CONTROL OF LEAF GROWTH 177 



similar substance may be closely linked to the primary light reaction, 

 although much more research is needed to clarify this point. 



The data on the participation of purines in leaf expansion presented 

 above (Table III) give us some additional information concerning the 

 diversity of the biochemical pathways traversed between the initial 

 light reaction and the final response. Basically, it appears that com- 

 pounds of group II, i.e., those which give the same amount of growth 

 in fight or darkness, obviate the need for light. These compounds then 

 must have their effect many reaction steps removed from the initial 

 act of light perception. This suggests, in leaf expansion at least, that 

 the initial light reaction is probably concerned with the activation of 

 a system leading to the production of adenine or a related purine, 

 which then may be further transformed to nucleic acids, coenzymes, 

 etc. It appears that compounds like those of group I, i.e., those which 

 simply cause growth over and above that caused by light, are not on 

 the principal pathway but that they in some way spare those com- 

 pounds which are so situated. A study of the interaction of compounds 

 of these two classes should contribute markedly to a better under- 

 standing of the biochemical pathways involved in leaf expansion and 

 the site of action of both types of compounds. The compounds of 

 group III, which show an inhibition in the presence of red light, are 

 unique, and there is neither a really satisfactory explanation for their 

 action nor an indication of the relative poshion in the sequence of 

 reactions at which they are effective. One possible explanation could 

 be that the compounds are light-sensitive and are simply destroyed by 

 light, since the growth in light in the presence of the chemical in most 

 cases is equal to or greater than growth in light without the chemical 

 (Table III). In view of the difference in responses elicited by the 4- as 

 compared to the 2- and 3-pyridylmethyl analogs, however, this pro- 

 posal seems rather unlikely. 



Whether compounds of the purine group are able to substitute for 

 light in any of the responses shown in Fig. 2 is not definitely known. 

 It does appear, however, that cobalt (Sahsbury, 1957; Thimann, 

 1956), auxin (Liverman, 1955), and gibberellic acid (Long, 1956; 

 Lockhart, 1956; Kahn et al, 1957) may be involved in a number of 

 the responses, and in some instances at least they appear to replace 

 fight. The available data on how these various substances affect 



