CONTROL OF LEAF GROWTH 171 



there are particular conditions under which we both will be correct. 

 Early in 1956 gibberellic acid began to be a topic of intense re- 

 search interest among plant physiologists. When it became available to 

 us in August 1956, it was immediately tested with regard to its effect 

 in causing leaf expansion. We observed that its promotive effect in 

 darkness was just about additive to the effect of red light, acting like 

 cobalt and like kinetin. Upon closer examination and after an exten- 

 sion to far-red reversal of the red effect, we noted a phenomenon, 

 Table V, which we had not observed previously with any kind of 



Control of Leaf Expansion by Gibberellic Acid" 



" Concentration, mg/1; increase in diameter, mm. 



compound, namely, that far-red light not only reverses the red light 

 effect but also appears to overcome a portion of the gibberellic acid 

 effect (Scott and Liverman, 1957). There are several other possible 

 interpretations of these results, but this is our own interpretation at 

 the present time. Although these results are not as clear-cut as they 

 might be, they do indicate a rather interesting aspect of light 

 physiology as controlled by chemicals. 



As a sidelight not directly related to seedling growth but which may 

 contribute to understanding the overall physiology of light action, I 

 would like to mention, in passing, some additional experiments which 

 we ran. We appear to have a summer dormancy problem in Texas 

 with many crops, but particularly with tomatoes, which appears to 

 be brought about by an excess of far-red light (Johnson et ai, 1956). 

 We reasoned that if our results with leaf discs were correct, we should 

 be able to reverse the dormancy of tomato fruits by spraying them 

 with gibberellic acid. The experiment worked, and a preliminary report 

 has been made (Liverman and Johnson, 1957). There is no positive 

 proof that this is the same response as observed with leaf discs, but 



