170 CHEMICAL AGENTS AND GROWTH 



rangement of the aromatic or heterocyclic nucleus around the 6-amino 

 group is quite important. 



This last requirement is stressed particularly in the phenyl and the 

 pyridylmethyl series of compounds. The distance of the benzene ring 

 from the nitrogen of 6-aminopurine markedly affects the response, i.e., 

 the benzyl substitution (in which the ring is one carbon removed from 

 the nitrogen) gives a group I type of response. The phenylethyl, 

 phenylpropyl, and phenylbutyl substitutions all give a group III type 

 response wherein growth in red light is less than in darkness. It thus 

 appears that the distance the ring is removed from the purine nucleus 

 is critical. With the pyridylmethyl series it appears that the orientation 

 of the ring nitrogen of pyridine with regard to the purine ring is 

 important; compare the 2-, 3-, and 4-pyridylmethyl substitutions. 

 From other data it appears that the pyridine orientation is concentra- 

 tion dependent; thus we must conduct more experiments before a final 

 conclusion may be drawn. 



Needless to say, we do not know the exact nature of this inhibitory 

 reaction nor how the chemicals of groups II and III alter the response 

 to light. We feel that adenine or some naturally occurring related 

 purine or purine derivative is intimately connected with the expansion 

 of etiolated leaves and that possibly it is the need for adenine or some 

 similar compound which is satisfied by light. Thus light might set in 

 motion a process that leads to the eventual formation of adenine or a 

 like compound, which may then be used in nucleic acid or nucleo- 

 protein synthesis. This connection needs to be studied in more detail 

 in order to gain a thorough understanding of its nature and to extend 

 the observations to systems other than our own. 



A discordant note in our observations should be mentioned at this 

 point. After the appearance in Science of our paper (Scott and Liver- 

 man, 1957) describing these experiments in a preliminary manner, 

 I received a letter from Dr. Carlos Miller (personal communication, 

 1957) saying he could not get an effect with adenine. We have since 

 exchanged seed and chemicals, with the same net result on his part. We 

 have not been able to get our system standardized since moving into 

 new quarters and, therefore, are not able to clear up the point. I 

 certainly feel that our results are correct, and I do not doubt those of 

 Dr. Miller. This difficulty probably can be resolved by showing that 



