CONTROL OF LEAF GROWTH 



169 



tissue. We then began a systematic study of the relation between 

 structure and activity of various adenine derivatives and of the many 

 kinetin-Hke compounds which were available. Table III summarizes 

 in a general way for a single concentration the relation between 

 structure and activity of examples taken from over 60 compounds 

 tested in this system. 



One may logically inquire whether this effect, particularly of adenine 

 and thiopurinesuccinic acid, is specific for this single concentration of 

 these two compounds or whether it applies throughout a concentra- 

 tion series. The basic question is, are these compounds specific for 

 the light reaction, or do they simply replace part of the light reaction, 

 and if we had used lower concentrations, would we have obtained a 

 different result? Table IV indicates that they are not simply replacing 



Table IV. Control of Leaf Expansion by Adenine and Thiopurinesuccinic Acid" 



Treatment 



Control 

 Adenine 



0.05 



0.5 



5.0 



10.0 



TPSA 



0.05 



0.5 



5.0 

 10.0 



Red 

 1.22 



2.20 

 2.79 

 2.31 

 1.96 



2.39 

 3.34 

 2.99 

 1.84 



Far Red 

 1.00 



2.19 

 2.81 

 2.29 

 2.05 



2.38 

 3.30 

 3.01 

 1.84 



° Concentration, mg/1 increase in diameter, mm. 



a part of the light reaction, but actually obviate the need for it. The 

 light action — either promotive or inhibitory — appears to be completely 

 erased by adenine. These results suggested to us the following relations 

 between structure and activity: 



1 . Response Group I. Replacement of one of the hydrogens of the 

 amino group of 6-aminopurine by a ring compound results in a pro- 

 motion which is at least partially additive to the red-light effect. 



2. Response Group II. Either nonreplacement or replacement of 

 one of the hydrogens by a short-chain aliphatic results in a disap- 

 pearance of any light effect. 



3. Response Group III. The exact configuration and spatial ar- 



