation. On the other hand in spring, towards the end of 

 rest, deciduous trees will normally not have any leaves. Under 

 particularly favourable conditions part of their leaves may 

 however be retained. We had occasion to observe that, in these 

 cases, the buds in the axil of such leaves will start to grow earlier 

 than buds the subtending leaves of which have dropped. Thus, 

 some substance or substances are formed in the leaves which 

 affect the rest of the adjoining bud. 



These phenomena are interpreted as due to a mechanism 

 involving the interplay of auxins formed in light and inhibitors, 

 some of which are destroyed by light, and most by cold. Before 

 the application of paper chromatography to growth substances 

 it was impossible to distinguish whether the disappaerance 

 during rest of auxin activity was due to a decrease of 'free' auxin, 

 as suggested by Bennett and Skoog^'- or, primarily, the difference 

 between the activity of growth-promoting and inhibiting sub- 

 stances, as we suggested^^. 



The first separations of these substances in woody plants were 

 carried out by Hembergi^ on Fraxinus and by Spiegel^^ in our 

 laboratory on Vitis. They showed that the concentration of 

 inhibitors clearly reflected the state of the rest period. Thus, in 

 the grape vine (Fig. 1), the inhibitors reached a pronounced 

 peak by the end of December, dropping to zero about two weeks 

 before bud burst. They also disappeared after rest had been 

 broken by cold treatment. Furthermore, comparing the curves 

 for two Vitis crosses, V. vinifera x V. rupestris 1202, which 

 requires little chilling, and V. vinifera X V. ber/andieri 41-B, which 

 requires severe chilling, we note that the general level of inhibitor 

 concentration is proportional to the chilling requirement. Also, 

 the inhibitor concentration in the V. rupestris cross reached zero 



