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DISCUSSION 



Dr. Sachs: Dr. Brian, could you comment on what you meant by 

 your evidence and that of Dr. Vlitos, concerning the role of the apical 

 bud in the intact plant? 



Dr. Brian: If you excise the apical bud from a pea plant and ap- 

 ply gibberellic acid, you don't get anything like the response that you 

 get when it is there. 



Dr. Sachs: I meant to ask about the size of the apical bud. 



Dr. Brian: A pretty large size. 



Dr. Sachs: In this case you perhaps removed the region where 

 growth actually takes place by cell division. Evidence indicates that 

 the whole region of shoot histogenesis exists within 2,000 to 3,000 mi- 

 crons, and perhaps up to 10,000 microns in some plants, below the 

 apical meristem. The apical meristem itself is completely inoperative 

 in shoot histogenesis. 



Dr. Brian: I would agree, but certainly the parts corresponding to 

 the section which increases in length by cell extension have not 

 been removed. 



Dr. Nitsch: I would like to make two comments. The first one 

 bears on what has just been said. Using oat seedlings, we have been 

 able to demonstrate that gibberellic acid acts mainly on the very 

 young, meristematic tissues, whereas auxins act on somewhat older 

 tissues, located further down the coleoptile or first internode (Nitsch, 

 J. P., and Nitsch, C. - Bui. Soc. Bot., France, in press). The other 

 comment concerns the changes caused by gibberellic acid in the 

 metabolism of the endogenous growth substances. We have been 

 studying this question ever since we discovered that the application 

 of GA to a sumac or a bean plant produces a great surge in the amount 

 of endogenous growth substances (Nitsch, J. P., and Nitsch, C. — hi: 

 Photoperiodism and Related Phenomena in Plants and Animals. R. B. 



